On Tue, Nov 14, 2017 at 10:34 PM, Ivan Levkivskyi <levkivskyi at 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.
>
>
>
> ..
> Local Variables:
> mode: indented-text
> indent-tabs-mode: nil
> sentence-end-double-space: t
> fill-column: 70
> coding: utf-8
> End:
>
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>
>
--
+ Koos Zevenhoven + http://twitter.com/k7hoven +
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