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bpo-43453: Update and re-add example to typing runtime_checkable #27013

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bpo-43453: Update and re-add example to typing runtime_checkable #27013

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updated and re-added example to `runtime_checkable`
akulakov Jul 4, 2021
db0cc5e
update refs
akulakov Jul 4, 2021
cc7d729
Update Doc/library/typing.rst
akulakov Jul 4, 2021
6630543
Updated phrasing per comments
akulakov Jul 4, 2021
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@akulakov
akulakov committed Jul 4, 2021
commit db0cc5eb01114e3133467e0cbb698d45f9c2b832
11 changes: 6 additions & 5 deletions Doc/library/typing.rst
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Expand Up @@ -1232,11 +1232,12 @@ These are not used in annotations. They are building blocks for creating generic

.. note::

:func:`runtime_checkable` will check only the presence of the required methods,
not their type signatures. For example, :class:`ssl.SSLObject` implements `__init__()`,
therefore it passes an :func:`builtins.issubclass()` check against :data:`Callable`.
However, the :meth:`ssl.SSLObject.__float__` method exists only to raise a TypeError
with a more informative message.
:func:`runtime_checkable` will check only the presence of the required
methods, not their type signatures. For example, :class:`ssl.SSLObject`
implements :func:`__init__`, therefore it passes an :func:`issubclass`
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Runtime protocol checking is an area which I'm not too familiar with so I may be wrong here, but isn't the requirement for Callable to define __call__, not __init__?

https://docs.python.org/3/library/collections.abc.html#collections.abc.Callable

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__call__ makes the instances callable, not the class itself. In my testing, a class meets the Callable check if it doesn't have __call__ defined, and it works if init is either explicitly defined or implicitly inherited from object. However, this class -- SSLObject -- cannot be called because init raises an exception. So this example illustrates that check for callable will tell you SSLObject is callable when in fact it's not callable in practice due to the dunder method raising an exception.

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[ins] In [89]: class A:
          ...:     def __init__(self):0
          ...:

[ins] In [90]: isinstance(A,Callable)
Out[90]: True

[ins] In [91]: class B:pass

[ins] In [92]: isinstance(B,Callable)
Out[92]: True

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Wow, interesting sleuthing. Thanks for investigating!

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@Fidget-Spinner Fidget-Spinner Jul 4, 2021 •
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I wonder if this has anything to play in it, but a class implicitly has __call__ from the type object:

>>> class A: ...
>>> A.__call__
<method-wrapper '__call__' of type object at 0x0000028F6E186DE0>
>>> type(A).__call__
<slot wrapper '__call__' of 'type' objects>

>>> type(A).__call__(A)
<__main__.A object at 0x0000028F6DE94280>
>>> A()
<__main__.A object at 0x0000028F6DE8BED0>

As you can see, the last two are equivalent. This corresponds to what the docs say about callables https://docs.python.org/3/reference/datamodel.html#object.__call__. Also mentioned here for callable() https://docs.python.org/3/library/functions.html#callable

The part I'm nitpicky about is saying ssl.SSLObject passes the issubclass check because it defines __init__. I think that's untrue -- it passes the check because all classes are callable (as mentioned in the docs for callable() above). I completely agree with the part about __init__ being used for raising exceptions.
I think the phrasing would be better if it became this:

        :func:`runtime_checkable` will check only the presence of the required
        methods, not their type signatures. For example, :class:`ssl.SSLObject`
        is a class, therefore it passes an :func:`issubclass`
        check against :data:`Callable`.  However, the
        :meth:`ssl.SSLObject.__init__` method exists only to raise a
        :exc:`TypeError` with a more informative message.

Sorry about the bikshedding!

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You're right, good catch! I think your edit is good, but I want to highlight the fact that init as implemented makes this class uncallable, maybe something like this:

        :func:`runtime_checkable` will check only the presence of the required
        methods, not their type signatures. For example, :class:`ssl.SSLObject`
        is a class, therefore it passes an :func:`issubclass`
        check against :data:`Callable`.  However, the
        :meth:`ssl.SSLObject.__init__` method exists only to raise a
        :exc:`TypeError` with a more informative message, therefore making
        it impossible to call (instantiate) :class:`ssl.SSLObject`.

WDYT?

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Great!

check against :data:`Callable`. However, the
:meth:`ssl.SSLObject.__init__` method exists only to raise a
:exc:`TypeError` with a more informative message.

.. versionadded:: 3.8

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