PEP 726: Module __setattr__ and __delattr__ (#3301)

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

.github/CODEOWNERS

@@ -602,6 +602,7 @@ pep-0721.rst  @encukou
 pep-0722.rst  @pfmoore
 pep-0723.rst  @AA-Turner
 pep-0725.rst  @pradyunsg
+pep-0726.rst  @AA-Turner
 pep-0727.rst  @JelleZijlstra
 # ...
 # pep-0754.txt

pep-0726.rst

@@ -0,0 +1,188 @@
+PEP: 726
+Title: Module ``__setattr__`` and ``__delattr__``
+Author: Sergey B Kirpichev <skirpichev@gmail.com>
+Sponsor: Adam Turner <python@quite.org.uk>
+Discussions-To: https://discuss.python.org/t/32640/
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 24-Aug-2023
+Python-Version: 3.13
+Post-History: `06-Apr-2023 <https://discuss.python.org/t/25506/>`__,
+              `31-Aug-2023 <https://discuss.python.org/t/32640/>`__,
+
+
+Abstract
+========
+
+This PEP proposes supporting user-defined ``__setattr__``
+and ``__delattr__`` methods on modules to extend customization
+of module attribute access beyond :pep:`562`.
+
+Motivation
+==========
+
+There are several potential uses of a module ``__setattr__``:
+
+1. To prevent setting an attribute at all (i.e. make it read-only)
+2. To validate the value to be assigned
+3. To intercept setting an attribute and update some other state
+
+Proper support for read-only attributes would also require adding the
+``__delattr__`` function to prevent their deletion.
+
+A typical workaround is assigning the ``__class__`` of a module object to a
+custom subclass of :py:class:`python:types.ModuleType` (see [1]_).
+Unfortunately, this also brings a noticeable speed regression
+(~2-3x) for attribute *access*.  It would be convenient to directly
+support such customization, by recognizing ``__setattr__`` and ``__delattr__``
+methods defined in a module that would act like normal
+:py:meth:`python:object.__setattr__` and :py:meth:`python:object.__delattr__`
+methods, except that they will be defined on module *instances*.
+
+For example
+
+.. code:: python
+
+   # mplib.py
+
+   CONSTANT = 3.14
+   prec = 53
+   dps = 15
+
+   def dps_to_prec(n):
+       """Return the number of bits required to represent n decimals accurately."""
+       return max(1, int(round((int(n)+1)*3.3219280948873626)))
+
+   def prec_to_dps(n):
+       """Return the number of accurate decimals that can be represented with n bits."""
+       return max(1, int(round(int(n)/3.3219280948873626)-1))
+
+   def validate(n):
+       n = int(n)
+       if n <= 0:
+           raise ValueError('non-negative integer expected')
+       return n
+
+   def __setattr__(name, value):
+       if name == 'CONSTANT':
+           raise AttributeError('Read-only attribute!')
+       if name == 'dps':
+           value = validate(value)
+           globals()['dps'] = value
+           globals()['prec'] = dps_to_prec(value)
+           return
+       if name == 'prec':
+           value = validate(value)
+           globals()['prec'] = value
+           globals()['dps'] = prec_to_dps(value)
+           return
+       globals()[name] = value
+
+   def __delattr__(name):
+       if name in ('CONSTANT', 'dps', 'prec'):
+           raise AttributeError('Read-only attribute!')
+       del globals()[name]
+
+.. code:: pycon
+
+  >>> import mplib
+  >>> mplib.foo = 'spam'
+  >>> mplib.CONSTANT = 42
+  Traceback (most recent call last):
+    ...
+  AttributeError: Read-only attribute!
+  >>> del mplib.foo
+  >>> del mplib.CONSTANT
+  Traceback (most recent call last):
+    ...
+  AttributeError: Read-only attribute!
+  >>> mplib.prec
+  53
+  >>> mplib.dps
+  15
+  >>> mplib.dps = 5
+  >>> mplib.prec
+  20
+  >>> mplib.dps = 0
+  Traceback (most recent call last):
+    ...
+  ValueError: non-negative integer expected
+
+
+Specification
+=============
+
+The ``__setattr__`` function at the module level should accept two
+arguments, the name of an attribute and the value to be assigned,
+and return :py:obj:`None` or raise an :exc:`AttributeError`.
+
+.. code:: python
+
+   def __setattr__(name: str, value: typing.Any, /) -> None: ...
+
+The ``__delattr__`` function should accept one argument,
+the name of an attribute, and return :py:obj:`None` or raise an
+:py:exc:`AttributeError`:
+
+.. code:: python
+
+   def __delattr__(name: str, /) -> None: ...
+
+The ``__setattr__`` and ``__delattr__`` functions are looked up in the
+module ``__dict__``.  If present, the appropriate function is called to
+customize setting the attribute or its deletion, else the normal
+mechanism (storing/deleting the value in the module dictionary) will work.
+
+Defining ``__setattr__`` or ``__delattr__`` only affect lookups made
+using the attribute access syntax---directly accessing the module
+globals is unaffected, e.g. ``sys.modules[__name__].some_global = 'spam'``.
+
+
+How to Teach This
+=================
+
+The "Customizing module attribute access" [1]_ section of the documentation
+will be expanded to include new functions.
+
+
+Reference Implementation
+========================
+
+The reference implementation for this PEP can be found in `CPython PR #108261
+<https://github.com/python/cpython/pull/108261>`__.
+
+
+Backwards compatibility
+=======================
+
+This PEP may break code that uses module level (global) names
+``__setattr__`` and ``__delattr__``, but the language reference
+explicitly reserves *all* undocumented dunder names, and allows
+"breakage without warning" [2]_.
+
+The performance implications of this PEP are small, since additional
+dictionary lookup is much cheaper than storing/deleting the value in
+the dictionary.  Also it is hard to imagine a module that expects the
+user to set (and/or delete) attributes enough times to be a
+performance concern.  On another hand, proposed mechanism allows to
+override setting/deleting of attributes without affecting speed of
+attribute access, which is much more likely scenario to get a
+performance penalty.
+
+
+Footnotes
+=========
+
+.. [1] Customizing module attribute access
+       (https://docs.python.org/3.11/reference/datamodel.html#customizing-module-attribute-access)
+
+.. [2] Reserved classes of identifiers
+       (https://docs.python.org/3.11/reference/lexical_analysis.html#reserved-classes-of-identifiers)
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.