From 663efa81fa23d36a67a0d25f67afc3e6ed6c64d5 Mon Sep 17 00:00:00 2001 From: Marc Mueller <30130371+cdce8p@users.noreply.github.com> Date: Sat, 31 Jan 2026 17:46:20 +0100 Subject: [PATCH 1/7] PEP 823: None-aware access operators --- .github/CODEOWNERS | 1 + peps/pep-0823.rst | 1330 ++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 1331 insertions(+) create mode 100644 peps/pep-0823.rst diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS index 1be6cc950fb..b068fe9c271 100644 --- a/.github/CODEOWNERS +++ b/.github/CODEOWNERS @@ -699,6 +699,7 @@ peps/pep-0819.rst @emmatyping peps/pep-0820.rst @encukou peps/pep-0821.rst @JelleZijlstra peps/pep-0822.rst @methane +peps/pep-0823.rst @gvanrossum # ... peps/pep-2026.rst @hugovk # ... diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst new file mode 100644 index 00000000000..c843724fc2e --- /dev/null +++ b/peps/pep-0823.rst @@ -0,0 +1,1330 @@ +PEP: 823 +Title: None-aware access operators +Author: Marc Mueller +Sponsor: Guido van Rossum +Discussions-To: Pending +Status: Draft +Type: Standards Track +Created: 31-Jan-2025 +Python-Version: 3.15 + + +Abstract +======== + +This PEP proposes adding two new operators. + +* The "``None``-aware attribute access" operator ``?.`` ("maybe dot") +* The "``None``-aware indexing" operator ``?[ ]`` ("maybe subscript") + +Both operators evaluate the left hand side, check if it is ``not None`` +and only then evaluate the full expression. They are roughly equivalent +to: + +.. code-block:: python + + # a.b?.c + _t.c if ((_t := a.b) is not None) else None + + # a.b?[c] + _t[c] if ((_t := a.b) is not None) else None + +See the `Specification`_ section for more details. + + +Terminology +=========== + +An attribute or value is ``optional`` + In the context of this PEP an attribute or value is considered + ``optional`` if it is always present but can be ``None``. + + .. code-block:: python-console + + >>> class A: + ... def __init__(self, val: int | None) -> None: + ... self.val = val + ... + >>> a = A(None) + >>> hasattr(a, "val") + True + +An attribute or value is ``missing`` + An attribute or value is considered ``missing`` if it is not present + at all. For ``typing.TypedDict`` these would be ``typing.NotRequired`` + keys when they are not preset. + + .. code-block:: python-console + + >>> class A: + ... val: int | None + ... + >>> a = A() + >>> hasattr(a, "val") + False + + +Motivation +========== + +First officially proposed ten years ago in (the now deferred) :pep:`505` +the idea to add ``None``-aware access operators has been along for +some time now, discussed at length in numerous threads, most recently +in [#discuss_revisit_505]_ and [#discuss_safe_navigation_op]_. This PEP +aims to capture the current state of discussion and proposes a specification +for addition to the Python language. In contrast to :pep:`505`, it will +only focus on the two access operators. See the `Deferred Ideas`_ section +for more details. + +``None``-aware access operators are not a new invention. Several other +modern programming languages have so called "``null``-aware" or +"optional chaining" operators, including TypeScript [#ts]_, +ECMAScript (a.k.a. JavaScript) [#js]_, C# [#csharp]_, Dart [#dart]_, +Swift [#swift]_, Kotlin [#kotlin]_, Ruby [#ruby]_, PHP [#php]_ and more. + +The general idea is to provide access operators which can traverse +``None`` values without raising exceptions. + +Nested objects with ``optional`` attributes +------------------------------------------- + +When writing Python code, it is common to encounter objects with ``optional`` +attributes. Accessing attributes, subscript or function calls can raise +``AttributeError`` or ``TypeError`` at runtime if the value is ``None``. +Several common patterns have developed to ensure these operations will +not raise. The goal for ``?.`` and ``?[ ]`` is to make reading and writing +these expressions much simpler while being predictable and doing the +correct things intuitively. + +.. code-block:: python + + from dataclasses import dataclass + + @dataclass + class Sensor: + machine: Machine | None + + @dataclass + class Machine: + line: Line | None + + @dataclass + class Line: + department: Department + + @dataclass + class Department: + engineer: Person | None + + @dataclass + class Person: + emails: list[str] | None + + def get_person_email(sensor: Sensor) -> str | None: + """Get first listed email address if it exists.""" + if ( + sensor.machine + and sensor.machine.line + and sensor.machine.line.department.engineer + and sensor.machine.line.department.engineer.emails + ): + return sensor.machine.line.department.engineer.emails[0] + return None + +A simple function which will most likely work just fine. However, there +are a few subtle issues. For one, each condition only checks for truthiness. +Would for example ``Machine`` overwrite ``__bool__`` to return ``False`` at +some point, the function would just return ``None``. This is problematic +since ``None`` is a valid return value already. Thus this would not raise +an exception in the caller and even type checkers would not be able to +detect it. The solution here is to compare with ``None`` instead. + +.. note:: + + It is assumed that if ``Person.emails is not None``, it will + always contain at least one item. This is done in order to avoid + confusion around potential error cases. The goal for this PEP is to + make the ``[ ]`` operator safe for ``optional`` attributes which + could raise a ``TypeError``. It is not to simplify accessing + elements in a sequence of unknown length which could raise an + ``IndexError`` instead. See `Add list.get(key, default=None)`_ in + the deferred ideas section for that. + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + if ( + sensor.machine is not None + and sensor.machine.line is not None + and sensor.machine.line.department.engineer is not None + and sensor.machine.line.department.engineer.emails is not None + ): + return sensor.machine.line.department.engineer.emails[0] + return None + +This is better, but here each attribute lookup is still performed +multiple times. If one of these attributes were a custom property or +a class would overwrite ``__getattribute__``, it could be possible +that the attribute values are different for each line. To resolve that +the lookup results need to be stored in a temporary variable. + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + if ( + (machine := sensor.machine) is not None + and (line := machine.line) is not None + and (engineer := line.department.engineer) is not None + and (emails := engineer.emails) is not None + ): + return emails[0] + return None + +Writing it like this is correct but, especially for deeply nested +object hierarchies, difficult to read and easy to get wrong. + +Alternative approaches include wrapping the whole expression with +a try-except block. While this would also achieve the desired +output, it as well has the potential to introduce errors which +might get unnoticed. E.g. if the ``Line.department`` attribute gets +deprecated, in the process making it ``optional`` and always return +``None``, the function would still succeed, even though the input changed +significantly. + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + try: + return sensor.machine.line.department.engineer.emails[0] + except AttributeError, TypeError: + return None + +Another approach would be to use a ``match`` statement instead. This +will work fine but is easy to get wrong as well. It is strongly +recommended to use keyword attributes as otherwise any change in +``__match_args__`` would cause the pattern match to fail. +If any attribute names change, the match statement needs to be +updated as well. Even IDEs can not reliably do that themselves since a +class pattern is not restricted to existing attributes and can instead +match any possible name. For sequence patterns it is also necessary +to remember the wildcard pattern. Lastly, using ``match`` is significantly +slower because for each class pattern an ``isinstance`` check is performed +first. This could be somewhat mitigated by using ``object(...)`` instead, +though reading the pattern would be considerably more difficult. + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + match sensor: + case Sensor( + machine=Machine( + line=Line( + department=Department( + engineer=Person( + emails=[email, *_]))))): + return email + case _: + return None + +In contrast to the code shown so far, the "``None``-aware attribute access" +and the "``None``-aware indexing" operators are designed to make writing +safe nested attribute access, subscript and function calls easy. + +To start, assume each attribute, subscript and function call is +``not-optional``: + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + return sensor.machine.line.department.engineer.email[0] + +Now insert ``?`` after each ``optional`` subexpression. IDEs and most +type checkers will be able to help with identifying these. *Spaces added +for clarity only, though still valid*: + +:: + + def get_person_email(sensor: Sensor) -> str | None: + return sensor.machine? .line? .department.engineer? .email? [0] + # ^^^^^^^^ ^^^^^ ^^^^^^^^^ ^^^^^^ + +The complete function would then be: + +:: + + def get_person_email(sensor: Sensor) -> str | None: + return sensor.machine?.line?.department.engineer?.email?[0] + +Which is roughly equivalent to the example code above if the temporary +variables ``_t1`` till ``_t4`` would not be created at runtime: + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + if ( + (_t1 := sensor.machine) is not None + and (_t2 := _t1.line) is not None + and (_t3 := _t2.department.engineer) is not None + and (_t4 := _t3.emails) is not None + ): + return _t4[0] + return None + +See `Specification`_ for more details on how the expression is evaluated. + +Parsing structured data +----------------------- + +The ``?.`` and ``?[ ]`` operators can also aid in the traversal of +structured data, oftentimes coming from JSON and parsed as nested +dicts and lists. It is worth noting though that the operators do not +handle ``missing`` attributes / data. For dictionaries a useful helper is +the ``.get(key, default=None)`` method with a default. Depending on +the specific use case, pattern matching might also be a viable +alternative here. + +.. code-block:: python + + from typing import NotRequired, TypedDict + + class Sensor(TypedDict): + machine: Machine | None + + class Machine(TypedDict): + # Note the 'NotRequired' here! + line: NotRequired[Line | None] + + class Line(TypedDict): + department: Department + + class Department(TypedDict): + engineer: Person | None + + class Person(TypedDict): + emails: list[str] | None + + def get_person_email(data: Sensor) -> str | None: + match data: + case { + "machine": { + "line": { + "department": { + "engineer": { + "emails": [email, *_], + } + } + } + } + }: + return email + case _: + return None + +Writing it using ``?.