docs: clarify that invariant-by-default applies to legacy TypeVar syntax#21108
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JelleZijlstra merged 2 commits intopython:masterfrom Mar 30, 2026
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Under PEP 695 class syntax variance is inferred, not invariant by default. Closes python#20366. Made-with: Cursor
docs/source/common_issues.rst
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| ``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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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..."
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Just fixed, thanks so much for the feedback! |
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Closes #20366.
The "invariant by default" rule applies to the legacy
TypeVarsyntax. Under PEP 695 (class MyClass[T]: ...), mypy infers variance from usage instead. Updated the wording incommon_issues.rstto make this distinction clear.