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[3.11] gh-102936: typing: document performance pitfalls of protocols decorated with @runtime_checkable (GH-102937) #102963

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Mar 23, 2023
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[3.11] gh-102936: typing: document performance pitfalls of protocols decorated with @runtime_checkable (GH-102937) #102963

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20 changes: 18 additions & 2 deletions Doc/library/typing.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1582,16 +1582,32 @@ These are not used in annotations. They are building blocks for creating generic

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

@runtime_checkable
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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