`` and ``?[ ]`` would look like this. Note that +``NotRequired`` is "translated" to ``.get("line")``. + +:: + + def get_person_email(data: Sensor) -> str | None: + return ( + data["machine"]?.get("line")? + ["department"]["engineer"]?["emails"]?[0] + ) + +Which is roughly equivalent to: + +.. code-block:: python + + def get_person_email(data: Sensor) -> str | None: + if ( + (_t1 := data["machne"]) is not None + and (_t2 := _t1.get("line")) is not None + and (_t3 := _t2["department"]["engineer"]) is not None + and (_t4 := _t3["emails"]) is not None + ): + return _t4[0] + return None + +Other common patterns +--------------------- + +A collection of additional patterns which could be improved with +``?.`` and ``?[ ]``. It is not the goal to list every foreseeable +use case but rather to help recognize these patterns which often +hide in plain sight. Attribute and function names have been shortened. + +.. note:: + + Most patterns below are **not** fully identical. As mentioned + `earlier `_, it is common + to use boolean expressions to filter out ``None`` values. Other + falsy values, e.g. ``False``, ``""``, ``0``, ``[]``, ``{}`` or custom + objects which overwrite ``__bool__``, are filtered out too though. + If code relied on this property, the expression cannot necessarily + be replaced with ``?.`` or ``.[ ]``. + +:: + + # In assignments + + x = a.b if (a is not None) else None + x = a?.b + +:: + + # In if statements often used as guard clause with early + # return or raising of an exception + + if not (a and a.b == val): ... + if not (a?.b == val): ... + + if not (a and a.lower()): ... + if not a?.lower(): ... + +:: + + # Misc expressions + + a and a.b and a.b.c + a?.b?.c + + a.b and a.b[0].c and a.b[0].c.d and a.b[0].c.d[0].e + a.b?[0].c?.d?[0].e + + d1: dict | None + d1 and key in d1 and d1[key] + d1?.get(key) + + d2: dict + key in d2 and d2[key][other] + d2.get(key)?[other] + + key in d2 and d2[key].do_something() + d2.get(key)?.do_something() + + (c := a.b) and c.startswith(key) + a.b?.startswith(key) + + (b := a.get(key)) and b.get(other) == 2 + a.get(key)?.get(other) == 2 + + (b := a.get(key)) and b.strip().lower() + a.get(key)?.strip().lower() + + +Specification +============= + +The maybe-dot and maybe-subscript operators +------------------------------------------- + +Two new operators are added, ``?.`` ("maybe-dot") and ``?[ ]`` +("maybe subscript"). Both operators first evaluate the left hand side +(the ``base``). The result is cached, so that the expression is not +evaluated again. It is checked if the result is not ``None`` and only +then is the remaining expression (the ``tail``) evaluated as if normal +attribute or subscript access were used. + +.. code-block:: python + + # base?.tail + (_t.tail) if ((_t := base) is not None) else None + +The ``base`` can be replace with any number of expressions, including +`Groups`_ while the ``tail`` is limited to attribute access, subscript, +their ``None``-aware variants and call expressions. + +.. code-block:: python + + # a.b?.c + _t.c if ((_t := a.b) is not None) else None + + # a.b?[c] + _t[c] if ((_t := a.b) is not None) else None + + # a.b?.c() + _t.c() if ((_t := a.b) is not None) else None + + # a?.b?.c.d + _t2.c.d if ( + (_t2 := _t1.b if ((_t1 := a) is not None) else None) is not None + ) else None + +Short-circuiting +**************** + +If the left hand side (the ``base``) for ``?.`` or ``?[ ]`` evaluates to +``None``, the remaining expression (the ``tail``) is skipped and the +result will be set to ``None`` instead. The ``AttributeError`` for +accessing a member of ``None`` or ``TypeError`` for trying to subscribe +to ``None`` are omitted. It is therefore not necessary to change +subsequent ``.`` or ``[ ]`` on the right hand side just because a +``?.`` or ``?[ ]`` is used prior. + +:: + + >>> a = None + >>> print(a?.b.c[0].some_function()) + None + +The ``None``-aware access operators will only short-circuit expressions +containing name, attribute access, subscript, their ``None``-aware +counterparts and call expressions. As a rule of thumb, short-circuiting +is broken once a (soft-) keyword is reached. + +:: + + >>> a = None + >>> print(a?.b.c) + None + >>> print(a?.b.c or "Hello") + 'Hello' + >>> 2 in a?.b.c + Traceback (most recent call last): + File "", line 1, in + 2 in a?.b.c + TypeError: argument of type 'NoneType' is not a container or iterable + >>> 2 in (a?.b.c or ()) + False + +Grouping +******** + +Using ``?.`` and ``?[ ]`` inside groups is possible. In addition to the +rules laid out in the `previous `_ section, +short-circuiting will also be broken at the end of a group. For example +the expression ``(a?.b).c`` will raise an ``AttributeError`` on ``.c`` +if ``a = None``. This is conceptually identical to extracting the group +contents and storing the result in a temporary variable before +substituting it back into the original expression. + +:: + + # (a?.b).c + + _t = a?.b + _t.c + +Common use cases for ``None``-aware access operators in groups are +boolean or conditional expressions which can provide a fallback value +in case the first part evaluates to ``None``. + +:: + + (a.b?.c or d).e?.func() + + # a.b?.c + _t2 = _t1.c if ((t1 := a.b) is not None) else None + + # (... or d) + _t3 = _t2 if _t2 else d + + # (...).e?.func() + _t4.func() if ((_t4 := _t3.e) is not None) else None + +Assignments +*********** + +``None``-aware access operators may only be used in a ``Load`` context. +Assignments are not permitted and will raise a ``SyntaxError``. + +:: + + >>> a?.b = 1 + File "", line 1 + a?.b = 1 + ^^^^ + SyntaxError: cannot assign to none aware expression + +It is however possible to use them in `groups `_, though care +must be taken so these can be evaluate properly. + +:: + + class D: + c = 0 + + a = None + d = D() + + (a?.b or d).c = 1 + + # This would be evaluated as + _t2 = t1.b if ((t1 := a) is not None) else None + + if _t2: + _t2.c = 1 + else: + d.c = 1 + +Await expressions +***************** + +``None``-aware access operations are permitted in ``await`` expressions. +It is up to the developer to make sure they do not evaluate to ``None`` +at runtime otherwise a ``TypeError`` is raised. This behavior is similar +to awaiting any other variable which can be ``None``. + +AST changes +----------- + +Two new AST nodes are added ``NoneAwareAttribute`` and ``NoneAwareSubscript``. +They are the counterparts to the existing ``Attribute`` and ``Subscript`` +nodes. Notably there is no ``expr_context`` attribute because the new nodes +do not support assignments themselves and thus the context will always be +``Load``. Furthermore, an optional ``group`` attribute is added for all +expression nodes. It is set to ``1`` if the expression is the topmost +node in a group, ``0`` otherwise. + +:: + + expr = ... + | Attribute(expr value, identifier attr, expr_context ctx) + | Subscript(expr value, expr slice, expr_context ctx) + ... + | NoneAwareAttribute(expr value, identifier attr) + | NoneAwareSubscript(expr value, expr slice) + + attributes (int? group, int lineno, int col_offset, + int? end_lineno, int? end_col_offset) + +Grammar changes +--------------- + +A new ``?`` token is added. In addition the ``primary`` grammar rule is +updated to include ``none_aware_attribute`` and ``none_aware_subscript``. + +.. code-block:: PEG + + primary: + | primary '.' NAME + | none_aware_attribute + | primary genexp + | primary '(' [arguments] ')' + | primary '[' slices ']' + | none_aware_subscript + | atom + + none_aware_attribute: + | primary '?' '.' NAME + + none_aware_subscript: + | primary '?' '[' slices ']' + +Multiline formatting +******************** + +Using two separate tokens to express ``?.`` and ``?[`` allows developers +to insert a space or line break as needed. For multiline expressions it +enables that ``?`` is appended to the ``optional`` subexpression whereas +``.`` or ``[`` could be moved to the next line. This is indented merely +as an option for developers. Everyone is free to choose a style that fits +their needs, especially code formatters might prefer a style which +conforms better to their existing preferences. An example of what +is possible: + +:: + + def get_person_email(data: Sensor) -> str | None: + return ( + data["machine"]? + .get("line")? + ["department"]["engineer"]? + ["emails"]? + [0] + ) + + +Backwards Compatibility +======================= + +``None``-aware access operators are **opt-in**. Existing programs will +continue to run as is. So far code which used either ``?.`` or ``?[ ]`` +raised a ``SyntaxError``. + + +Security Implications +===================== + +There are no new security implications from this proposal. + + +How to Teach This +================= + +After students know how the attribute access ``.`` and subscript ``[ ]`` +operators work, they may learn about the "``None``-aware" versions for +both. + +Students may find it helpful to think of ``?.`` and ``?[ ]`` as a +combination of two different actions. First the ``?`` postfix represents +an ``is not None`` check on the subexpression with short-circuiting +if the check fails. If it succeeds, the attribute and subscript access +are performed like normal. + +Experienced developers may find that, after learning about the PEP, they +start to notice the patterns described in the `Motivation`_ section in +their own code bases. + + +Reference Implementation +======================== + +A reference implementation is available at +https://github.com/cdce8p/cpython/tree/pep-XXX. A online demo can be +tested at https://pepXXX-demo.pages.dev/. + + +Deferred Ideas +============== + +Coalesce ``??`` and coalesce assignment operator ``??=`` +-------------------------------------------------------- + +:pep:`505` also suggested the addition of a "``None`` coalescing" operator +``??`` and a "coalescing assignment" operator ``??