8000 two-arg iter() with None sentinel does not remove ‘Optional’ · Issue #3201 · python/typeshed · GitHub
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two-arg iter() with None sentinel does not remove ‘Optional’ #3201

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wbolster-eiq opened this issue Aug 20, 2019 · 2 comments · Fixed by #3291
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two-arg iter() with None sentinel does not remove ‘Optional’ #3201

wbolster-eiq opened this issue Aug 20, 2019 · 2 comments · Fixed by #3291
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stubs: false positive Type checkers report false errors

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@wbolster-eiq
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wbolster-eiq commented Aug 20, 2019 •
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[moved from https://github.com/python/mypy/issues/7371]

The 2-arg form of iter() takes a callable and a sentinel value. Quoting its docstring:

iter(callable, sentinel) -> iterator

[...], the callable is called until it returns the sentinel.

When the sentinel is the literal None, this means that a callable that returns an Optional[T] is transformed into a Iterator[T]. However, mypy does not detect that, and reveal_type on the iter() result claims its Revealed type is 'typing.Iterator[Union[T, None]]', which allows for None values, which is impossible.

Here's a small example:

from typing import Optional

def maybe_int() -> Optional[int]:
    return 123  # or maybe None

for n in iter(maybe_int, None):
    print(n + n)

Even though n is always an int inside the loop (never None), mypy (version 0.720) is not aware of that, and complains:

error: Unsupported operand types for + ("int" and "None")
error: Unsupported operand types for + ("None" and "int")
error: Unsupported left operand type for + ("None")
Both left and right operands are unions

Since the code is perfectly safe, mypy should infer that n is of type int (not Optional[int]) and should not report any errors about adding int and None values.
@wbolster-eiq wbolster-eiq mentioned this issue Aug 20, 2019
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@wbolster-eiq
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wbolster-eiq commented Aug 20, 2019 •
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current definition:

@overload
def iter(__function: Callable[[], _T], __sentinel: _T) -> Iterator[_T]: ...

@srittau
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srittau commented Aug 20, 2019

Changing the overload to use two type vars could work, but I haven't tested it:

@overload
def iter(__function: Callable[[], Union[_T, _U]], __sentinel: _U) -> Iterator[_T]: ...

@srittau srittau added size-small stubs: false positive Type checkers report false errors labels Aug 20, 2019
utkarsh2102 added a commit to utkarsh2102/typeshed that referenced this issue Oct 1, 2019
@utkarsh2102
builtins.pyi: Use two type vars
2e5873b 
Fixes: python#3201
@utkarsh2102 utkarsh2102 mentioned this issue Oct 1, 2019
Merged
utkarsh2102 added a commit to utkarsh2102/typeshed that referenced this issue Oct 1, 2019
@utkarsh2102
builtins.pyi: Use two type vars
82ee154 
Fixes: python#3201
utkarsh2102 added a commit to utkarsh2102/typeshed that referenced this issue Oct 3, 2019
@utkarsh2102
builtins.pyi: Use two type vars
a2ffcbf 
Fixes: python#3201
srittau pushed a commit that referenced this issue Oct 3, 2019
@utkarsh2102 @srittau
builtins.pyi: Use two type vars (#3291)
fa571fb 
Fixes: #3201
@wbolster wbolster mentioned this issue Oct 8, 2019
Merged
srittau pushed a commit that referenced this issue Oct 9, 2019
@wbolster @srittau
Make 2-arg iter() retrun type match passed callable's return type (#3326
824e94a 
)

This is a continuation of #3291, which was the initial fix for #3201.

The 2-arg version of iter() turns a callable into an iterator. The
changes made in #3291 introduce an Any return type for both the
callable's return type and the iterator's type, while in reality the
return type of the function is always the same as the iterator's type.
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builtins.pyi: Use two type vars utkarsh2102/typeshed
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