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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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0335b69
gh-99505: Fix docs for determining the metaclass
robsdedude Aug 26, 2024
4e3e456
Improve formatting
robsdedude Aug 27, 2024
8cd038a
Amend missed case
robsdedude Aug 27, 2024
723be69
Merge branch 'main' into fix-issue-99505
robsdedude Feb 14, 2026
a05cb5a
Fix typo
robsdedude Feb 14, 2026
470dfde
Use "class of class" instead of "metaclass of class"
robsdedude Feb 14, 2026
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34 changes: 21 additions & 13 deletions Doc/reference/datamodel.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -2825,19 +2825,27 @@ 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 class of the first
base is used as candidate;
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@merwok merwok Feb 13, 2026

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Technically it would be the class (or type) of the first base (itself a class), not its metaclass, but not sure how to phrase that elegantly…

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@robsdedude robsdedude Feb 14, 2026

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Thanks, a good point. I would've interpreted "the metaclass of a class C" as type(C) and not the technically correct type(type(C)). I think "class of the first base" and similar is better here. It's more accurate and still comprehensible, I find.

* 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 current candidate is a subclass of the class of the current base,
continue with the next base;
* if the class of the current base is a subclass of the current candidate ,
continue with the next base;
* else raise a :exc:`TypeError`.


.. _prepare:
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