=``. As the ``None``-aware +access operators have their own use cases, the coalescing operators were +moved into a separate document, see PEP-824. Both proposals can be +adopted independently of one another. + +``None``-aware function calls +----------------------------- + +The ``None``-aware access operators work for attribute and index access. +It seems natural to ask if there should be a variant which works for +function invocations. It might be written as ``a.foo?()`` which would +be equivalent to: + +.. code-block:: python + + _t1() if ((_t1 := a.foo) is not None) else None + +This has been deferred on the basis that the proposed operators are +intended to help for nested objects with ``optional`` attributes and +the parsing of structured data, *not* the traversal of arbitrary class +hierarchies. + +A workaround would be to write ``a.foo?.__call__(arguments)``. + +Add ``list.get(key, default=None)`` +----------------------------------- + +It was suggested to add a ``.get(key, default=None)`` method to ``list`` +and ``tuple`` objects, similar to the existing ``dict.get`` method. This +could further make parsing of structured data easier since it would no +longer be necessary to check if a ``list`` or ``tuple`` is long enough +before trying to access the n-th element avoiding a possible +``IndexError``. While potentially useful, the idea is out of the scope +for this PEP. + + +Rejected Ideas +============== + +Exception-aware operators +------------------------- + +Arguably, the reason to short-circuit an expression when ``None`` is +encountered is to avoid the ``AttributeError`` or ``TypeError`` that +would be raised under normal circumstances. Instead of testing for +``None``, it was suggested that ``?.`` and ``?[ ]`` could instead handle +``AttributeError`` and ``TypeError`` and skip the remainder of the +expression. Similar to nested try-except blocks. + +While this would technically work, it's not at all clear what the result +should be if an error is caught. Furthermore, this approach would hide +genuine issues like a misspelled attribute which would have raised an +``AttributeError``. There are also already established patterns to +handle these kinds of errors in the form of ``getattr`` and +``.get(key, default=None)``. + +As catching exceptions would be unexpected and hide potential errors, +it is rejected. + +Add a ``maybe`` keyword +----------------------- + +The ``None``-aware access operators only check for ``None`` in the place +there are used. If multiple attributes in an expression can return ``None``, +it might be necessary to add them multiple times ``a?.b.c?[0].d?.e()``. +It was suggested to instead add a new soft-keyword ``maybe`` to prefix +the expression: ``maybe a.b.c[0].d.e()``. A ``None`` check would then be +added for each attribute and item access automatically. + +While this might be easier to write at first, it introduces new issues. +When using explicit ``?.`` and ``?[ ]`` operators, the input space is well +defined. Only ``a``, ``.c`` and ``.d`` are expected to possibly be ``None``. +If ``.b`` all of the sudden is also ``None``, it would still raise an +``AttributeError`` since it was unexpected. That would not happen for +``maybe``. This behavior is problematic since it can subtly hide real +issues. As the expression output can already be ``None``, the space of +potential outputs didn't change and as such no error would appear. + +If it is the intend to catch all ``AttributeError`` and ``TypeError``, +a try-except block can be used instead. + +As the ``?.`` and ``?[ ]`` would allow developers to be more explicit in +their intend, this suggestion is rejected. + +Remove short-circuiting +----------------------- + +It was suggested to remove the `Short-circuiting`_ behavior completely +because it might be too difficult to understand. Developers should +instead change any subsequent attribute access or subscript to their +``None``-aware variants. + +:: + + # before + a.b.optional?.c.d.e + + # after + a.b.optional?.c?.d?.e + +The idea has some of the same challenges as `Add a maybe keyword`_. +By forcing the use of ``?.`` or ``?[ ]`` for attributes which are +``not-optional``, it will be difficult to know if the ``not-optional`` +attributes ``.c`` or ``.d`` suddenly started to return ``None`` as well. +The ``AttributeError`` would have been silenced. + +Another issue especially for longer expressions is that **all** +subsequent attribute access and subscript operators need to be changed +as soon as just one attribute in a long chain is ``optional``. Missing +just one can instantly cause a new ``AttributeError`` or ``TypeError``. + +``?`` Unary Postfix operator +---------------------------- + +To generalize the ``None``-aware behavior and limit the number of new +operators introduced, a unary, postfix operator ``?`` was considered. +``?.`` or ``?[ ]`` could then be considered to be two separate operators. + +While this might have made teaching the operators a bit easier, just one +instead of two new operators, it may also be **too general**, in a sense +that it can be combine with any other operator. For example it is not +clear what the following expressions would mean: + +:: + + >>> x? + 1 + >>> x? -= 1 + >>> x? == 1 + >>> ~x? + >>> [*x?] + +Even if a default meaning of ``is not None else None`` is assumed, the +expressions are likely to raise errors at some point. + +:: + + >>> x? + 1 + >>> (_t1 if ((_t1 := x) is not None) else None) + 1 + +This degree of generalization is not useful. The ``None``-aware access +operators where intentionally chosen to make it easier to access +values in nested objects with ``optional`` attributes. + +If future PEPs want to introduce new operators to access attributes or +call methods, e.g. a chaining operator, it would be advisable to consider +if a ``None``-aware variant for it could be useful, at that time. + +Builtin function for traversal +------------------------------ + +There are a number of libraries which provide some kind of object +traversal functions. The most popular likely being ``glom`` [#glom]_. +Others include ``jmespath`` [#jmespath]_ and ``nonesafe`` [#nonesafe]_. +The idea is usually to pass an object and the lookup attributes as +string to a function which handles the evaluation. It was suggested +to add a ``traverse`` or ``deepget`` function to the stdlib. + +.. code-block:: python-console + + # pip install glom + >>> from glom import glom + >>> data = {"a": {"b": {"c": "d"}}} + >>> glom(data, "a.b.c") + 'd' + >>> glom(data, "a.b.f.g", default=2) + 2 + +While these libraries do work and have its use cases, especially +``glom`` provides an excellent interface to extract and combine multiple +data points from deeply nested objects, they do also have some +disadvantages. Passing the lookup attributes as a string means that often +times there are no more IDE suggestions. Type checking these expressions +is also limited. Furthermore, normal function calls can not provide +short-circuiting, so they would still need to be combined with assignment +and conditional expressions. + +Maybe function +-------------- + +Another suggestion was to add a ``maybe`` function which would return +either an instance of ``Something`` or an instance of ``Nothing``. +``Nothing`` would override the dunder methods in order to allow chaining +on ``optional`` attributes. + +A Python package called ``pymaybe`` [#pymaybe]_ provides a rough +approximation. An example could look like this: + +.. code-block:: python-console + + # pip install pymaybe + >>> from pymaybe import maybe + >>> data = {"a": {"b": {"c": "d"}}} + >>> type(maybe(data)["a"]["b"]["c"]) + + >>> maybe(data)["a"]["b"]["c"] + 'd' + >>> type(maybe(data)["a"]["b"]["c"]["e"]) + + >>> maybe(data)["a"]["b"]["c"]["e"] + None + +While this could work, ``Something`` and ``Nothing`` are only wrapper +classes for the actual values which adds its own challenges. For example +to filter out ``None`` in a subsequent operation an ``is not None`` +check would always return ``True`` and instead ``.is_some()`` would need +to be used. This would make adopting it across a large codebase +difficult and limit its usefulness. Additionally any pure Python +implementation can not really short-circuit the expression. The best +it can do is to implement no-ops on the wrapper classes. + +As such a builtin ``maybe`` function to support accessing nested objects +with ``optional`` attributes is rejected. + +Result object +------------- + +It was suggested to introduce a ``Result`` object similar to how +``asyncio.Future`` works today. Expressions marked with a special +keyword or syntax would then return an instance of ``Result`` instead +of the evaluated expression. The actual value could then be retrieved +by calling ``.result()`` or ``.exception()`` on it. With that it could +be possible to gracefully handle ``None``-aware expression as well. + +While this is an interesting idea, it would be a disruptive change how +expressions need to be written and evaluated today. + +An advantages of the ``?.`` and ``?[ ]`` operators is that they do not +change the result much aside from adding ``None`` as a possible return +value of an expression. As such they are a better solution for the use +cases outlined in the `Motivation`_ section. + +No-Value Protocol +----------------- + +The ``None``-aware access operators could be generalized to user-defined +types by defining a protocol to indicate when a value represents +"no value". Such a protocol may be a dunder method ``__has_value__(self)`` +that returns ``True`` if the value should be treated as having a value +and ``False`` if the value should be treated as no value. + +In the specification section, all uses of ``x is not None`` would be +replaced with ``x.__has_value__()``. + +There are a few obvious candidates like ``math.nan`` and ``NotImplemented``. +However, while these could be interpreted as representing no value, the +interpretation is **domain specific**. For the language itself they *should* +still be treated as values. For example ``math.nan.imag`` is well defined +(it is ``0.0``) and so short-circuiting ``math.nan?.imag`` to return +``None`` would be incorrect. + +As ``None`` is already defined by the language as being the value that +represents "no value" the idea is rejected. + +Use existing syntax or keyword +------------------------------ + +Some comments suggested to use existing syntax like ``->`` for the +``None``-aware access operators, e.g. ``a->b.c``. + +Though possible, the ``->`` operator is already used in Python for +something completely different. Additionally, a majority of other +languages which support "``null``-aware" or "optional chaining" +operators use ``?.