PEP: 562 Title: Module __getattr__ and __dir__ Author: Ivan Levkivskyi Status: Final Type: Standards Track Content-Type: text/x-rst Created: 09-Sep-2017 Python-Version: 3.7 Post-History: 09-Sep-2017 Resolution: https://mail.python.org/pipermail/python-dev/2017-December/151033.html Abstract ======== It is proposed to support ``__getattr__`` and ``__dir__`` function defined on modules to provide basic customization of module attribute access. Rationale ========= It is sometimes convenient to customize or otherwise have control over access to module attributes. A typical example is managing deprecation warnings. Typical workarounds are assigning ``__class__`` of a module object to a custom subclass of ``types.ModuleType`` or replacing the ``sys.modules`` item with a custom wrapper instance. It would be convenient to simplify this procedure by recognizing ``__getattr__`` defined directly in a module that would act like a normal ``__getattr__`` method, except that it will be defined on module *instances*. For example:: # lib.py from warnings import warn deprecated_names = ["old_function", ...] def _deprecated_old_function(arg, other): ... def __getattr__(name): if name in deprecated_names: warn(f"{name} is deprecated", DeprecationWarning) return globals()[f"_deprecated_{name}"] raise AttributeError(f"module {__name__!r} has no attribute {name!r}") # main.py from lib import old_function # Works, but emits the warning Another widespread use case for ``__getattr__`` would be lazy submodule imports. Consider a simple example:: # lib/__init__.py import importlib __all__ = ['submod', ...] def __getattr__(name): if name in __all__: return importlib.import_module("." + name, __name__) raise AttributeError(f"module {__name__!r} has no attribute {name!r}") # lib/submod.py print("Submodule loaded") class HeavyClass: ... # main.py import lib lib.submod.HeavyClass # prints "Submodule loaded" There is a related proposal :pep:`549` that proposes to support instance properties for a similar functionality. The difference is this PEP proposes a faster and simpler mechanism, but provides more basic customization. An additional motivation for this proposal is that :pep:`484` already defines the use of module ``__getattr__`` for this purpose in Python stub files, see :pep:`484#stub-files`. In addition, to allow modifying result of a ``dir()`` call on a module to show deprecated and other dynamically generated attributes, it is proposed to support module level ``__dir__`` function. For example:: # lib.py deprecated_names = ["old_function", ...] __all__ = ["new_function_one", "new_function_two", ...] def new_function_one(arg, other): ... def new_function_two(arg, other): ... def __dir__(): return sorted(__all__ + deprecated_names) # main.py import lib dir(lib) # prints ["new_function_one", "new_function_two", "old_function", ...] Specification ============= The ``__getattr__`` function at the module level should accept one argument which is the name of an attribute and return the computed value or raise an ``AttributeError``:: def __getattr__(name: str) -> Any: ... If an attribute is not found on a module object through the normal lookup (i.e. ``object.__getattribute__``), then ``__getattr__`` is searched in the module ``__dict__`` before raising an ``AttributeError``. If found, it is called with the attribute name and the result is returned. Looking up a name as a module global will bypass module ``__getattr__``. This is intentional, otherwise calling ``__getattr__`` for builtins will significantly harm performance. The ``__dir__`` function should accept no arguments, and return a list of strings that represents the names accessible on module:: def __dir__() -> List[str]: ... If present, this function overrides the standard ``dir()`` search on a module. The reference implementation for this PEP can be found in [2]_. Backwards compatibility and impact on performance ================================================= This PEP may break code that uses module level (global) names ``__getattr__`` and ``__dir__``. (But the language reference explicitly reserves *all* undocumented dunder names, and allows "breakage without warning"; see [3]_.) The performance implications of this PEP are minimal, since ``__getattr__`` is called only for missing attributes. Some tools that perform module attributes discovery might not expect ``__getattr__``. This problem is not new however, since it is already possible to replace a module with a module subclass with overridden ``__getattr__`` and ``__dir__``, but with this PEP such problems can occur more often. Discussion ========== Note that the use of module ``__getattr__`` requires care to keep the referred objects pickleable. For example, the ``__name__`` attribute of a function should correspond to the name with which it is accessible via ``__getattr__``:: def keep_pickleable(func): func.__name__ = func.__name__.replace('_deprecated_', '') func.__qualname__ = func.__qualname__.replace('_deprecated_', '') return func @keep_pickleable def _deprecated_old_function(arg, other): ... One should be also careful to avoid recursion as one would do with a class level ``__getattr__``. To use a module global with triggering ``__getattr__`` (for example if one wants to use a lazy loaded submodule) one can access it as:: sys.modules[__name__].some_global or as:: from . import some_global Note that the latter sets the module attribute, thus ``__getattr__`` will be called only once. References ========== .. [2] The reference implementation (https://github.com/ilevkivskyi/cpython/pull/3/files) .. [3] Reserved classes of identifiers (https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers) Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: