On Tue, Nov 14, 2017 at 10:34 PM, Ivan Levkivskyi levkivskyi@gmail.com wrote: ​[..]​

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

​Deprecating functions is already possible, so I assume the reason for this would be performance? If so, are you sure this would help for performance? ​ ​Deprecating module attributes / globals is indeed difficult to do at present. This PEP would allow deprecation warnings for accessing attributes, which is nice! However, as thread-unsafe as it is, many modules use module attributes to configure the state of the module. In that case, the user is more likely to *set* the attribute that to *get* it. Is this outside the scope of the PEP?

​[..]​

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.

​I'm not surprised that the comparison is in favor of this PEP ;-).​

​[..]​

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: ...

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

The Rationale (quoted in the beginning of this email) easily leaves a different impression of this.​

​[..] ​

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__``.

Off-topic: In some sense, I'm happy to hear something about pickleability. But in some sense not.

I think there are three kinds of people regarding pickleability:

1. Those who don't care about anything being pickleable

2. Those ​who care about some things being picklable

​3. ​Those who care about all things being picklable

Personally, I'd like to belong to group 3, but because group 3 cannot even attempt to coexist with groups 1 and 2, I actually belong to group 1 most of the time.

​––Koos ​

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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