Use empty context as fallback for return statements #19767
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sterliakov
approved these changes
Aug 31, 2025
mypy/checker.py
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| # If there are errors with the original type context, try re-inferring in empty context. | ||
| original_messages = msg.filtered_errors() | ||
| original_type_map = type_map | ||
| with self.msg.filter_errors(filter_errors=True, save_filtered_errors=True) as msg: |
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How should [deprecated] errors be handled here? They can be context-dependent in case of some scary overload
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Probably just filter_deprecated=True in both branches to avoid false positives?
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Yes, I think we need to filter deprecated as well.
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Diff from mypy_primer, showing the effect of this PR on open source code: spark (https://github.com/apache/spark)
+ python/pyspark/core/rdd.py:2210: error: Unused "type: ignore" comment [unused-ignore]
prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/flows.py:1980: error: Argument "on_completion" to "Flow" has incompatible type "Optional[list[FlowStateHook[[VarArg(Any), KwArg(Any)], Any]]]"; expected "Optional[list[FlowStateHook[P, R]]]" [arg-type]
- src/prefect/flows.py:1981: error: Argument "on_failure" to "Flow" has incompatible type "Optional[list[FlowStateHook[[VarArg(Any), KwArg(Any)], Any]]]"; expected "Optional[list[FlowStateHook[P, R]]]" [arg-type]
- src/prefect/flows.py:1982: error: Argument "on_cancellation" to "Flow" has incompatible type "Optional[list[FlowStateHook[[VarArg(Any), KwArg(Any)], Any]]]"; expected "Optional[list[FlowStateHook[P, R]]]" [arg-type]
- src/prefect/flows.py:1983: error: Argument "on_crashed" to "Flow" has incompatible type "Optional[list[FlowStateHook[[VarArg(Any), KwArg(Any)], Any]]]"; expected "Optional[list[FlowStateHook[P, R]]]" [arg-type]
- src/prefect/flows.py:1984: error: Argument "on_running" to "Flow" has incompatible type "Optional[list[FlowStateHook[[VarArg(Any), KwArg(Any)], Any]]]"; expected "Optional[list[FlowStateHook[P, R]]]" [arg-type]
Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/context/menu.py:200: error: Argument "default" to "resolve_to_member" of "MenuContext" has incompatible type "None"; expected "User" [arg-type]
hydra-zen (https://github.com/mit-ll-responsible-ai/hydra-zen)
- src/hydra_zen/wrapper/_implementations.py:1756: error: Unsupported operand types for + ("list[None]" and "list[str]") [operator]
jax (https://github.com/google/jax)
+ jax/_src/sharding_impls.py:189: error: Unused "type: ignore" comment [unused-ignore]
ibis (https://github.com/ibis-project/ibis)
- ibis/backends/sql/__init__.py:515: error: Item "Iterable[str]" of "str | Iterable[str]" has no attribute "format" [union-attr]
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ilevkivskyi
pushed a commit
that referenced
this pull request
Sep 2, 2025
Fixes #19716. It is a follow-up to #19767 and was missed there due to malformed test stubs (the same testcase fails when run by full mypy against typeshed, I did not notice missing AwaitExpr because of the passing test...). I synced typevar variance with typeshed definitions and added AwaitExpr to the list of context-dependent exprs. Cc @ilevkivskyi
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Fixes #16924
Fixes #15886
Mypy uses external type context first, this can cause bad type inference in return statements (see example in test case added), usually we recommend a workaround to users like replacing:
with
But this is a bit ugly, and more importantly we can essentially automatically try this workaround. This is what this PR adds. I checked performance impact, and don't see any (but for some reason noise level on my desktop is much higher now).