python-peps/pep-0562.rst

PEP: 562
Title: Module __getattr__
Author: Ivan Levkivskyi <levkivskyi@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 09-Sep-2017
Python-Version: 3.7
Post-History: 09-Sep-2017


Abstract
========

It is proposed to support ``__getattr__`` 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 substituting ``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.submodule.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 [1]_.


Specification
=============

The ``__getattr__`` function at the module level should accept one argument
which is a name of an attribute and return the computed value or raise
an ``AttributeError``::

  def __getattr__(name: str) -> Any: ...

This function will be called only if ``name`` is not found in the module
through the normal attribute lookup.

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) name ``__getattr__``.
The performance implications of this PEP are minimal, since ``__getattr__``
is called only for missing attributes.


References
==========

.. [1] PEP 484 section about ``__getattr__`` in stub files
   (https://www.python.org/dev/peps/pep-0484/#stub-files)

.. [2] The reference implementation
   (https://github.com/ilevkivskyi/cpython/pull/3/files)


Copyright
=========

This document has been placed in the public domain.



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