Generator optional parameters

[YodaEmbedding]

Can the last two parameters of Generator[YieldType, SendType, ReturnType] be made optional? Usually, one uses generators without them:

def infinite_stream(start: int) -> Generator[int, None, None]:
    while True:
        yield start
        start += 1

Iterator[YieldType] also works, of course, but Generator[YieldType] is more specific.

See also: python/mypy#4221

[gvanrossum]

PEP 484 currently doesn't support optional arguments to generics (note that Tuple[int, str] is a special form, not just a generic type). But you can easily define a type alias, e.g.

from typing import *

T = TypeVar("T")

SimpleGenerator = Generator[T, None, None]

def foo() -> SimpleGenerator[int]:
    yield 0
    yield 1.1

This gives an error on yield 1.1:

t.py:9: error: Incompatible types in "yield" (actual type "float", expected type "int")

Using various reveal_type() calls I can also confirm that the type of foo is correct:

t.py:11: note: Revealed type is 'def () -> typing.Generator[builtins.int, None, None]'
t.py:12: note: Revealed type is 'typing.Generator[builtins.int, None, None]'
t.py:13: note: Revealed type is 'builtins.int*'

[JelleZijlstra]

This is a duplicate of #307. (Generator is probably among the few standard ABCs for which default type parameters would make sense.)

[srittau]

@JelleZijlstra These issues aren't duplicates. #307 is about TypeVar(T, default=...), this issue about e.g. Generator[int].

[JelleZijlstra]

But you could use #307 to fix this issue. I guess they're not exact duplicates though.

[srittau]

But it's a duplicate of https://bugs.python.org/issue31700. :) Closing again.