PEP 562 -- Module __getattr__ and __dir__

PEP: 562 Title: Module __getattr__ and __dir__ Author: Ivan Levkivskyi <levkivskyi at gmail.com> Status: Final Type: Standards Track 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__} has no attribute {name}") # 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 . 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 .

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 .) 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.