Respect explicit return type of __new__() by ilevkivskyi · Pull Request #21441 · python/mypy

ilevkivskyi · 2026-05-08T14:09:32Z

With this PR will still give an error if the explicit return __new__() is not a subtype of current class, but now we will actually use it. There are two exceptions (to preserve backwards compatibility):

If the return type is Any with still use current class as the return type.
If the explicit return type comes from a superclass and is a supertype of implicit return type.

class A:
    def __new__(cls): ...
reveal_type(A())  # still __main__.A

class B:
    def __new__(cls) -> B:
        return cls()
class C(B): ...
reveal_type(C())  # still __main__.C

This uses a more principled implementation than some earlier attempts: adding a new dedicated attribute to CallableType for this purpose. Some comments:

This PR has a bit for boilerplate, but this is expected. When adding a new attribute to a type, one needs to update most visitors.
While doing the above I noticed that CallableType.type_guard and CallableType.type_is were not handled in dependency visitors (neither coarse-grained nor fine-grained). IMO they definitely should be handled there, so I now handle them.
I try to reduce the size of CallableType a bit to compensate for new attribute by removing (rarely used) min_args attribute. I also use compact flags serialization.
I skimmed the code base and updated all places where we "casually" use CallableType.ret_type for various type object edge cases.
Note that I don't assert that instance_type is set when is_type_obj() returns True. This is mostly to avoid breaking 3rd party plugins.
I didn't add a test case for each edge case, but I added (improved) test cases from Believe more __new__ return types #16020 plus some more. Suggestions for more test cases are welcome.
It looks like this exposes a pre-existing bug where we leaked type variables in type[T]. It is relatively niche edge case, but it looks important for NumPy stubs, so I am trying to fix it here.

ilevkivskyi · 2026-05-08T14:51:54Z

mypy_primer looks sad, but I guess this is what people want. I spot-checked the new errors, and they all look correct. The only one suspicious thing in one new Any error in pandas-stubs.

cc @Dr-Irv for all the new errors in pandas. IIUC these are because of things like this https://github.com/pandas-dev/pandas/blob/47f6ccdbec98998bd675324fb6f049794ad03acf/pandas/_libs/tslibs/timestamps.pyi#L44-L60

class Timestamp(datetime):
    ...
    def __new__(  # type: ignore[misc]
        cls: type[Self],
        # ...
    ) -> Self | NaTType: ...

Dr-Irv · 2026-05-08T15:04:49Z

mypy_primer looks sad, but I guess this is what people want. I spot-checked the new errors, and they all look correct. The only one suspicious thing in one new Any error in pandas-stubs.

Yes, I agree that is suspicious.

cc @Dr-Irv for all the new errors in pandas. IIUC these are because of things like this https://github.com/pandas-dev/pandas/blob/47f6ccdbec98998bd675324fb6f049794ad03acf/pandas/_libs/tslibs/timestamps.pyi#L44-L60
class Timestamp(datetime):
    ...
    def __new__(  # type: ignore[misc]
        cls: type[Self],
        # ...
    ) -> Self | NaTType: ...

We should probably copy the PYI for Timestamp and Timedelta and NAType from pandas-stubs into the pandas source at some point to address this.

I have on my TODO list to first make the pandas source compatible with the latest release of mypy. So when these changes in this PR get released, I'll do a similar task then.

bzoracler · 2026-05-08T18:02:50Z

If the return type is Any with still use current class as the return type.

If the explicit return type comes from a superclass and is a supertype of implicit return type.

Would the following alternatives also preserve backwards compatibility just as well, but reflect explicit intention better?

If the return type is unannotated (i.e. an explicit Any is not used):

class A:
    def __new__(cls, arg: object): ...

class B:
    def __new__(cls, arg: object) -> Any: ...

class C:
    def __new__(cls, arg: object) -> "bad + annotation": ...

reveal_type(A())  # `__main__.A`
reveal_type(B())  # `Any`
reveal_type(C())  # `Any`

If the explicit return type comes from a superclass and the superclass's __new__'s return type is the same as the class:
```
class A:
    def __new__(cls) -> A: ...
class B(A):
    def __new__(cls) -> A: ...

class C(B): ...
class D(A): ...

reveal_type(C())  # Would it be possible for this to be `__main__.A`?
reveal_type(D())  # `__main__.D`
```
This pattern came up when I was trying to write stubs for symengine.py a while back (maybe it affects SymPy too), where they have a root Expr class, and most of Expr's subclasses have a __new__ that frequently returns something that is not an instance of the subclass.

