[Python-ideas] PEP 562

[Ionel Cristian Mărieș]

Wouldn't a better approach be a way to customize the type of the module?
That would allow people to define behavior for almost anything (__call__,
__getattr__, __setattr__, __dir__, various operators etc). This question
shouldn't exist "why can't I customize behavior X in a module when I can do
it for a class". Why go half-way.


Thanks,
-- Ionel Cristian Mărieș, http://blog.ionelmc.ro

On Sun, Sep 10, 2017 at 9:48 PM, Ivan Levkivskyi <levkivskyi at gmail.com>
wrote:

> I have written a short PEP as a complement/alternative to PEP 549.
> I will be grateful for comments and suggestions. The PEP should
> appear online soon.
>
> --
> Ivan
>
> ***********************************************************
>
> PEP: 562
> Title: Module __getattr__
> Author: Ivan Levkivskyi <levkivskyi at 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
>
> 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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>
>
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