``. Some exceptions being Ruby [#ruby]_ with ``&.`` +or PHP [#php]_ with ``?->``. The ``?`` character does not have an +assigned meaning in Python just yet. As such it makes sense to adopt +the most common spelling for the ``None``-aware access operators. +Especially considering that it also works well with the "normal" +``.`` and ``[ ]`` operators. + +Defer ``None``-aware indexing operator +-------------------------------------- + +A point of discussion was the ``?[ ]`` operator. Some thought it might be +missed to easily in an expression ``a.b?[c]``. To move the discussion +forward, it was suggested to defer the operator for later. + +Though it is often helpful to reduce the scope to move forward at all, +the ``?[ ]`` operator is necessary to efficiently get items from +``optional`` objects. While for dictionaries a suitable alternative is to +use ``d?.get(key)``, for general objects developers would have needed +to defer to ``o?.__getitem__(key)``. + +Furthermore, any future PEP just for a ``?[ ]`` would have likely needed +to included a lot of the arguments and objections listed in this one +again. As such it makes sense to include both operators in the same PEP. + +While adding ``list.get(key, default=None)`` as suggested in +`Add list.get(key, default=None)`_ would reduce the need for ``?[ ]`` +for lists and tuples and as such would be a valuable addition to the +language itself, it doesn't remove the need for arbitrary objects which +implement a custom ``__getitem__`` method. + +Ignore groups for short-circuiting +---------------------------------- + +An earlier version of this PEP suggested the short-circuiting +behavior should be indifferent towards grouping. It was assumed that +short-circuiting would be broken already for more complex group +expressions like ``(a?.b or c).d`` by the behavior outline in the +`Short-circuiting`_ section, while for simpler ones like ``(a?.b).c`` +the grouping was considered trivial and the expression would be equal to +``a?.b.c``. The advantage being that developers would not have to +look for groupings when evaluating simpler expressions. As long as +any ``None``-aware access operator was used and the expression was not +broken by a (soft-) keyword, it would return ``None`` instead of raising +an ``AttributeError`` or ``TypeError``. + +This suggestion was rejected in favor of the specification outline in +the `Grouping`_ section since it violates the substitution principle. +An expression ``(a?.b).c`` should behave the same whether or not ``a?.b`` +is written inline inside a group or defined as a separate variable. + +:: + + (a?.b).c + + _t = a?.b + _t.c + +Furthermore, defining the short-circuiting behavior that way would have +been a deviation from the already established behavior in +languages like JS [#js_short_circuiting]_ and C# [#csharp]_. + +Change ``primary`` rule to be right-recursive +--------------------------------------------- + +The ``primary`` grammar rule as it is defined [#py_grammar]_ is +left-recursive. This can make it difficult to reason about especially +with regards to the `Short-circuiting`_ and `Grouping`_ behavior. + +It was therefore proposed to make the ``None``-aware access operators +part of the ``primary`` rule right-recursive instead. The expression +``a.b?.c[0].func()`` would then roughly be parsed as: + +.. code-block:: python + + NoneAwareAttribute( + base=Attribute( + expr=Name(identifier="a"), + identifier="b" + ) + tail=[ + AttributeTail(identifier="c"), + SubscriptTail(slice=Constant(value=0)), + AttributeTail(identifier="func") + CallTail(args=[], keyword=[]) + ] + ) + +While this approach would clearly define which parts of an expression +would be short-circuited, it has several drawbacks. To implement it at +least three additional AST nodes have to be added ``AttributeTail``, +``SubscriptTail`` and ``CallTail``. As these are right-recursive now, +reusing code from the ``Attribute``, ``Subscript`` and ``Call`` nodes +might prove difficult. Not to mention that the ``NoneAware*`` nodes +would contain parts that are both left- and right-recursive which +would be confusing. + +In comparison the proposed `Grammar changes`_ are intentional kept to +a minimum. The ``None``-aware access operators should behave more or +less like a drop-in replacement for ``.`` and ``[ ]``, only with the +behavior outline in this PEP. + + +Common objections +================= + +Difficult to read +----------------- + +A common objection raised during the discussion was that ``None``-aware +operators are difficult to read in expressions and add line noise. They +might be too easy to miss besides "normal" attribute access and subscript +operators. + +This is a valid concern. Especially for long lines, it is not difficult +to imaging a ``?`` hiding somewhere. However, as with all proposals the +downsides have to be weight against the alternatives. As shown in the +`Motivation`_ section, accessing nested values from objects with +``optional`` attributes can be quite cumbersome. Getting all steps right, +often involves a combination of assignment expressions, temporary +variables and chained conditionals. Especially for beginners this can be +overwhelming and it is frequently just faster to repeat each subexpression +as well as only relying on the implicit ``bool(...) is True`` check instead +of ``is not None``, which as shown can fail in unexpected ways. It is also +a bit slower. Even if the conditional expression is written correctly, +reading it again is far from simple. + +In contrast, ``?.`` and ``?[ ]`` leave the core of the expression mostly +untouched. It is thus fairly strait forward to see what is happening. +Furthermore, it will be easier to write since one can start from the +normal attribute access and subscript operators and just insert ``?`` +as needed. The Python error messages for accessing a member or subscript +of ``None`` can help here, similarly type checkers and IDEs will be +able to assist. For conditional expressions it is necessary to first +split it up the expression, combine both parts with ``and``, add an +assignment expression (do not miss the brackets!) and add the +``is not None`` check. The whole process is a lot more involved. + +Easy to get ``?.`` wrong +------------------------ + +It was pointed out that it is too easy to switch up the characters in +``?.`` . + +During the PEP discussion numerous alternatives have been proposed. This +contributed to a sense of not knowing which is the "current" or "right" +one. The author believes that this is only temporary and will resolve +itself once a PEP has been accepted. Any spelling mistake will also +raise a ``SyntaxError``. + +As described in the `How to Teach This`_ section, it can be helpful to +think of ``?.`` and ``?[ ]`` as a combination of two different actions +which need to be performed in a certain order. The ``?`` postfix +represents an ``is not None`` check on the subexpression and as such +should always come first. + +Not obvious what ``?.`` and ``?[ ]`` do +--------------------------------------- + +A lot of the discussion centered around the interpretation of the +``None``-aware operators, should they only work for ``optional``, +i.e. perform ``is not None`` checks, or also work for ``missing`` +attributes, i.e. do ``getattr(obj, attr, None) is not None``. + +It was agreed that the ``None``-aware operators should only handle +``optional`` attributes, see the `Exception-aware operators`_ section +section for more details why the latter interpretation was rejected. + +Similar to `Easy to get ?. wrong`_ the discussion created a lot of +confusion what the agreed upon interpretation should be. This will also +resolve itself once a PEP has been accepted. + +``?.`` and ``?[ ]`` should handle missing attributes +---------------------------------------------------- + +Some comments pointed out that the ``None``-aware operators should +handle ``missing`` attributes to be useful. + +As shown in the `Motivation`_ section, the operators are not designed +to handle arbitrary data, rather to make it easier to work with nested +objects with ``optional`` attributes. If arbitrary data handling is the +goal, other language concepts are likely better suited, like try-except, +a match statement or data traversal libraries from PyPI. + +See the `Exception-aware operators`_ section for more details why +this was rejected. + +Short circuiting is difficult to understand +------------------------------------------- + +Some have pointed the `Short-circuiting`_ behavior might be difficult +to understand and suggested to remove it in favor of a simplified +proposal. + +It is true that using short-circuiting that way is uncommon in Python and +as such unknown. That closest analogs would be boolean expressions like +``a or b``, ``a and b`` which do also short-circuit the expression if +the first value is truthy or falsy respectively. However, while +the details are complex, the behavior itself is rather intuitive. +To understand it, it is often enough to know that once a subexpression +for an ``optional`` value evaluates to ``None``, the result will be +``None`` as well. Any subsequent attribute access, subscript or call +will be skipped. In the example below, if ``a.b`` is ``None``, so will +be ``a.b?.c``: + +:: + + a.b?.c + ^^^ + +On a technical level, removing short-circuiting would make it difficult +to detect if ``not-optional`` attributes suddenly started to return +``None`` as well. See `Remove short-circuiting`_ in the rejected ideas +section for more details on that. + +Lastly, the short-circuiting behavior as proposed is identical to that +of other languages like TS [#ts]_, JS [#js_short_circuiting]_ and +C# [#csharp]_. + +Just use ... +------------ + +A common reply towards the proposal was to use an existing language +concept instead of introducing a new syntax. A few alternatives have +been proposed. + +... a conditional expression +**************************** + +Conditional expressions provide the basis for this PEP. As shown in the +`Motivation`_ section, each operator can effectively be written as such. +The ``None``-aware access operators could therefore simply be considered +syntactic sugar. While this is true, this PEP has highlighted repeatedly +that conditional expressions can often times get fairly complex and are +difficult to get right to the point that it is not uncommon for +developers to prefer a less safe and slower alternative with repetitions +just because it is easier to write. The ``?.