Handle `hash = None` in somewhat more intuitive fashion by sterliakov · Pull Request #19168 · python/mypy · GitHub

Fixes #4266. Fixes #18622. Refs #2455.

This is the first step towards checking hashability of dict keys (and Hashable compatibility in general). This PR implements the following rules:

__hash__ = None is only allowed on classes who inherited __hash__ directly from object (no redefinitions in MRO)
def __hash__ is allowed on subclasses whose parents declare __hash__ = None - it is a relatively common solution to derive a hashable subclass, and LSP violation only means assert SomeCls.__hash__ is None will not pass for subclasses. This sounds safe enough.
Classes defining __eq__ without __hash__ get their __hash__ assigned to None to match runtime semantics. Protocols are exempt from this as defining __eq__ on protocol just requires classes that support __eq__, not necessarily unhashable ones.

Refs #19043.

…k in subclasses, only allowed to override `object.__hash__`

…-dunder-hash-consistently

xarray should either always use __hash__ = None or __hash__: ClassVar[None]. Using __hash__: None everywhere should also work, I suppose, but doesn't have proper semantics. All errors are correct, and "cannot override instance variable..." will be fixed by using ClassVar[None] everywhere. Another one (argument "name"...) is a true positive, Mapping is not Hashable, the call should be rejected.
pandas: ByT is not hashable because its member Period is not. And it's a bug, at runtime Period is hashable, they're simply missing a __hash__ definition on it. If we can contact pandas-stubs maintainers, would be nice to let them know.
I'm lost in static_frame, this seems reasonable, there's also pyright-ignore on that line, but I'm failing to trace precise types in a 10 KLOC file (and not even self-contained in that file).
All other primer hits are correct; noticing that str and Mapping/MutableMapping are incompatible is also a great outcome.

…-dunder-hash-consistently

Diff from mypy_primer, showing the effect of this PR on open source code:

beartype (https://github.com/beartype/beartype)
+ beartype/_check/metadata/metadecor.py:289: error: Unused "type: ignore" comment  [unused-ignore]
+ beartype/_check/metadata/metacheck.py:97: error: Unused "type: ignore" comment  [unused-ignore]

mongo-python-driver (https://github.com/mongodb/mongo-python-driver)
+ bson/regex.py:112: error: Unused "type: ignore" comment  [unused-ignore]

prefect (https://github.com/PrefectHQ/prefect)
+ src/prefect/_internal/concurrency/calls.py:273: error: Unused "type: ignore" comment  [unused-ignore]

colour (https://github.com/colour-science/colour)
- colour/utilities/structures.py:515: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- colour/io/luts/sequence.py:266: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- colour/io/luts/operator.py:411: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- colour/io/luts/lut.py:354: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]

asynq (https://github.com/quora/asynq)
+ asynq/tests/test_tools.py:183: error: Unused "type: ignore" comment  [unused-ignore]

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/python_api.py:88: error: Unused "type: ignore" comment  [unused-ignore]
+ src/_pytest/_code/source.py:50: error: Unused "type: ignore" comment  [unused-ignore]
+ src/_pytest/_code/code.py:77: error: Unused "type: ignore" comment  [unused-ignore]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/arrays/base.py:2207: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/generic.py:1868: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/indexes/base.py:5204: error: Unused "type: ignore" comment  [unused-ignore]

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/datastructures/structures.py:648: error: Unused "type: ignore" comment  [unused-ignore]
+ src/werkzeug/datastructures/headers.py:108: error: Unused "type: ignore" comment  [unused-ignore]
+ src/werkzeug/routing/rules.py:912: error: Unused "type: ignore" comment  [unused-ignore]

jax (https://github.com/google/jax)
+ jax/_src/core.py:535: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/core.py:875: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/earray.py:33: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/prng.py:301: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/sparse/_base.py:34: error: Unused "type: ignore" comment  [unused-ignore]

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ pandas-stubs/core/generic.pyi:67: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas-stubs/core/frame.pyi:375: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas-stubs/core/indexes/base.pyi:75: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas-stubs/core/groupby/generic.pyi:209: error: Return type "Iterator[tuple[ByT, Series[S2]]]" of "__iter__" incompatible with return type "Iterator[tuple[Hashable, Series[S2]]]" in supertype "pandas.core.groupby.groupby.BaseGroupBy"  [override]
+ pandas-stubs/core/groupby/generic.pyi:449: error: Return type "Iterator[tuple[ByT, DataFrame]]" of "__iter__" incompatible with return type "Iterator[tuple[Hashable, DataFrame]]" in supertype "pandas.core.groupby.groupby.BaseGroupBy"  [override]

ibis (https://github.com/ibis-project/ibis)
- ibis/common/bases.py:167: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- ibis/common/bases.py:205: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- ibis/common/bases.py:249: error: Signature of "__hash__" incompatible with supertype "Slotted"  [override]
- ibis/common/bases.py:249: note:      Superclass:
- ibis/common/bases.py:249: note:          None
- ibis/common/bases.py:249: note:      Subclass:
- ibis/common/bases.py:249: note:          def __hash__(self) -> int
- ibis/common/collections.py:153: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- ibis/common/collections.py:291: error: Signature of "__hash__" incompatible with supertype "Mapping"  [override]
- ibis/common/collections.py:291: note:      Superclass:
- ibis/common/collections.py:291: note:          None
- ibis/common/collections.py:291: note:      Subclass:
- ibis/common/collections.py:291: note:          def __hash__(self) -> int
- ibis/common/collections.py:313: note:      Superclass:
- ibis/common/collections.py:313: note:          None
- ibis/common/collections.py:313: note:      Subclass:
- ibis/common/collections.py:313: note:          def __hash__(self) -> int
- ibis/common/collections.py:313: error: Signature of "__hash__" incompatible with supertype "Mapping"  [override]
- ibis/common/deferred.py:180: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- ibis/common/patterns.py:660: error: Signature of "__hash__" incompatible with supertype "Slotted"  [override]
- ibis/common/patterns.py:660: note:      Superclass:
- ibis/common/patterns.py:660: note:          None
- ibis/common/patterns.py:660: note:      Subclass:
- ibis/common/patterns.py:660: note:          def __hash__(self) -> int
- ibis/common/patterns.py:937: error: Signature of "__hash__" incompatible with supertype "Slotted"  [override]
- ibis/common/patterns.py:937: note:      Superclass:
- ibis/common/patterns.py:937: note:          None
- ibis/common/patterns.py:937: note:      Subclass:
- ibis/common/patterns.py:937: note:          def __hash__(self) -> int
- ibis/common/grounds.py:164: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- ibis/common/grounds.py:220: error: Signature of "__hash__" incompatible with supertype "Comparable"  [override]
- ibis/common/grounds.py:220: note:      Superclass:
- ibis/common/grounds.py:220: note:          None
- ibis/common/grounds.py:220: note:      Subclass:
- ibis/common/grounds.py:220: note:          def __hash__(self) -> int
- ibis/common/grounds.py:220: error: Signature of "__hash__" incompatible with supertype "Annotable"  [override]
- ibis/common/grounds.py:220: note:      Superclass:
- ibis/common/grounds.py:220: note:          None
- ibis/common/grounds.py:220: note:      Subclass:
- ibis/common/grounds.py:220: note:          def __hash__(self) -> int
- ibis/common/egraph.py:124: error: Incompatible types in assignment (expression has type "None", base class "object" defined the type as "Callable[[], int]")  [assignment]
- ibis/expr/datatypes/core.py:941: error: Definition of "__hash__" in base class "Concrete" is incompatible with definition in base class "Mapping"  [misc]
- ibis/expr/schema.py:25: error: Definition of "__hash__" in base class "Concrete" is incompatible with definition in base class "Mapping"  [misc]
- ibis/common/tests/test_grounds.py:142: error: Definition of "__hash__" in base class "Concrete" is incompatible with definition in base class "Mapping"  [misc]

mkdocs (https://github.com/mkdocs/mkdocs)
+ mkdocs/utils/templates.py:47: error: Subclass of "ExtraScriptValue" and "str" cannot exist: would have incompatible method signatures  [unreachable]

schema_salad (https://github.com/common-workflow-language/schema_salad)
+ schema_salad/jsonld_context.py: note: In function "pred":
+ schema_salad/jsonld_context.py:53:31: error: Subclass of "str" and "MutableMapping[Any, Any]" cannot exist: would have incompatible method signatures  [unreachable]
+ schema_salad/jsonld_context.py:54:21: error: Statement is unreachable  [unreachable]
+ schema_salad/schema.py: note: In function "get_anon_name":
+ schema_salad/schema.py:422:27: error: Subclass of "str" and "Mapping[Any, Any]" cannot exist: would have incompatible method signatures  [unreachable]
+ schema_salad/schema.py:423:17: error: Statement is unreachable  [unreachable]

static-frame (https://github.com/static-frame/static-frame)
+ static_frame/core/series.py:3346: error: Incompatible return value type (got "IndexBase", expected "TLabel | ndarray[Any, Any]")  [return-value]
+ static_frame/core/frame.py:7056: error: Argument 1 to "index_from_optional_constructors" has incompatible type "TypeBlocks"; expected "TIndexInitializer"  [arg-type]
+ static_frame/core/frame.py:7056: note: Following member(s) of "TypeBlocks" have conflicts:
+ static_frame/core/frame.py:7056: note:     Expected:
+ static_frame/core/frame.py:7056: note:         def __iter__(self) -> Iterator[TLabel]
+ static_frame/core/frame.py:7056: note:     Got:
+ static_frame/core/frame.py:7056: note:         def __iter__(self) -> Iterator[TypeBlocks]
+ static_frame/core/frame.py:7056: note: Following member(s) of "TypeBlocks" have conflicts:
+ static_frame/core/frame.py:7056: note:     Expected:
+ static_frame/core/frame.py:7056: note:         def __iter__(self) -> Iterator[Sequence[TLabel]]
+ static_frame/core/frame.py:7056: note:     Got:
+ static_frame/core/frame.py:7056: note:         def __iter__(self) -> Iterator[TypeBlocks]
+ static_frame/core/frame.py:7298: error: Argument 1 to "index_from_optional_constructors" has incompatible type "TypeBlocks"; expected "TIndexInitializer"  [arg-type]
+ static_frame/core/frame.py:7298: note: Following member(s) of "TypeBlocks" have conflicts:
+ static_frame/core/frame.py:7298: note:     Expected:
+ static_frame/core/frame.py:7298: note:         def __iter__(self) -> Iterator[TLabel]
+ static_frame/core/frame.py:7298: note:     Got:
+ static_frame/core/frame.py:7298: note:         def __iter__(self) -> Iterator[TypeBlocks]
+ static_frame/core/frame.py:7298: note: Following member(s) of "TypeBlocks" have conflicts:
+ static_frame/core/frame.py:7298: note:     Expected:
+ static_frame/core/frame.py:7298: note:         def __iter__(self) -> Iterator[Sequence[TLabel]]
+ static_frame/core/frame.py:7298: note:     Got:
+ static_frame/core/frame.py:7298: note:         def __iter__(self) -> Iterator[TypeBlocks]

