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mypy is confused by return values from generators #695
Description
This is fine:
from typing import Iterator
def foo() -> Iterator[int]:
yield 42
x = next(foo()) + 1000
print(x)
But now try this:
from typing import Iterator
def foo() -> Iterator[int]:
yield 42
return 'abc'
This complains about the return statement:
g.py, line 5: Incompatible return value type: expected typing.Iterator[builtins.int], got builtins.str
It's matching the generator return value (which is only accessible by a caller that uses yield from) to the declared return type (which is just the type of what you get by calling foo().
Activity
spkersten commentedon May 21, 2015
I've been fixing this (see spkersten/generator). Type checking return already works in that branch, only
yield fromand some special casing for coroutines need a bit more work.gvanrossum commentedon Jan 13, 2016
@spkersten, do you still have that branch? If not I'm probably going to look into this -- there also seems overlap with #642 and #1011.
gvanrossum commentedon Jan 14, 2016
FWIW, I think that the example using
return 'abc'should be rejected, but the error message should indicate that a generator declared to return an Iterator should not return a value in a return statement. However, if the function return is declared as-> Generator[X, Y, Z]then the return expression should be type-checked against Z, and the value returned by a yield expression should be type-checked against Y. IOW, ue these mnemonics:We could treat Iterator[X] as Generator[X, Y, Z] except I still think that even
return Noneshould be disallowed -- onlyreturnshould be allowed if the function declaration doesn't use Generator (because that's the pre-3.3 way).gvanrossum commentedon Jan 15, 2016
@sakana will address this next. (@jukka: can you add @sakana as a committer? It's David. :-)
ddfisher commentedon Jan 15, 2016
I'm @ddfisher now!
JukkaL commentedon Jan 15, 2016
Added @ddfisher.