`` and ``?[ ]`` operators +will provide a better alternative for these exact situations. + +... a match statement +********************* + +Match statements can be a great option to parse any kind of data and, +as shown in the `Motivation`_ section, could also be used to deal with +``optional`` attributes. However, getting them right can be equally +tricky as the conditional expression. There are a few pitfalls to watch +out for, only using keyword attributes for class patterns, making sure +the attribute names are correct since the class pattern can also match +``missing`` once and as such will not emit an error if it is misspelled, +and the performance impact from an often times unnecessary ``isinstance`` +check. Furthermore, great care must be taking during refactorings as +patterns often can not be updated automatically. + +While the match statement has been available in Python since 3.10, +anecdotal evidence suggests that developers still prefer other +alternatives for ``optional`` attributes, at least for simple, +strait-forward expression. Pattern matching starts to become much more +useful once multiple attributes or values need to be checked at the +same time. + +... try ... except ... +********************** + +Especially for deeply nested data, a common suggestion was to use +try-except: + +.. code-block:: python + + def get_person_email(sensor: Sensor) -> str | None: + try: + return sensor.machine.line.department.engineer.emails[0] + except AttributeError, TypeError: + return None + +While this will likely work, it does not provide nearly the same level +as granularity as a conditional expression would. It is often desired +to raise an exception if something unexpected changes. The try-except +block would simply swallow it. + +... a traversal library +*********************** + +Data traversal libraries like ``glom`` [#glom]_ can be used to access +data from nested objects with ``optional`` attributes. However, as +highlighted in the `Builtin function for traversal`_ section passing +the attribute lookup as a string usually means developers will not +get any more IDE suggestions and type checking these expressions is +also fairly limited. Furthermore, normal functions do not support +short-circuiting for the remaining expression. + +Proliferation of ``None`` in code bases +--------------------------------------- + +One of the reasons why :pep:`505` stalled was that some expressed their +concern how ``None``-aware access operators will effect the code written +by developers. If it is easier to work with ``optional`` attributes, +this will encourage developers to use them more. They believe that e.g. +returning an ``optional`` (can be ``None``) value from a function is +usually an anti-pattern. In an ideal world the use of ``None`` would +be limited as much as possible, for example with early data validation. + +It is certainly true that new language features effect how the language +as a whole develops. Therefore any changes should be considered carefully. +However, just because ``None`` represents an anti-pattern for some, has +not prevented the community as a whole from using it extensively. Rather +the lack of ``None``-aware access operators has stopped developers from +writing concise expressions to access ``optional`` attributes and instead +often leads to code which is difficult to understand and can contain +subtly errors, see the `Motivation`_ section for more details. + +``None`` is not special enough +------------------------------ + +Some mentioned that ``None`` is not special enough to warrant dedicated +operators. + +"``null``-aware" or "optional chaining" operators have been added to a +number of other modern programming languages. Furthermore, adding +``None``-aware access operators is something which was suggested numerous +times since :pep:`505` was first proposed ten years ago. + +In Python ``None`` is frequently used to indicate the absence of +something better or a missing value, e.g. ``dict.get(key)``. Other +languages do often have two separate sentinels for that, e.g. JavaScript +with ``null`` and ``undefined``. This only contributes further to the +prevalence of ``None`` in Python. + +``?`` last available ASCII character +------------------------------------ + +Another objections was that ``?`` is one of the last available ASCII +characters for new syntax. It was suggested to use something else. + +While this is true, the use of ``?.`` and ``?[ ]`` for "null"- / +``None``-aware operators in other languages means that it would be +difficult to us ``?`` for anything else. + +Furthermore, it is common for developers to use / be fluent in multiple +programming languages. It is up the Python language specification to +provide a meaning for these operators which roughly matches those in +other languages while still respecting the norms in Python itself. + + +Footnotes +========= + +.. [#discuss_revisit_505] discuss.python.org: Revisiting PEP 505 - None-aware operators + (https://discuss.python.org/t/revisiting-pep-505-none-aware-operators/74568) +.. [#discuss_safe_navigation_op] discuss.python.org: Introducing a Safe Navigation Operator in Python + (https://discuss.python.org/t/introducing-a-safe-navigation-operator-in-python/35480) +.. [#ts] TypeScript: Optional Chaining + (https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#optional-chaining) +.. [#js] JavaScript: Optional chaining (?.) + (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining) +.. [#js_short_circuiting] JavaScript: Optional chaining (?.) - Short-circuiting + (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining#short-circuiting) +.. [#csharp] C# Reference: Member access operators + (https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/member-access-operators#null-conditional-operators--and-) +.. [#dart] Dart: Other operators + (https://dart.dev/language/operators#other-operators) +.. [#swift] Swift: Optional Chaining + (https://docs.swift.org/swift-book/documentation/the-swift-programming-language/optionalchaining/) +.. [#kotlin] Kotlin: Safe call operator + (https://kotlinlang.org/docs/null-safety.html#safe-call-operator) +.. [#ruby] Ruby: Safe navigation operator + (https://ruby-doc.org/core-2.6/doc/syntax/calling_methods_rdoc.html#label-Safe+navigation+operator) +.. [#php] PHP: Nullsafe operator + (https://wiki.php.net/rfc/nullsafe_operator) +.. [#glom] PyPI: glom + (https://pypi.org/project/glom/) +.. [#jmespath] PyPI: jmespath + (https://pypi.org/project/jmespath/) +.. [#nonesafe] PyPI: nonesafe + (https://pypi.org/project/nonesafe/) +.. [#pymaybe] PyPI: pymaybe + (https://pypi.org/project/pymaybe/) +.. [#py_grammar] Python documentation: Full Grammar specification + (https://docs.python.org/3/reference/grammar.html) + + +Copyright +========= + +This document is placed in the public domain or under the +CC0-1.0-Universal license, whichever is more permissive. From 56a58b5c9869c5fb701aae04db2cda5b1e94b19a Mon Sep 17 00:00:00 2001 From: Marc Mueller <30130371+cdce8p@users.noreply.github.com> Date: Sat, 31 Jan 2026 22:51:52 +0100 Subject: [PATCH 2/7] Fix typos --- peps/pep-0823.rst | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst index c843724fc2e..7b101aad781 100644 --- a/peps/pep-0823.rst +++ b/peps/pep-0823.rst @@ -67,7 +67,7 @@ An attribute or value is ``missing`` Motivation ========== -First officially proposed ten years ago in (the now deferred) :pep:`505` +First officially proposed ten years ago in (the now deferred) :pep:`505`, the idea to add ``None``-aware access operators has been along for some time now, discussed at length in numerous threads, most recently in [#discuss_revisit_505]_ and [#discuss_safe_navigation_op]_. This PEP @@ -760,11 +760,11 @@ If ``.b`` all of the sudden is also ``None``, it would still raise an issues. As the expression output can already be ``None``, the space of potential outputs didn't change and as such no error would appear. -If it is the intend to catch all ``AttributeError`` and ``TypeError``, +If it is the intent to catch all ``AttributeError`` and ``TypeError``, a try-except block can be used instead. As the ``?.`` and ``?[ ]`` would allow developers to be more explicit in -their intend, this suggestion is rejected. +their intent, this suggestion is rejected. Remove short-circuiting ----------------------- @@ -1237,21 +1237,21 @@ Proliferation of ``None`` in code bases --------------------------------------- One of the reasons why :pep:`505` stalled was that some expressed their -concern how ``None``-aware access operators will effect the code written +concern how ``None``-aware access operators will affect the code written by developers. If it is easier to work with ``optional`` attributes, this will encourage developers to use them more. They believe that e.g. returning an ``optional`` (can be ``None``) value from a function is usually an anti-pattern. In an ideal world the use of ``None`` would be limited as much as possible, for example with early data validation. -It is certainly true that new language features effect how the language +It is certainly true that new language features affect how the language as a whole develops. Therefore any changes should be considered carefully. However, just because ``None`` represents an anti-pattern for some, has not prevented the community as a whole from using it extensively. Rather the lack of ``None``-aware access operators has stopped developers from writing concise expressions to access ``optional`` attributes and instead often leads to code which is difficult to understand and can contain -subtly errors, see the `Motivation`_ section for more details. +subtle errors, see the `Motivation`_ section for more details. ``None`` is not special enough ------------------------------ From 732a99b0c3e3fb2a90c7dbd56cf2606926a5d8d4 Mon Sep 17 00:00:00 2001 From: Marc Mueller <30130371+cdce8p@users.noreply.github.com> Date: Sun, 1 Feb 2026 11:53:12 +0100 Subject: [PATCH 3/7] Minor changes --- peps/pep-0823.rst | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-) diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst index 7b101aad781..8cb645f3493 100644 --- a/peps/pep-0823.rst +++ b/peps/pep-0823.rst @@ -23,11 +23,11 @@ to: .. code-block:: python - # a.b?.c - _t.c if ((_t := a.b) is not None) else None + # a.b?.c.d + _t.c.d if ((_t := a.b) is not None) else None - # a.b?[c] - _t[c] if ((_t := a.b) is not None) else None + # a.b?