xarray (https://github.com/pydata/xarray)
+ xarray/core/_typed_ops.py:93: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/_typed_ops.py:362: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/_typed_ops.py:736: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/_typed_ops.py:1164: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/_typed_ops.py:1376: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/_typed_ops.py:1502: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/dataset.py:1510: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/dataset.py: note: In class "Dataset":
+ xarray/core/dataset.py:1510: error: Cannot override instance variable (previously declared on base class "DatasetOpsMixin") with class variable  [misc]
+ xarray/core/dataset.py:1510: note: Error code "misc" not covered by "type: ignore" comment
+ xarray/core/dataset.py: note: At top level:
+ xarray/core/dataarray.py:1364: error: Unused "type: ignore" comment  [unused-ignore]
+ xarray/core/dataarray.py: note: In member "rename" of class "DataArray":
+ xarray/core/dataarray.py:2601: error: Argument "name" to "_replace" of "DataArray" has incompatible type "Hashable | Mapping[Any, Hashable]"; expected "Hashable | Default | None"  [arg-type]
+ xarray/core/dataarray.py: note: At top level:
+ xarray/core/datatree.py:303: error: Unused "type: ignore" comment  [unused-ignore]

sterliakov added 2 commits May 29, 2025 18:13

Make behaviour of __hash__ = None more intuitive: can be rolled bac…

25d10b1

…k in subclasses, only allowed to override `object.__hash__`

Missing get_proper_type

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sterliakov added 3 commits June 1, 2025 16:31

Allow any such overrides

31a5c2a

Fix stub for mypyc test

22a2729

Adjust remaining test, replace ellipsis with pass for consistency

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Do not add implicit __hash__ = None to protocols

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sterliakov added 3 commits June 3, 2025 20:53

Merge remote-tracking branch 'upstream/master' into feature/st-handle…

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…-dunder-hash-consistently

Support in assignments too

bd953b4

Handle allow_incompatible_override consistenly instead

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sterliakov marked this pull request as ready for review June 6, 2025 22:39

sterliakov changed the title experiment: handle __hash__ = None in somewhat more intuitive fashion Handle __hash__ = None in somewhat more intuitive fashion Jun 7, 2025

sterliakov mentioned this pull request Jun 7, 2025

Fix and simplify error de-duplication #19247

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sterliakov and others added 2 commits June 11, 2025 03:52

Merge branch 'master' into feature/st-handle-dunder-hash-consistently

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Sync test after merge

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ktbarrett mentioned this pull request Jun 22, 2025

Various fixes cocotb/cocotb#4765

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Merge branch 'master' into feature/st-handle-dunder-hash-consistently

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Merge branch 'master' into feature/st-handle-dunder-hash-consistently

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sterliakov requested review from A5rocks and JukkaL July 4, 2025 01:53

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Merge branch 'master' into feature/st-handle-dunder-hash-consistently

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Merge branch 'master' into feature/st-handle-dunder-hash-consistently

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Merge remote-tracking branch 'upstream/master' into feature/st-handle…

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…-dunder-hash-consistently

hauntsaninja added the upnext label Jan 18, 2026

Handle `hash = None` in somewhat more intuitive fashion by sterliakov · Pull Request #19168 · python/mypy · GitHub

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Handle hash = None in somewhat more intuitive fashion - #19168

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Handle `__hash__ = None` in somewhat more intuitive fashion by sterliakov · Pull Request #19168 · python/mypy · GitHub

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Handle __hash__ = None in somewhat more intuitive fashion - #19168

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Handle `hash = None` in somewhat more intuitive fashion by sterliakov · Pull Request #19168 · python/mypy · GitHub

Handle hash = None in somewhat more intuitive fashion - #19168

Handle hash = None in somewhat more intuitive fashion#19168
sterliakov wants to merge 16 commits into
python:masterfrom
sterliakov:feature/st-handle-dunder-hash-consistently

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