Closed
Description
Tried to write type stubs for lazy package. Lazy provides a descriptor that invokes the underlying method only once and caches the result in the instance.
The inferred type is <nothing>, so mypy crashes when checking expressions.
from typing import Any, Generic, Callable, TypeVar
T = TypeVar('T')
class lazy:
def __init__(self, func: Callable[[Any], T]) -> None: ...
def __get__(self, inst: Any, inst_cls: Any) -> T: ...
class Haystack:
@lazy
def hasNeedles(self) -> bool:
# some expensive lookup we want to execute only once
return True
reveal_type(Haystack().hasNeedles)
# checking this expression crashes
x = Haystack().hasNeedles or True<...>
note: Revealed type is '<nothing>'
<...>
version: 0.761
Traceback (most recent call last):
File "mypy/checkexpr.py", line 3645, in accept
File "mypy/nodes.py", line 1736, in accept
File "mypy/checkexpr.py", line 2029, in visit_op_expr
File "mypy/checkexpr.py", line 2662, in check_boolean_op
If lazy was defined as a function, type gets inferred correctly.
def lazy(func: Callable[[Any], T]) -> T: ...note: Revealed type is 'builtins.bool*'