[c].d + _t[c].d if ((_t := a.b) is not None) else None See the `Specification`_ section for more details. @@ -89,7 +89,7 @@ Nested objects with ``optional`` attributes ------------------------------------------- When writing Python code, it is common to encounter objects with ``optional`` -attributes. Accessing attributes, subscript or function calls can raise +attributes. Accessing attributes, subscripts or function calls can raise ``AttributeError`` or ``TypeError`` at runtime if the value is ``None``. Several common patterns have developed to ensure these operations will not raise. The goal for ``?.`` and ``?[ ]`` is to make reading and writing @@ -377,6 +377,7 @@ hide in plain sight. Attribute and function names have been shortened. if not (a and a.b == val): ... if not (a?.b == val): ... + if a?.b != val: ... if not (a and a.lower()): ... if not a?.lower(): ... @@ -469,7 +470,7 @@ subsequent ``.`` or ``[ ]`` on the right hand side just because a The ``None``-aware access operators will only short-circuit expressions containing name, attribute access, subscript, their ``None``-aware -counterparts and call expressions. As a rule of thumb, short-circuiting +counterparts, and call expressions. As a rule of thumb, short-circuiting is broken once a (soft-) keyword is reached. :: @@ -494,7 +495,7 @@ Using ``?.`` and ``?[ ]`` inside groups is possible. In addition to the rules laid out in the `previous `_ section, short-circuiting will also be broken at the end of a group. For example the expression ``(a?.b).c`` will raise an ``AttributeError`` on ``.c`` -if ``a = None``. This is conceptually identical to extracting the group +if ``a is None``. This is conceptually identical to extracting the group contents and storing the result in a temporary variable before substituting it back into the original expression. @@ -550,8 +551,7 @@ must be taken so these can be evaluate properly. (a?.b or d).c = 1 # This would be evaluated as - _t2 = t1.b if ((t1 := a) is not None) else None - + _t2 = _t1.b if ((_t1 := a) is not None) else None if _t2: _t2.c = 1 else: From 52ec4273af0001d51cad69a586f720281107cc16 Mon Sep 17 00:00:00 2001 From: Marc Mueller <30130371+cdce8p@users.noreply.github.com> Date: Mon, 2 Feb 2026 00:07:26 +0100 Subject: [PATCH 4/7] Add links to reference implementation and demo --- peps/pep-0823.rst | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst index 8cb645f3493..53443bed73b 100644 --- a/peps/pep-0823.rst +++ b/peps/pep-0823.rst @@ -671,8 +671,8 @@ Reference Implementation ======================== A reference implementation is available at -https://github.com/cdce8p/cpython/tree/pep-XXX. A online demo can be -tested at https://pepXXX-demo.pages.dev/. +https://github.com/cdce8p/cpython/tree/pep823-none-aware-access-operators. +A online demo can be tested at https://pep823-and-pep824-demo.pages.dev/. Deferred Ideas From 5ea0ea819c0cfaa7b4def2117626e9a35a999b72 Mon Sep 17 00:00:00 2001 From: Guido van Rossum Date: Thu, 7 May 2026 14:23:59 -0700 Subject: [PATCH 5/7] Move pep-0823.rst to 3.16 Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com> --- peps/pep-0823.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst index 53443bed73b..5db7f1e817f 100644 --- a/peps/pep-0823.rst +++ b/peps/pep-0823.rst @@ -6,7 +6,7 @@ Discussions-To: Pending Status: Draft Type: Standards Track Created: 31-Jan-2025 -Python-Version: 3.15 +Python-Version: 3.16 Abstract From daf15f43de6962ce8a6d0c1a8457c1ab7131e0ba Mon Sep 17 00:00:00 2001 From: Marc Mueller <30130371+cdce8p@users.noreply.github.com> Date: Tue, 16 Jun 2026 14:30:33 +0200 Subject: [PATCH 6/7] Rewrite motivation section --- peps/pep-0823.rst | 277 +++++++++++----------------------------------- 1 file changed, 67 insertions(+), 210 deletions(-) diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst index 5db7f1e817f..8c6544d51b9 100644 --- a/peps/pep-0823.rst +++ b/peps/pep-0823.rst @@ -91,259 +91,116 @@ Nested objects with ``optional`` attributes When writing Python code, it is common to encounter objects with ``optional`` attributes. Accessing attributes, subscripts or function calls can raise ``AttributeError`` or ``TypeError`` at runtime if the value is ``None``. -Several common patterns have developed to ensure these operations will -not raise. The goal for ``?.`` and ``?[ ]`` is to make reading and writing -these expressions much simpler while being predictable and doing the -correct things intuitively. +To ensure these operations will not raise, a common option is to add +conditional ``is not None`` checks. Even for fairly simple objects, +this often adds nesting and code duplication. Consider the following +simplified example: .. code-block:: python from dataclasses import dataclass @dataclass - class Sensor: - machine: Machine | None + class Data: + customer: Customer | None + total_customers: int @dataclass - class Machine: - line: Line | None + class Customer: + user: User | None + shopping_card: list[Item] @dataclass - class Line: - department: Department - - @dataclass - class Department: - engineer: Person | None - - @dataclass - class Person: - emails: list[str] | None - - def get_person_email(sensor: Sensor) -> str | None: - """Get first listed email address if it exists.""" - if ( - sensor.machine - and sensor.machine.line - and sensor.machine.line.department.engineer - and sensor.machine.line.department.engineer.emails - ): - return sensor.machine.line.department.engineer.emails[0] - return None - -A simple function which will most likely work just fine. However, there -are a few subtle issues. For one, each condition only checks for truthiness. -Would for example ``Machine`` overwrite ``__bool__`` to return ``False`` at -some point, the function would just return ``None``. This is problematic -since ``None`` is a valid return value already. Thus this would not raise -an exception in the caller and even type checkers would not be able to -detect it. The solution here is to compare with ``None`` instead. - -.. note:: - - It is assumed that if ``Person.emails is not None``, it will - always contain at least one item. This is done in order to avoid - confusion around potential error cases. The goal for this PEP is to - make the ``[ ]`` operator safe for ``optional`` attributes which - could raise a ``TypeError``. It is not to simplify accessing - elements in a sequence of unknown length which could raise an - ``IndexError`` instead. See `Add list.get(key, default=None)`_ in - the deferred ideas section for that. - -.. code-block:: python - - def get_person_email(sensor: Sensor) -> str | None: - if ( - sensor.machine is not None - and sensor.machine.line is not None - and sensor.machine.line.department.engineer is not None - and sensor.machine.line.department.engineer.emails is not None - ): - return sensor.machine.line.department.engineer.emails[0] + class User: + name: str + + def get_customer_name(data: Data) -> str | None: + """Get customer name in lower case if it exists.""" + customer = data.customer + if customer is not None: + user = customer.user + if user is not None: + return user.name.lower() return None -This is better, but here each attribute lookup is still performed -multiple times. If one of these attributes were a custom property or -a class would overwrite ``__getattribute__``, it could be possible -that the attribute values are different for each line. To resolve that -the lookup results need to be stored in a temporary variable. +This could also be written using assignment expressions: .. code-block:: python - def get_person_email(sensor: Sensor) -> str | None: + def get_customer_name(data: Data) -> str | None: if ( - (machine := sensor.machine) is not None - and (line := machine.line) is not None - and (engineer := line.department.engineer) is not None - and (emails := engineer.emails) is not None + (customer := data.customer) is not None + and (user := customer.user) is not None ): - return emails[0] + return user.name.lower() return None -Writing it like this is correct but, especially for deeply nested -object hierarchies, difficult to read and easy to get wrong. +While both options are correct, it requires more effort than necessary +to understand the simple checks being added here. Additionally, even +writing these statements and expressions can become quite cumbersome, +to a point that it is not uncommon to skip certain parts: be it the +explicit ``is not None`` and instead defaulting to implicit +boolean checks or the assignments to temporary variables and instead +repeating the previous expressions. -Alternative approaches include wrapping the whole expression with -a try-except block. While this would also achieve the desired -output, it as well has the potential to introduce errors which -might get unnoticed. E.g. if the ``Line.department`` attribute gets -deprecated, in the process making it ``optional`` and always return -``None``, the function would still succeed, even though the input changed -significantly. - -.. code-block:: python - - def get_person_email(sensor: Sensor) -> str | None: - try: - return sensor.machine.line.department.engineer.emails[0] - except AttributeError, TypeError: - return None - -Another approach would be to use a ``match`` statement instead. This -will work fine but is easy to get wrong as well. It is strongly -recommended to use keyword attributes as otherwise any change in -``__match_args__`` would cause the pattern match to fail. -If any attribute names change, the match statement needs to be -updated as well. Even IDEs can not reliably do that themselves since a -class pattern is not restricted to existing attributes and can instead -match any possible name. For sequence patterns it is also necessary -to remember the wildcard pattern. Lastly, using ``match`` is significantly -slower because for each class pattern an ``isinstance`` check is performed -first. This could be somewhat mitigated by using ``object(...)`` instead, -though reading the pattern would be considerably more difficult. - -.. code-block:: python - - def get_person_email(sensor: Sensor) -> str | None: - match sensor: - case Sensor( - machine=Machine( - line=Line( - department=Department( - engineer=Person( - emails=[email, *_]))))): - return email - case _: - return None - -In contrast to the code shown so far, the "``None``-aware attribute access" -and the "``None``-aware indexing" operators are designed to make writing -safe nested attribute access, subscript and function calls easy. - -To start, assume each attribute, subscript and function call is -``not-optional``: - -.. code-block:: python - - def get_person_email(sensor: Sensor) -> str | None: - return sensor.machine.line.department.engineer.email[0] - -Now insert ``?