Improving type support for math.prod (#13572)

stdlib/@tests/test_cases/check_math.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+from decimal import Decimal
+from fractions import Fraction
+from math import prod
+from typing import Any, Literal, Union
+from typing_extensions import assert_type
+
+
+class SupportsMul:
+    def __mul__(self, other: Any) -> SupportsMul:
+        return SupportsMul()
+
+
+class SupportsRMul:
+    def __rmul__(self, other: Any) -> SupportsRMul:
+        return SupportsRMul()
+
+
+class SupportsMulAndRMul:
+    def __mul__(self, other: Any) -> SupportsMulAndRMul:
+        return SupportsMulAndRMul()
+
+    def __rmul__(self, other: Any) -> SupportsMulAndRMul:
+        return SupportsMulAndRMul()
+
+
+literal_list: list[Literal[0, 1]] = [0, 1, 1]
+
+assert_type(prod([2, 4]), int)
+assert_type(prod([3, 5], start=4), int)
+
+assert_type(prod([True, False]), int)
+assert_type(prod([True, False], start=True), int)
+assert_type(prod(literal_list), int)
+
+assert_type(prod([SupportsMul(), SupportsMul()], start=SupportsMul()), SupportsMul)
+assert_type(prod([SupportsMulAndRMul(), SupportsMulAndRMul()]), Union[SupportsMulAndRMul, Literal[1]])
+
+assert_type(prod([5.6, 3.2]), Union[float, Literal[1]])
+assert_type(prod([5.6, 3.2], start=3), Union[float, int])
+
+assert_type(prod([Fraction(7, 2), Fraction(3, 5)]), Union[Fraction, Literal[1]])
+assert_type(prod([Fraction(7, 2), Fraction(3, 5)], start=Fraction(1)), Fraction)
+assert_type(prod([Decimal("3.14"), Decimal("2.71")]), Union[Decimal, Literal[1]])
+assert_type(prod([Decimal("3.14"), Decimal("2.71")], start=Decimal("1.00")), Decimal)
+assert_type(prod([complex(7, 2), complex(3, 5)]), Union[complex, Literal[1]])
+assert_type(prod([complex(7, 2), complex(3, 5)], start=complex(1, 0)), complex)
+
+
+# mypy and pyright infer the types differently for these, so we can't use assert_type
+# Just test that no error is emitted for any of these
+prod([5.6, 3.2])  # mypy: `float`; pyright: `float | Literal[0]`
+prod([2.5, 5.8], start=5)  # mypy: `float`; pyright: `float | int`
+
+# These all fail at runtime
+prod([SupportsMul(), SupportsMul()])  # type: ignore
+prod([SupportsRMul(), SupportsRMul()], start=SupportsRMul())  # type: ignore
+prod([SupportsRMul(), SupportsRMul()])  # type: ignore
+
+# TODO: these pass pyright with the current stubs, but mypy erroneously emits an error:
+# prod([3, Fraction(7, 22), complex(8, 0), 9.83])
+# prod([3, Decimal("0.98")])

stdlib/_typeshed/__init__.pyi

@@ -117,6 +117,12 @@ class SupportsSub(Protocol[_T_contra, _T_co]):
 class SupportsRSub(Protocol[_T_contra, _T_co]):
     def __rsub__(self, x: _T_contra, /) -> _T_co: ...
 
+class SupportsMul(Protocol[_T_contra, _T_co]):
+    def __mul__(self, x: _T_contra, /) -> _T_co: ...
+
+class SupportsRMul(Protocol[_T_contra, _T_co]):
+    def __rmul__(self, x: _T_contra, /) -> _T_co: ...
+
 class SupportsDivMod(Protocol[_T_contra, _T_co]):
     def __divmod__(self, other: _T_contra, /) -> _T_co: ...
 

stdlib/math.pyi

@@ -1,6 +1,7 @@
 import sys
+from _typeshed import SupportsMul, SupportsRMul
 from collections.abc import Iterable
-from typing import Final, Protocol, SupportsFloat, SupportsIndex, TypeVar, overload
+from typing import Any, Final, Literal, Protocol, SupportsFloat, SupportsIndex, TypeVar, overload
 from typing_extensions import TypeAlias
 
 _T = TypeVar("_T")
@@ -99,10 +100,29 @@ elif sys.version_info >= (3, 9):
 
 def perm(n: SupportsIndex, k: SupportsIndex | None = None, /) -> int: ...
 def pow(x: _SupportsFloatOrIndex, y: _SupportsFloatOrIndex, /) -> float: ...
+
+_PositiveInteger: TypeAlias = Literal[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]
+_NegativeInteger: TypeAlias = Literal[-1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -15, -16, -17, -18, -19, -20]
+_LiteralInteger = _PositiveInteger | _NegativeInteger | Literal[0]  # noqa: Y026  # TODO: Use TypeAlias once mypy bugs are fixed
+
+_MultiplicableT1 = TypeVar("_MultiplicableT1", bound=SupportsMul[Any, Any])
+_MultiplicableT2 = TypeVar("_MultiplicableT2", bound=SupportsMul[Any, Any])
+
+class _SupportsProdWithNoDefaultGiven(SupportsMul[Any, Any], SupportsRMul[int, Any], Protocol): ...
+
+_SupportsProdNoDefaultT = TypeVar("_SupportsProdNoDefaultT", bound=_SupportsProdWithNoDefaultGiven)
+
+# This stub is based on the type stub for `builtins.sum`.
+# Like `builtins.sum`, it cannot be precisely represented in a type stub
+# without introducing many false positives.
+# For more details on its limitations and false positives, see #13572.
+# Instead, just like `builtins.sum`, we explicitly handle several useful cases.
 @overload
-def prod(iterable: Iterable[SupportsIndex], /, *, start: SupportsIndex = 1) -> int: ...  # type: ignore[overload-overlap]
+def prod(iterable: Iterable[bool | _LiteralInteger], /, *, start: int = 1) -> int: ...  # type: ignore[overload-overlap]
 @overload
-def prod(iterable: Iterable[_SupportsFloatOrIndex], /, *, start: _SupportsFloatOrIndex = 1) -> float: ...
+def prod(iterable: Iterable[_SupportsProdNoDefaultT], /) -> _SupportsProdNoDefaultT | Literal[1]: ...
+@overload
+def prod(iterable: Iterable[_MultiplicableT1], /, *, start: _MultiplicableT2) -> _MultiplicableT1 | _MultiplicableT2: ...
 def radians(x: _SupportsFloatOrIndex, /) -> float: ...
 def remainder(x: _SupportsFloatOrIndex, y: _SupportsFloatOrIndex, /) -> float: ...
 def sin(x: _SupportsFloatOrIndex, /) -> float: ...