I'm hoping that "__new__'s return type being the same as a class" would handle the vast majority of backwards compatibility cases because it was common before typing.Self was introduced.

ilevkivskyi · 2026-05-08T20:08:19Z

@bzoracler Yeah, I actually was thinking about both these options myself. Reasons why I didn't do this:

We have a specific test checking that explicit def __new__() -> Any is ignored, see testNewReturnType2. I don't know the motivation, so decided to simply not touch that.
The definition of "the same as the class" gets a little blurry in case of generic classes. For example, is plain -> C (and not say -> C[T]) considered the same, etc.

However, I don't have strong feelings on either, so if others will chime in, I am happy reconsider.

ilevkivskyi · 2026-05-08T21:45:14Z

It looks like this exposes a pre-existing bug where we leaked type variables in type[T]. It is relatively niche edge case, but it looks important for NumPy stubs, so I am trying to fix it here.

github-actions · 2026-05-08T21:59:27Z

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

zope.interface (https://github.com/zopefoundation/zope.interface)
- src/zope/interface/common/tests/test_collections.py:88: error: Incompatible types in assignment (expression has type "set[type[array[int]]]", base class "VerifyClassMixin" defined the type as "tuple[()]")  [assignment]
+ src/zope/interface/common/tests/test_collections.py:88: error: Incompatible types in assignment (expression has type "set[type[array[_T]]]", base class "VerifyClassMixin" defined the type as "tuple[()]")  [assignment]

comtypes (https://github.com/enthought/comtypes)
- comtypes/server/register.py:226: error: Incompatible types in assignment (expression has type "reversed[tuple[int, str]]", variable has type "list[tuple[int, str]]")  [assignment]
+ comtypes/server/register.py:226: error: Incompatible types in assignment (expression has type "Iterator[tuple[int, str]]", variable has type "list[tuple[int, str]]")  [assignment]

discord.py (https://github.com/Rapptz/discord.py)
- discord/abc.py:1815: error: Incompatible types in assignment (expression has type "reversed[MessagePin]", variable has type "list[MessagePin]")  [assignment]
+ discord/abc.py:1815: error: Incompatible types in assignment (expression has type "Iterator[MessagePin]", variable has type "list[MessagePin]")  [assignment]