`` after each ``optional`` subexpression. IDEs and most -type checkers will be able to help with identifying these. *Spaces added -for clarity only, though still valid*: +The goal for ``?.`` and ``?[ ]`` is to make reading and writing these +kinds of expressions much simpler while being predictable and doing +the correct things intuitively. Using these operators, the function +could instead be written as: :: - def get_person_email(sensor: Sensor) -> str | None: - return sensor.machine? .line? .department.engineer? .email? [0] - # ^^^^^^^^ ^^^^^ ^^^^^^^^^ ^^^^^^ - -The complete function would then be: + def get_customer_name(data: Data) -> str | None: + return data.customer?.user?.name.lower() -:: - - def get_person_email(sensor: Sensor) -> str | None: - return sensor.machine?.line?.department.engineer?.email?[0] - -Which is roughly equivalent to the example code above if the temporary -variables ``_t1`` till ``_t4`` would not be created at runtime: - -.. code-block:: python - - def get_person_email(sensor: Sensor) -> str | None: - if ( - (_t1 := sensor.machine) is not None - and (_t2 := _t1.line) is not None - and (_t3 := _t2.department.engineer) is not None - and (_t4 := _t3.emails) is not None - ): - return _t4[0] - return None - -See `Specification`_ for more details on how the expression is evaluated. +Here ``?`` are inserted after each ``optional`` subexpression to change +the attribute access to "``None``-aware attribute access" operators. +In contrast to before, it is not necessary to add additional ``if`` +statements, assignments or nesting, so starting from "normal" +attribute access operators and changing these to ``?.`` where +necessary afterwards, is an easy way to write these expressions. Parsing structured data ----------------------- The ``?.`` and ``?[ ]`` operators can also aid in the traversal of structured data, oftentimes coming from JSON and parsed as nested -dicts and lists. It is worth noting though that the operators do not -handle ``missing`` attributes / data. For dictionaries a useful helper is -the ``.get(key, default=None)`` method with a default. Depending on -the specific use case, pattern matching might also be a viable -alternative here. +dicts and lists. It is worth noting though that the operators do +**NOT** handle ``missing`` attributes / data. In these cases, at +least for dictionaries a useful helper method is ``dict.get(key)``. .. code-block:: python from typing import NotRequired, TypedDict - class Sensor(TypedDict): - machine: Machine | None + class Data(TypedDict): + # Note the 'NotRequired' here, i.e. 'customer' can be **missing**. + # That is different from the previous example! - class Machine(TypedDict): - # Note the 'NotRequired' here! - line: NotRequired[Line | None] + customer: NotRequired[Customer | None] + total_customers: int - class Line(TypedDict): - department: Department + class Customer(TypedDict): + user: User | None + shopping_card: list[Item] - class Department(TypedDict): - engineer: Person | None + class User(TypedDict): + name: str - class Person(TypedDict): - emails: list[str] | None + def get_customer_name(data: Data) -> str | None: + customer = data.get("customer") + if customer is not None: + user = customer["user"] + if user is not None: + return user.name.lower() + return None - def get_person_email(data: Sensor) -> str | None: - match data: - case { - "machine": { - "line": { - "department": { - "engineer": { - "emails": [email, *_], - } - } - } - } - }: - return email - case _: - return None - -Writing it using ``?.`` and ``?[ ]`` would look like this. Note that -``NotRequired`` is "translated" to ``.get("line")``. +Writing it using ``?.`` and ``?[ ]`` would look like this: :: - def get_person_email(data: Sensor) -> str | None: - return ( - data["machine"]?.get("line")? - ["department"]["engineer"]?["emails"]?[0] - ) - -Which is roughly equivalent to: - -.. code-block:: python - - def get_person_email(data: Sensor) -> str | None: - if ( - (_t1 := data["machne"]) is not None - and (_t2 := _t1.get("line")) is not None - and (_t3 := _t2["department"]["engineer"]) is not None - and (_t4 := _t3["emails"]) is not None - ): - return _t4[0] - return None + def get_customer_name(data: Data) -> str | None: + return data.get("customer")?["user"]?.name.lower() Other common patterns --------------------- From af1db0355753b05a9a5a4fec2aaea3eb39225828 Mon Sep 17 00:00:00 2001 From: Marc Mueller <30130371+cdce8p@users.noreply.github.com> Date: Tue, 16 Jun 2026 15:20:45 +0200 Subject: [PATCH 7/7] Improvements --- peps/pep-0823.rst | 158 ++++++++++++++++++++++++++++------------------ 1 file changed, 95 insertions(+), 63 deletions(-) diff --git a/peps/pep-0823.rst b/peps/pep-0823.rst index 8c6544d51b9..74571808bd8 100644 --- a/peps/pep-0823.rst +++ b/peps/pep-0823.rst @@ -5,7 +5,7 @@ Sponsor: Guido van Rossum Discussions-To: Pending Status: Draft Type: Standards Track -Created: 31-Jan-2025 +Created: 16-Jun-2026 Python-Version: 3.16 @@ -51,23 +51,22 @@ An attribute or value is ``optional`` An attribute or value is ``missing`` An attribute or value is considered ``missing`` if it is not present - at all. For ``typing.TypedDict`` these would be ``typing.NotRequired`` - keys when they are not preset. + at all. .. code-block:: python-console - >>> class A: - ... val: int | None + >>> class B: + ... ... ... - >>> a = A() - >>> hasattr(a, "val") + >>> b = B() + >>> hasattr(b, "val") False Motivation ========== -First officially proposed ten years ago in (the now deferred) :pep:`505`, +First officially proposed over ten years ago in (the now deferred) :pep:`505`, the idea to add ``None``-aware access operators has been along for some time now, discussed at length in numerous threads, most recently in [#discuss_revisit_505]_ and [#discuss_safe_navigation_op]_. This PEP @@ -233,7 +232,6 @@ hide in plain sight. Attribute and function names have been shortened. # return or raising of an exception if not (a and a.b == val): ... - if not (a?.b == val): ... if a?.b != val: ... if not (a and a.lower()): ... @@ -243,6 +241,10 @@ hide in plain sight. Attribute and function names have been shortened. # Misc expressions + # Each of these results still needs to be checked for the + # negative case, though falsy values are usually enough. + # It often does not matter whether it is 'False' or 'None'. + a and a.b and a.b.c a?.b?.c @@ -270,6 +272,14 @@ hide in plain sight. Attribute and function names have been shortened. a.get(key)?.strip().lower() + # Sometimes actions should only be performed + # if a variable is not None + + if timeout_handle is not None: + timeout_handle.cancel() + timeout_handle?.cancel() + + Specification ============= @@ -286,10 +296,11 @@ attribute or subscript access were used. .. code-block:: python # base?.tail - (_t.tail) if ((_t := base) is not None) else None + _t.tail if ((_t := base) is not None) else None The ``base`` can be replace with any number of expressions, including -`Groups`_ while the ``tail`` is limited to attribute access, subscript, +`Parenthesized ones `_ +while the ``tail`` is limited to attribute access, subscript, their ``None``-aware variants and call expressions. .. code-block:: python @@ -326,9 +337,10 @@ subsequent ``.`` or ``[ ]`` on the right hand side just because a None The ``None``-aware access operators will only short-circuit expressions -containing name, attribute access, subscript, their ``None``-aware -counterparts, and call expressions. As a rule of thumb, short-circuiting -is broken once a (soft-) keyword is reached. +containing primary expressions (name, attribute access, subscript, +their ``None``-aware counterparts, and call expressions). As a rule of +thumb, short-circuiting is broken once an operator other than +``.``, ``[ ]``, ``?.``, ``?[ ]`` is reached. :: @@ -345,8 +357,8 @@ is broken once a (soft-) keyword is reached. >>> 2 in (a?.b.c or ()) False -Grouping -******** +Parenthesized expressions - groupings +************************************* Using ``?.`` and ``?[ ]`` inside groups is possible. In addition to the rules laid out in the `previous `_ section, @@ -389,13 +401,22 @@ Assignments are not permitted and will raise a ``SyntaxError``. :: >>> a?.b = 1 - File "", line 1 + File "", line 1 a?.b = 1 ^^^^ SyntaxError: cannot assign to none aware expression -It is however possible to use them in `groups `_, though care -must be taken so these can be evaluate properly. + >>> (a?.b) = 1 + File "", line 1 + (a?.b) = 1 + ^^^^ + SyntaxError: cannot assign to none aware expression here. Maybe you meant '==' instead of '='? + +It is however possible to use them in **non-trivial** +`Parenthesized expressions `_ +since in these the "``None``-aware" expressions are in a ``Load`` +context. Though care must be taken so they can be evaluated properly, +e.g. with a fallback value. :: @@ -419,7 +440,7 @@ Await expressions ``None``-aware access operations are permitted in ``await`` expressions. It is up to the developer to make sure they do not evaluate to ``None`` -at runtime otherwise a ``TypeError`` is raised. This behavior is similar +at runtime, otherwise a ``TypeError`` is raised. This behavior is similar to awaiting any other variable which can be ``None``. AST changes @@ -430,7 +451,7 @@ They are the counterparts to the existing ``Attribute`` and ``Subscript`` nodes. Notably there is no ``expr_context`` attribute because the new nodes do not support assignments themselves and thus the context will always be ``Load``. Furthermore, an optional ``group`` attribute is added for all -expression nodes. It is set to ``1`` if the expression is the topmost +expression nodes. It is set to ``1`` if an expression is the topmost node in a group, ``0`` otherwise. :: @@ -482,13 +503,11 @@ is possible: :: - def get_person_email(data: Sensor) -> str | None: + def get_customer_name(data: Data) -> str | None: return ( - data["machine"]? - .get("line")? - ["department"]["engineer"]? - ["emails"]? - [0] + data.customer? + .user? + .name.lower() ) @@ -510,7 +529,7 @@ How to Teach This ================= After students know how the attribute access ``.`` and subscript ``[ ]`` -operators work, they may learn about the "``None``-aware" versions for +operators work, they may learn about the "``None``-aware" variants for both. Students may find it helpful to think of ``?.