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JelleZijlstra merged 2 commits into from
Mar 30, 2026
Merged

docs: clarify that invariant-by-default applies to legacy TypeVar syntax #21108

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docs: clarify that invariant-by-default applies to legacy TypeVar syntax
Mar 30, 2026
c048835
address review: start new paragraph for invariance error example
Mar 30, 2026
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10 changes: 6 additions & 4 deletions docs/source/common_issues.rst
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Expand Up @@ -303,10 +303,12 @@ See :ref:`type-narrowing` for more information.
Invariance vs covariance
------------------------

Most mutable generic collections are invariant, and mypy considers all
user-defined generic classes invariant by default
(see :ref:`variance-of-generics` for motivation). This could lead to some
unexpected errors when combined with type inference. For example:
Most mutable generic collections are invariant. When using the legacy
``TypeVar`` syntax, mypy considers all user-defined generic classes invariant
by default (see :ref:`variance-of-generics` for motivation). When using the
:pep:`695` syntax (``class MyClass[T]: ...``), variance is inferred from
usage rather than defaulting to invariant. This could lead to some unexpected
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@JelleZijlstra JelleZijlstra Mar 30, 2026

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The last sentence reads oddly now, I think "this" refers to the fact that mutable sequences are invariant, which we mentioned several sentences ago. I would start a new paragraph here with something like "The fact that mutable sequences are usually invariant can lead to unexpected errors..."

errors when combined with type inference. For example:

.. code-block:: python

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