sympy (https://github.com/sympy/sympy)
+ sympy/core/evalf.py:997: error: Argument 1 to "evalf_add" has incompatible type "Expr"; expected "Add"  [arg-type]
+ sympy/core/function.py:449: error: Unused "type: ignore" comment  [unused-ignore]
+ sympy/printing/smtlib.py:188: error: Argument 1 to "_print_Unequality" of "SMTLibPrinter" has incompatible type "Unequality | BooleanFalse | BooleanTrue"; expected "Unequality"  [arg-type]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/tseries/frequencies.py:298: error: Incompatible return value type (got "Timestamp | NaTType", expected "Timestamp")  [return-value]
- pandas/core/dtypes/cast.py:698: error: Non-overlapping identity check (left operand type: "Timestamp", right operand type: "NaTType")  [comparison-overlap]
- pandas/core/dtypes/cast.py:711: error: Non-overlapping identity check (left operand type: "Timedelta", right operand type: "NaTType")  [comparison-overlap]
+ pandas/core/dtypes/cast.py:159: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "str | bytes | date | datetime | timedelta | <7 more items> | Interval[Any] | complex | integer[Any] | floating[Any] | complexfloating[Any, Any]")  [assignment]
+ pandas/core/dtypes/cast.py:161: error: Incompatible types in assignment (expression has type "Timedelta | NaTType", variable has type "str | bytes | date | datetime | timedelta | <7 more items> | Interval[Any] | complex | integer[Any] | floating[Any] | complexfloating[Any, Any]")  [assignment]
+ pandas/core/arrays/_ranges.py:147: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Timestamp | None")  [assignment]
+ pandas/core/arrays/datetimes.py:832: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "unit"  [union-attr]
+ pandas/core/arrays/datetimes.py:3177: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Timestamp | None")  [assignment]
+ pandas/core/arrays/datetimes.py:3177: note: Error code "assignment" not covered by "type: ignore[arg-type]" comment
- pandas/core/arrays/datetimes.py:3178: error: Non-overlapping identity check (left operand type: "Timestamp", right operand type: "NaTType")  [comparison-overlap]
+ pandas/core/arrays/datetimes.py:3178: error: Non-overlapping identity check (left operand type: "Timestamp | None", right operand type: "NaTType")  [comparison-overlap]
+ pandas/core/arrays/datetimes.py:3179: error: Item "None" of "Timestamp | None" has no attribute "as_unit"  [union-attr]
+ pandas/core/arrays/datetimes.py:3185: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Timestamp | None")  [assignment]
+ pandas/core/arrays/datetimes.py:3185: note: Error code "assignment" not covered by "type: ignore[arg-type]" comment
- pandas/core/arrays/datetimes.py:3186: error: Non-overlapping identity check (left operand type: "Timestamp", right operand type: "NaTType")  [comparison-overlap]
+ pandas/core/arrays/datetimes.py:3186: error: Non-overlapping identity check (left operand type: "Timestamp | None", right operand type: "NaTType")  [comparison-overlap]
+ pandas/core/arrays/datetimes.py:3187: error: Item "None" of "Timestamp | None" has no attribute "as_unit"  [union-attr]
+ pandas/core/arrays/period.py:385: error: Incompatible return value type (got "Period | NaTType", expected "Period")  [return-value]
+ pandas/core/arrays/period.py:1227: error: Argument 1 to "_add_timedelta_arraylike" of "PeriodArray" has incompatible type "ndarray[tuple[Any, ...], dtype[generic[date | int | timedelta | None]]]"; expected "TimedeltaArray | ndarray[tuple[Any, ...], dtype[timedelta64[timedelta | int | None]]]"  [arg-type]
+ pandas/core/arrays/period.py:1342: error: Argument 1 to "delta_to_tick" has incompatible type "Timedelta | NaTType"; expected "timedelta"  [arg-type]
+ pandas/core/window/rolling.py:2023: error: Incompatible types in assignment (expression has type "int | signedinteger[_64Bit]", variable has type "int | None")  [assignment]
+ pandas/core/tools/datetimes.py:633: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_julian_date"  [union-attr]
+ pandas/core/indexes/timedeltas.py:324: error: Argument 1 to "to_offset" has incompatible type "Timedelta | NaTType"; expected "timedelta | str | PeriodDtypeBase"  [arg-type]
+ pandas/core/indexes/timedeltas.py:324: note: Error code "arg-type" not covered by "type: ignore[operator]" comment
+ pandas/core/indexes/period.py:673: error: Incompatible return value type (got "Period | NaTType", expected "Period")  [return-value]
+ pandas/core/indexes/period.py:700: error: Item "NaTType" of "Period | NaTType" has no attribute "asfreq"  [union-attr]
+ pandas/core/indexes/datetimes.py:943: error: Unsupported left operand type for % ("NaTType")  [operator]
+ pandas/core/indexes/datetimes.py:943: note: Left operand is of type "Timedelta | NaTType"
+ pandas/core/indexes/datetimes.py:1120: error: Item "NaTType" of "Period | NaTType" has no attribute "start_time"  [union-attr]
+ pandas/core/indexes/datetimes.py:1123: error: Unsupported operand types for + ("NaTType" and "int")  [operator]
+ pandas/core/indexes/datetimes.py:1123: note: Left operand is of type "Period | NaTType"
+ pandas/core/indexes/datetimes.py:1148: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "datetime")  [assignment]
+ pandas/core/indexes/datetimes.py:1154: error: "datetime" has no attribute "tz_localize"  [attr-defined]
+ pandas/core/indexes/datetimes.py:1156: error: Incompatible return value type (got "tuple[datetime, Resolution]", expected "tuple[Timestamp, Resolution]")  [return-value]
+ pandas/core/indexes/datetimes.py:1735: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "unit"  [union-attr]
+ pandas/core/indexes/datetimes.py:1736: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "unit"  [union-attr]
+ pandas/core/indexes/datetimelike.py:1107: error: Argument 1 to "to_offset" has incompatible type "Timedelta | NaTType"; expected "timedelta | str | PeriodDtypeBase"  [arg-type]
+ pandas/core/resample.py:2524: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Literal['epoch', 'start', 'start_day', 'end', 'end_day'] | Timestamp")  [assignment]
+ pandas/core/resample.py:2648: error: Argument "offset" to "_get_timestamp_range_edges" has incompatible type "Timedelta | NaTType | None"; expected "Timedelta | None"  [arg-type]
+ pandas/core/resample.py:2840: error: Argument "offset" to "_get_period_range_edges" has incompatible type "Timedelta | NaTType | None"; expected "Timedelta | None"  [arg-type]
+ pandas/core/resample.py:2959: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Literal['epoch', 'start', 'start_day', 'end', 'end_day'] | Timestamp")  [assignment]
+ pandas/core/resample.py:2975: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Timestamp")  [assignment]
+ pandas/core/resample.py:2977: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Timestamp")  [assignment]
+ pandas/core/resample.py:2979: error: Incompatible types in assignment (expression has type "Timestamp | NaTType", variable has type "Timestamp")  [assignment]
+ pandas/core/resample.py:3091: error: No overload variant of "__mul__" of "signedinteger" matches argument type "Tick"  [operator]