`` and ``?[ ]`` as a @@ -539,10 +558,10 @@ Coalesce ``??`` and coalesce assignment operator ``??=`` -------------------------------------------------------- :pep:`505` also suggested the addition of a "``None`` coalescing" operator -``??`` and a "coalescing assignment" operator ``??=``. As the ``None``-aware -access operators have their own use cases, the coalescing operators were -moved into a separate document, see PEP-824. Both proposals can be -adopted independently of one another. +``??`` and a "``None`` coalescing assignment" operator ``??=``. As the +``None``-aware access operators have their own use cases, the coalescing +operators were moved into a separate document, see PEP-824. +Both proposals can be adopted independently of one another. ``None``-aware function calls ----------------------------- @@ -563,10 +582,10 @@ hierarchies. A workaround would be to write ``a.foo?.__call__(arguments)``. -Add ``list.get(key, default=None)`` ------------------------------------ +Add ``list.get(key)`` +--------------------- -It was suggested to add a ``.get(key, default=None)`` method to ``list`` +It was suggested to add a ``.get(key)`` method to ``list`` and ``tuple`` objects, similar to the existing ``dict.get`` method. This could further make parsing of structured data easier since it would no longer be necessary to check if a ``list`` or ``tuple`` is long enough @@ -593,7 +612,7 @@ should be if an error is caught. Furthermore, this approach would hide genuine issues like a misspelled attribute which would have raised an ``AttributeError``. There are also already established patterns to handle these kinds of errors in the form of ``getattr`` and -``.get(key, default=None)``. +``dict.get(key)``. As catching exceptions would be unexpected and hide potential errors, it is rejected. @@ -615,7 +634,7 @@ If ``.b`` all of the sudden is also ``None``, it would still raise an ``AttributeError`` since it was unexpected. That would not happen for ``maybe``. This behavior is problematic since it can subtly hide real issues. As the expression output can already be ``None``, the space of -potential outputs didn't change and as such no error would appear. +potential outputs did not change and as such no error would appear. If it is the intent to catch all ``AttributeError`` and ``TypeError``, a try-except block can be used instead. @@ -821,14 +840,14 @@ the ``?[ ]`` operator is necessary to efficiently get items from use ``d?.get(key)``, for general objects developers would have needed to defer to ``o?.__getitem__(key)``. -Furthermore, any future PEP just for a ``?[ ]`` would have likely needed +Furthermore, any future PEP just for a ``?[ ]`` would likely have needed to included a lot of the arguments and objections listed in this one again. As such it makes sense to include both operators in the same PEP. -While adding ``list.get(key, default=None)`` as suggested in -`Add list.get(key, default=None)`_ would reduce the need for ``?[ ]`` +While adding ``list.get(key)`` as suggested in +`Add list.get(key)`_ would reduce the need for ``?[ ]`` for lists and tuples and as such would be a valuable addition to the -language itself, it doesn't remove the need for arbitrary objects which +language itself, it does not remove the need for arbitrary objects which implement a custom ``__getitem__`` method. Ignore groups for short-circuiting @@ -843,13 +862,14 @@ the grouping was considered trivial and the expression would be equal to ``a?.b.c``. The advantage being that developers would not have to look for groupings when evaluating simpler expressions. As long as any ``None``-aware access operator was used and the expression was not -broken by a (soft-) keyword, it would return ``None`` instead of raising -an ``AttributeError`` or ``TypeError``. +broken by any other unrelated operator, it would return ``None`` +instead of raising an ``AttributeError`` or ``TypeError``. This suggestion was rejected in favor of the specification outline in -the `Grouping`_ section since it violates the substitution principle. -An expression ``(a?.b).c`` should behave the same whether or not ``a?.b`` -is written inline inside a group or defined as a separate variable. +the `Parenthesized expressions - groupings`_ section since it violates +the substitution principle. An expression ``(a?.b).c`` should behave +the same whether or not ``a?.b`` is written inline inside a group or +defined as a separate variable. :: @@ -867,7 +887,9 @@ Change ``primary`` rule to be right-recursive The ``primary`` grammar rule as it is defined [#py_grammar]_ is left-recursive. This can make it difficult to reason about especially -with regards to the `Short-circuiting`_ and `Grouping`_ behavior. +with regards to the `Short-circuiting`_ and +`Parenthesized expressions `_ +behavior. It was therefore proposed to make the ``None``-aware access operators part of the ``primary`` rule right-recursive instead. The expression @@ -889,7 +911,7 @@ part of the ``primary`` rule right-recursive instead. The expression ) While this approach would clearly define which parts of an expression -would be short-circuited, it has several drawbacks. To implement it at +would be short-circuited, it has several drawbacks. To implement it, at least three additional AST nodes have to be added ``AttributeTail``, ``SubscriptTail`` and ``CallTail``. As these are right-recursive now, reusing code from the ``Attribute``, ``Subscript`` and ``Call`` nodes @@ -923,9 +945,9 @@ often involves a combination of assignment expressions, temporary variables and chained conditionals. Especially for beginners this can be overwhelming and it is frequently just faster to repeat each subexpression as well as only relying on the implicit ``bool(...) is True`` check instead -of ``is not None``, which as shown can fail in unexpected ways. It is also -a bit slower. Even if the conditional expression is written correctly, -reading it again is far from simple. +of ``is not None``, which can fail in unexpected ways. It is also +a bit slower. Even if the statements and expressions are written +correctly, reading them again is far from simple. In contrast, ``?.`` and ``?[ ]`` leave the core of the expression mostly untouched. It is thus fairly strait forward to see what is happening. @@ -964,7 +986,7 @@ A lot of the discussion centered around the interpretation of the i.e. perform ``is not None`` checks, or also work for ``missing`` attributes, i.e. do ``getattr(obj, attr, None) is not None``. -It was agreed that the ``None``-aware operators should only handle +It was decided that the ``None``-aware operators should only handle ``optional`` attributes, see the `Exception-aware operators`_ section section for more details why the latter interpretation was rejected. @@ -994,8 +1016,8 @@ Some have pointed the `Short-circuiting`_ behavior might be difficult to understand and suggested to remove it in favor of a simplified proposal. -It is true that using short-circuiting that way is uncommon in Python and -as such unknown. That closest analogs would be boolean expressions like +It is true that using short-circuiting that way is uncommon in Python. +That closest analogs would be boolean expressions like ``a or b``, ``a and b`` which do also short-circuit the expression if the first value is truthy or falsy respectively. However, while the details are complex, the behavior itself is rather intuitive. @@ -1015,7 +1037,7 @@ to detect if ``not-optional`` attributes suddenly started to return ``None`` as well. See `Remove short-circuiting`_ in the rejected ideas section for more details on that. -Lastly, the short-circuiting behavior as proposed is identical to that +Lastly, the short-circuiting behavior as proposed matches that of other languages like TS [#ts]_, JS [#js_short_circuiting]_ and C# [#csharp]_. @@ -1042,9 +1064,19 @@ will provide a better alternative for these exact situations. ... a match statement ********************* -Match statements can be a great option to parse any kind of data and, -as shown in the `Motivation`_ section, could also be used to deal with -``optional`` attributes. However, getting them right can be equally +Match statements can be a great option to parse any kind of data and +could also be used to deal with ``optional`` attributes. + +.. code-block:: python + + def get_customer_name(data: Data) -> str | None: + match data.customer: + case Customer(user=User(name=name)): + return name.lower() + case _: + return None + +However, getting them right can be equally tricky as the conditional expression. There are a few pitfalls to watch out for, only using keyword attributes for class patterns, making sure the attribute names are correct since the class pattern can also match @@ -1068,9 +1100,9 @@ try-except: .. code-block:: python - def get_person_email(sensor: Sensor) -> str | None: + def get_customer_name(data: Data) -> str | None: try: - return sensor.machine.line.department.engineer.emails[0] + return data.customer.user.name.lower() except AttributeError, TypeError: return None @@ -1108,7 +1140,7 @@ not prevented the community as a whole from using it extensively. Rather the lack of ``None``-aware access operators has stopped developers from writing concise expressions to access ``optional`` attributes and instead often leads to code which is difficult to understand and can contain -subtle errors, see the `Motivation`_ section for more details. +subtle errors. ``None`` is not special enough ------------------------------ @@ -1119,7 +1151,7 @@ operators. "``null``-aware" or "optional chaining" operators have been added to a number of other modern programming languages. Furthermore, adding ``None``-aware access operators is something which was suggested numerous -times since :pep:`505` was first proposed ten years ago. +times since :pep:`505` was first proposed over ten years ago. In Python ``None`` is frequently used to indicate the absence of something better or a missing value, e.g. ``dict.get(key)``. Other @@ -1135,7 +1167,7 @@ characters for new syntax. It was suggested to use something else. While this is true, the use of ``?.`` and ``?[ ]`` for "null"- / ``None``-aware operators in other languages means that it would be -difficult to us ``?`` for anything else. +difficult to use ``?`` for anything else. Furthermore, it is common for developers to use / be fluent in multiple programming languages. It is up the Python language specification to