PEP: 702 Title: Marking deprecations using the type system Author: Jelle Zijlstra Discussions-To: https://mail.python.org/archives/list/typing-sig@python.org/thread/AKTFUYW3WDT7R7PGRIJQZMYHMDJNE4QH/ Status: Draft Type: Standards Track Topic: Typing Content-Type: text/x-rst Created: 30-Dec-2022 Python-Version: 3.12 Post-History: `01-Jan-2023 `__ Abstract ======== This PEP adds an ``@typing.deprecated()`` decorator that marks a class or function as deprecated, enabling static checkers to warn when it is used. Motivation ========== As software evolves, new functionality is added and old functionality becomes obsolete. Library developers want to work towards removing obsolete code while giving their users time to migrate to new APIs. Python provides a mechanism for achieving these goals: the :exc:`DeprecationWarning` warning class, which is used to show warnings when deprecated functionality is used. This mechanism is widely used: as of the writing of this PEP, the CPython main branch contains about 150 distinct code paths that raise :exc:`!DeprecationWarning`. Many third-party libraries also use :exc:`!DeprecationWarning` to mark deprecations. In the `top 5000 PyPI packages `__, there are: - 1911 matches for the regex ``warnings\.warn.*\bDeprecationWarning\b``, indicating use of :exc:`!DeprecationWarning` (not including cases where the warning is split over multiple lines); - 1661 matches for the regex ``^\s*@deprecated``, indicating use of some sort of deprecation decorator. However, the current mechanism is often insufficient to ensure that users of deprecated functionality update their code in time. For example, the removal of various long-deprecated :mod:`unittest` features had to be `reverted `__ from Python 3.11 to give users more time to update their code. Users may run their test suite with warnings disabled for practical reasons, or deprecations may be triggered in code paths that are not covered by tests. Providing more ways for users to find out about deprecated functionality can speed up the migration process. This PEP proposes to leverage static type checkers to communicate deprecations to users. Such checkers have a thorough semantic understanding of user code, enabling them to detect and report deprecations that a single ``grep`` invocation could not find. In addition, many type checkers integrate with IDEs, enabling users to see deprecation warnings right in their editors. Rationale ========= At first glance, deprecations may not seem like a topic that type checkers should touch. After all, type checkers are concerned with checking whether code will work as is, not with potential future changes. However, the analysis that type checkers perform on code to find type errors is very similar to the analysis that would be needed to detect usage of many deprecations. Therefore, type checkers are well placed to find and report deprecations. Other languages already have similar functionality: * GCC supports a ``deprecated`` `attribute `__ on function declarations. This powers CPython's ``Py_DEPRECATED`` macro. * GraphQL `supports `__ marking fields as ``@deprecated``. * Kotlin `supports `__ a ``Deprecated`` annotation. * Scala `supports `__ an ``@deprecated`` annotation. * Swift `supports `__ using the ``@available`` attribute to mark APIs as deprecated. * TypeScript `uses `__ the ``@deprecated`` JSDoc tag to issue a hint marking use of deprecated functionality. Several users have requested support for such a feature: * `typing-sig thread `__ * `Pyright feature request `__ * `mypy feature request `__ There are similar existing third-party tools: * `Deprecated `__ provides a decorator to mark classes, functions, or methods as deprecated. Access to decorated objects raises a runtime warning, but is not detected by type checkers. * `flake8-deprecated `__ is a linter plugin that warns about use of deprecated features. However, it is limited to a short, hard-coded list of deprecations. Specification ============= A new decorator ``@deprecated()`` is added to the :mod:`typing` module. This decorator can be used on a class, function or method to mark it as deprecated. This includes :class:`typing.TypedDict` and :class:`typing.NamedTuple` definitions. With overloaded functions, the decorator may be applied to individual overloads, indicating that the particular overload is deprecated. The decorator may also be applied to the overload implementation function, indicating that the entire function is deprecated. The decorator takes a single argument of type ``str``, which is a message that should be shown by the type checker when it encounters a usage of the decorated object. The message must be a string literal. The content of deprecation messages is up to the user, but it may include the version in which the deprecated object is to be removed, and information about suggested replacement APIs. Type checkers should produce a diagnostic whenever they encounter a usage of an object marked as deprecated. For deprecated overloads, this includes all calls that resolve to the deprecated overload. For deprecated classes and functions, this includes: * References through module, class, or instance attributes (``module.deprecated_object``, ``module.SomeClass.deprecated_method``, ``module.SomeClass().deprecated_method``) * Any usage of deprecated objects in their defining module (``x = deprecated_object()`` in ``module.py``) * If ``import *`` is used, usage of deprecated objects from the module (``from module import *; x = deprecated_object()``) * ``from`` imports (``from module import deprecated_object``) There are some additional scenarios where deprecations could come into play: * An object implements a :class:`typing.Protocol`, but one of the methods required for protocol compliance is deprecated. * A class uses the ``@override`` decorator from :pep:`698` to assert that its method overrides a base class method, but the base class method is deprecated. As these scenarios appear complex and relatively unlikely to come up in practice, this PEP does not mandate that type checkers detect them. Example ------- As an example, consider this library stub named ``library.pyi``: .. code-block:: python from typing import deprecated @deprecated("Use Spam instead") class Ham: ... @deprecated("It is pining for the fiords") def norwegian_blue(x: int) -> int: ... @overload @deprecated("Only str will be allowed") def foo(x: int) -> str: ... @overload def foo(x: str) -> str: ... Here is how type checkers should handle usage of this library: .. code-block:: python from library import Ham # error: Use of deprecated class Ham. Use Spam instead. import library library.norwegian_blue(1) # error: Use of deprecated function norwegian_blue. It is pining for the fiords. map(library.norwegian_blue, [1, 2, 3]) # error: Use of deprecated function norwegian_blue. It is pining for the fiords. library.foo(1) # error: Use of deprecated overload for foo. Only str will be allowed. library.foo("x") # no error Runtime behavior ---------------- At runtime, the decorator sets an attribute ``__deprecated__`` on the decorated object. The value of the attribute is the message passed to the decorator. The decorator returns the original object. Notably, it does not issue a runtime :exc:`DeprecationWarning`. For compatibility with :func:`typing.get_overloads`, the ``@deprecated`` decorator should be placed after the ``@overload`` decorator. Type checker behavior --------------------- This PEP does not specify exactly how type checkers should present deprecation diagnostics to their users. However, some users (e.g., application developers targeting only a specific version of Python) may not care about deprecations, while others (e.g., library developers who want their library to remain compatible with future versions of Python) would want to catch any use of deprecated functionality in their CI pipeline. Therefore, it is recommended that type checkers provide configuration options that cover both use cases. As with any other type checker error, it is also possible to ignore deprecations using ``# type: ignore`` comments. Deprecation policy ------------------ We propose that CPython's deprecation policy (:pep:`387`) is updated to require that new deprecations use the functionality in this PEP to alert users about the deprecation, if possible. Concretely, this means that new deprecations should be accompanied by a change to the ``typeshed`` repo to add the ``@deprecated`` decorator in the appropriate place. This requirement does not apply to deprecations that cannot be expressed using this PEP's functionality. Backwards compatibility ======================= Creating a new decorator poses no backwards compatibility concerns. As with all new typing functionality, the ``@deprecated`` decorator will be added to the ``typing_extensions`` module, enabling its use in older versions of Python. How to teach this ================= For users who encounter deprecation warnings in their IDE or type checker output, the messages they receive should be clear and self-explanatory. Usage of the ``@deprecated`` decorator will be an advanced feature mostly relevant to library authors. The decorator should be mentioned in relevant documentation (e.g., :pep:`387` and the :exc:`DeprecationWarning` documentation) as an additional way to mark deprecated functionality. Reference implementation ======================== A runtime implementation of the ``@deprecated`` decorator is `available `__. The ``pyanalyze`` type checker has `prototype support `__ for emitting deprecation errors. Rejected ideas ============== Runtime warnings ---------------- Users might expect usage of the ``@deprecated`` decorator to issue a :exc:`DeprecationWarning` at runtime. However, this would raise a number of thorny issues: * When the decorator is applied to a class or an overload, the warning would not be raised as expected. For classes, the warning could be raised on instantiation, but this would not cover usage in type annotations or :func:`isinstance` checks. * Users may want to control the :func:`~warnings.warn` call in more detail (e.g., changing the warning class). * ``typing.py`` generally aims to avoid affecting runtime behavior. Users who want to use ``@deprecated`` while also issuing a runtime warning can use the ``if TYPE_CHECKING:`` idiom, for example: .. code-block:: python from typing import TYPE_CHECKING import functools import warnings if TYPE_CHECKING: from typing import deprecated else: def deprecated(msg): def decorator(func): @functools.wraps(func) def wrapper(*args, **kwargs): warnings.warn(msg, DeprecationWarning, stacklevel=2) return func(*args, **kwargs) wrapper.__deprecated__ = msg return wrapper return decorator Deprecation of modules and attributes ------------------------------------- This PEP covers deprecations of classes, functions and overloads. This allows type checkers to detect many but not all possible deprecations. To evaluate whether additional functionality would be worthwhile, I `examined `__ all current deprecations in the CPython standard library. I found: * 74 deprecations of functions, methods and classes (supported by this PEP) * 28 deprecations of whole modules (largely due to :pep:`594`) * 9 deprecations of function parameters (supported by this PEP through decorating overloads) * 1 deprecation of a constant * 38 deprecations that are not easily detectable in the type system (for example, for calling :func:`asyncio.get_event_loop` without an active event loop) Modules could be marked as deprecated by adding a ``__deprecated__`` module-level constant. However, the need for this is limited, and it is relatively easy to detect usage of deprecated modules simply by grepping. Therefore, this PEP omits support for whole-module deprecations. As a workaround, users could mark all module-level classes and functions with ``@deprecated``. For deprecating module-level constants, object attributes, and function parameters, a ``Deprecated[type, message]`` type modifier, similar to ``Annotated`` could be added. However, this would create a new place in the type system where strings are just strings, not forward references, complicating the implementation of type checkers. In addition, my data show that this feature is not commonly needed. Acknowledgments =============== A call with the typing-sig meetup group led to useful feedback on this proposal. Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.