+ pandas/core/resample.py:3091: note: Possible overload variants:
+ pandas/core/resample.py:3091: note:     def __mul__(self, int | signedinteger[_8Bit] | numpy.bool[builtins.bool] | signedinteger[_64Bit], /) -> signedinteger[_64Bit]
+ pandas/core/resample.py:3091: note:     def __mul__(self, float, /) -> float64
+ pandas/core/resample.py:3091: note:     def __mul__(self, complex, /) -> complex128
+ pandas/core/resample.py:3091: note:     def __mul__(self, signedinteger[Any], /) -> signedinteger[Any]
+ pandas/core/resample.py:3091: note:     def __mul__(self, integer[Any], /) -> Any
+ pandas/core/resample.py:3091: note: Left operand is of type "int | signedinteger[_64Bit]"
+ pandas/core/resample.py:3139: error: Incompatible return value type (got "tuple[Timestamp | NaTType, Timestamp | NaTType]", expected "tuple[Timestamp, Timestamp]")  [return-value]
+ pandas/io/excel/_odfreader.py:220: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "time"  [union-attr]
+ pandas/tseries/holiday.py:262: error: Incompatible types in assignment (expression has type "Timestamp | NaTType | None", variable has type "Timestamp | None")  [assignment]
+ pandas/tseries/holiday.py:264: error: Incompatible types in assignment (expression has type "Timestamp | NaTType | None", variable has type "Timestamp | None")  [assignment]
+ pandas/tests/tslibs/test_timedeltas.py:24: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:277: error: Unsupported operand types for - ("NaTType" and "Nano")  [operator]
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:277: note: Left operand is of type "Timestamp | NaTType"
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:278: error: Unsupported operand types for - ("NaTType" and "Nano")  [operator]
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:278: note: Left operand is of type "Timestamp | NaTType"
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:288: error: Unsupported operand types for - ("NaTType" and "Nano")  [operator]
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:288: note: Left operand is of type "Timestamp | NaTType"
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:289: error: Unsupported operand types for - ("NaTType" and "Nano")  [operator]
+ pandas/tests/tseries/offsets/test_custom_business_hour.py:289: note: Left operand is of type "Timestamp | NaTType"
+ pandas/tests/tseries/offsets/test_business_day.py:78: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/tseries/offsets/test_business_day.py:79: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/series/indexing/test_setitem.py:512: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/series/indexing/test_setitem.py:513: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/scalar/test_nat.py:557: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/scalar/test_nat.py:558: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/scalar/timedelta/test_timedelta.py:266: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "max"  [union-attr]
+ pandas/tests/scalar/interval/test_arithmetic.py:116: error: Value of type variable "_OrderableT" of "Interval" cannot be "Timestamp | NaTType"  [type-var]
+ pandas/tests/scalar/interval/test_arithmetic.py:119: error: Value of type variable "_OrderableT" of "Interval" cannot be "Timedelta | NaTType"  [type-var]
+ pandas/tests/indexes/test_engines.py:62: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/indexes/test_engines.py:63: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:72: error: List item 0 has incompatible type "list[Timestamp | NaTType]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:79: error: List item 1 has incompatible type "list[Timestamp | NaTType]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:86: error: List item 2 has incompatible type "list[Timedelta | NaTType]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:93: error: List item 3 has incompatible type "list[Period | Any]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:94: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:95: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:96: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:97: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:98: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:202: error: List item 0 has incompatible type "list[float | Timestamp | NaTType]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:212: error: List item 1 has incompatible type "list[float | Timestamp | NaTType]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:222: error: List item 2 has incompatible type "list[float | Timedelta | NaTType]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:232: error: List item 3 has incompatible type "list[float | Period | Any]"; expected "ndarray[tuple[Any, ...], dtype[Any]]"  [list-item]
+ pandas/tests/groupby/methods/test_rank.py:233: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:234: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:236: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:237: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/groupby/methods/test_rank.py:238: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/arrays/test_timedeltas.py:215: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/arrays/test_timedeltas.py:216: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/arrays/test_timedeltas.py:238: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/arrays/test_datetimes.py:669: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "to_period"  [union-attr]
+ pandas/tests/arrays/test_datetimelike.py:187: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "time"  [union-attr]
+ pandas/tests/arithmetic/conftest.py:70: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/arithmetic/conftest.py:72: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/arithmetic/conftest.py:87: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/arithmetic/conftest.py:106: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/arithmetic/conftest.py:131: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/plotting/_matplotlib/converter.py:153: error: Item "NaTType" of "Timestamp | NaTType" has no attribute "time"  [union-attr]
+ pandas/plotting/_matplotlib/converter.py:309: error: Item "NaTType" of "Period | NaTType" has no attribute "ordinal"  [union-attr]
+ pandas/conftest.py:1575: error: Incompatible return value type (got "Timestamp | NaTType", expected "Timestamp")  [return-value]
+ pandas/tests/arithmetic/test_timedelta64.py:87: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/arithmetic/test_period.py:42: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/arithmetic/test_period.py:59: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]
+ pandas/tests/arithmetic/test_numeric.py:227: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_timedelta64"  [union-attr]
+ pandas/tests/arithmetic/test_numeric.py:228: error: Item "NaTType" of "Timedelta | NaTType" has no attribute "to_pytimedelta"  [union-attr]

