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gh-102936: typing: document performance pitfalls of protocols decorated with @runtime_checkable #102937

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Mar 23, 2023
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gh-102936: typing: document performance pitfalls of protocols decorated with @runtime_checkable #102937

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Typing: document performance pitfalls of protocols decorated with `@r…
AlexWaygood Mar 22, 2023
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A thread named Bob
AlexWaygood Mar 23, 2023
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20 changes: 18 additions & 2 deletions Doc/library/typing.rst
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Expand Up @@ -1584,16 +1584,32 @@ These are not used in annotations. They are building blocks for creating generic

assert isinstance(open('/some/file'), Closable)

@runtime_checkable
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I think many users aren't too familiar with OSError's constructor / we can't use kwargs and it's unclear what the second argument is. Maybe we could use int and HasNumerator instead?

(I know it's not writeable, but OSError's errno also is maybe not writeable? Writing to errno doesn't affect repr)

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Yeah, I was really struggling to think of a good example for this! I'll ponder some more and see if I can come up with something better...

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Perhaps unsurprisingly, not too many good cases of writeable attributes in stdlib. Maybe Thread.name? It's explicitly documented as something you can write to.

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That works! I'll make the change tomorrow (it's late here)

class Named(Protocol):
name: str

import threading
assert isinstance(threading.Thread(name='Bob'), Named)

.. note::

:func:`runtime_checkable` will check only the presence of the required
methods, not their type signatures. For example, :class:`ssl.SSLObject`
:func:`!runtime_checkable` will check only the presence of the required
methods or attributes, not their type signatures or types.
For example, :class:`ssl.SSLObject`
is a class, therefore it passes an :func:`issubclass`
check against :data:`Callable`. However, the
``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`.

.. note::

An :func:`isinstance` check against a runtime-checkable protocol can be
surprisingly slow compared to an ``isinstance()`` check against
a non-protocol class. Consider using alternative idioms such as
:func:`hasattr` calls for structural checks in performance-sensitive
code.

.. versionadded:: 3.8

Other special directives
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