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gh-99505: Fix docs for determining the metaclass #123349

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gh-99505: Fix docs for determining the metaclass #123349

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gh-99505: Fix docs for determining the metaclass
robsdedude Aug 26, 2024
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Improve formatting
robsdedude Aug 27, 2024
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Amend missed case
robsdedude Aug 27, 2024
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35 changes: 22 additions & 13 deletions Doc/reference/datamodel.rst
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Expand Up @@ -2462,19 +2462,28 @@ Determining the appropriate metaclass
.. index::
single: metaclass hint

The appropriate metaclass for a class definition is determined as follows:

* if no bases and no explicit metaclass are given, then :func:`type` is used;
* if an explicit metaclass is given and it is *not* an instance of
:func:`type`, then it is used directly as the metaclass;
* if an instance of :func:`type` is given as the explicit metaclass, or
bases are defined, then the most derived metaclass is used.

The most derived metaclass is selected from the explicitly specified
metaclass (if any) and the metaclasses (i.e. ``type(cls)``) of all specified
base classes. The most derived metaclass is one which is a subtype of *all*
of these candidate metaclasses. If none of the candidate metaclasses meets
that criterion, then the class definition will fail with ``TypeError``.
The appropriate metaclass for a class definition is determined in two steps:

1) a candidate metaclass is determined as follows:

* if no bases and no explicit metaclass are given, then :func:`type` is used
as candidate;
* if an explicit metaclass is given and it is an instance of :func:`type`,
then it is used as candidate;
* if bases are defined but no explicit metaclass, the metaclass of the first
base is used as candidate;
* if an explicit metaclass is given and it is *not* an instance of
:func:`type`, then it is used directly as the metaclass and step 2 is skipped.

2) bases, if present, are traversed left-to-right, and the most derived
metaclass is determined as follows:

* if the metaclass of the current candidate is a subtype of the metaclass of
the current base, updated the candidate to the metaclass of the base and
continue to the next base;
* if the current candidate is a subtype of the metaclass of the current base,
continue to the next base;
* else raise a :exc:`TypeError`.


.. _prepare:
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