... (truncated 8 lines) ...

jax (https://github.com/google/jax)
- jax/experimental/pallas/ops/tpu/splash_attention/splash_attention_mask_info.py:729: error: Incompatible types in assignment (expression has type "ndarray[tuple[Any, ...], dtype[numpy.bool[builtins.bool]]]", variable has type "list[ndarray[tuple[Any, ...], dtype[Any]]] | None")  [assignment]
+ jax/experimental/pallas/ops/tpu/splash_attention/splash_attention_mask_info.py:729: error: Incompatible types in assignment (expression has type "ndarray[tuple[Any, ...], dtype[bool[Any]]]", variable has type "list[ndarray[tuple[Any, ...], dtype[Any]]] | None")  [assignment]

starlette (https://github.com/encode/starlette)
+ starlette/testclient.py:129: error: Incompatible types in assignment (expression has type "tuple[MemoryObjectSendStream[MutableMapping[str, Any]], MemoryObjectReceiveStream[MutableMapping[str, Any]]]", variable has type "create_memory_object_stream[MutableMapping[str, Any]]")  [assignment]
+ starlette/testclient.py:131: error: Incompatible types in assignment (expression has type "tuple[MemoryObjectSendStream[MutableMapping[str, Any]], MemoryObjectReceiveStream[MutableMapping[str, Any]]]", variable has type "create_memory_object_stream[MutableMapping[str, Any]]")  [assignment]
+ starlette/testclient.py:678: error: Incompatible types in assignment (expression has type "tuple[MemoryObjectSendStream[MutableMapping[str, Any] | None], MemoryObjectReceiveStream[MutableMapping[str, Any] | None]]", variable has type "create_memory_object_stream[MutableMapping[str, Any] | None]")  [assignment]
+ starlette/testclient.py:680: error: Incompatible types in assignment (expression has type "tuple[MemoryObjectSendStream[MutableMapping[str, Any]], MemoryObjectReceiveStream[MutableMapping[str, Any]]]", variable has type "create_memory_object_stream[MutableMapping[str, Any]]")  [assignment]
+ starlette/middleware/base.py:189: error: Incompatible types in assignment (expression has type "tuple[MemoryObjectSendStream[MutableMapping[str, Any]], MemoryObjectReceiveStream[MutableMapping[str, Any]]]", variable has type "create_memory_object_stream[MutableMapping[str, Any]]")  [assignment]

ilevkivskyi · 2026-05-08T22:02:19Z

OK, mypy_primer looks good now. @hauntsaninja @JukkaL this is ready for review now. You may also want to say what do you think about this comment #21441 (comment)

hauntsaninja · 2026-05-08T22:27:08Z

I mostly agree with bzoracler:

if there is an explicit return Any, ideally we should reveal Any
the biggest reason for back compat is code that predates Self, so the heuristic for return typing matching current class makes sense to me

Will look at the diff in a little bit!

ilevkivskyi added 7 commits May 5, 2026 23:51

Preparatory refactor

b5ea90c

Add basic logic

a43ecc1

Add boilerplate

c930a95

Do not use ret_type in various special cases

8c3e017

Store precise type

b01ffea

Some more tweaks

b76c746

Update docstring

73bb465

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ilevkivskyi requested review from JukkaL and hauntsaninja May 8, 2026 14:44

Fix one more casual ret_type

4434f73

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Do not leak type variables in type[T]

b921130

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Respect explicit return type of new()#21441

Respect explicit return type of new()#21441
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