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rhettinger merged 2 commits into from
Apr 5, 2021
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bpo-20503: Show how isinstance() works with ABC registered classes. #25175

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bpo-20503: Show how isinstance() works with ABC registered classes.
rhettinger Apr 4, 2021
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Remove __main__ references
rhettinger Apr 4, 2021
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35 changes: 35 additions & 0 deletions Doc/faq/programming.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1420,6 +1420,41 @@ single class, e.g. ``isinstance(obj, (class1, class2, ...))``, and can also
check whether an object is one of Python's built-in types, e.g.
``isinstance(obj, str)`` or ``isinstance(obj, (int, float, complex))``.

Note that :func:`isinstance` also checks for virtual inheritance from an
:term:`abstract base class`. So, the test will return ``True`` for a
registered class even if hasn't directly or indirectly inherited from it. To
test for "true inheritance", scan the :term:`MRO` of the class:

.. testcode::

from collections.abc import Mapping

class P:
pass

class C(P):
pass

Mapping.register(P)

.. doctest::

>>> c = C()
>>> isinstance(c, C) # direct
True
>>> isinstance(c, P) # indirect
True
>>> isinstance(c, Mapping) # virtual
True

# Actual inheritance chain
>>> type(c).__mro__
(<class 'C'>, <class 'P'>, <class 'object'>)

# Test for "true inheritance"
>>> Mapping in type(c).__mro__
False

Note that most programs do not use :func:`isinstance` on user-defined classes
very often. If you are developing the classes yourself, a more proper
object-oriented style is to define methods on the classes that encapsulate a
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