Mypy doesnt do type inference in "walrus" expressions (i.e. := )

[samuelbryant]

I'm running python3.8 and mypy 0.750 and I realized that mypy doesn't do automatic type inference in if expressions involving the walrus operator the way it does with ordinary expressions. Specifically, where mypy would infer that a value is not None after passing an if value is not None expression, the corresponding walrus expression does not work

Example of valid mypy code without the walrus operator:

def method(arg: int):
    pass
int_map: Dict[str, int] = {}    
value = int_map.get(key, None)
if value is not None:
    method(value) # okay

With the walrus operator, the following code should not produce a mypy error:

def method(arg: int):
    pass
int_map: Dict[str, int] = {}    
if (value := int_map.get(key, None)) is not None:
    method(value) # triggers "... has incompatible type Optional[int]"; expected "int"

but it does

Apologies if this is a duplicate

[samuelbryant]

if someone gives me a hint for where in the source code something like this should be implemented, I'd be happy to try and write the patch myself

[JelleZijlstra]

This is a duplicate of #7316, so I'll close this one. The fix should be in one of the places I touched in #6899, but I'm not sure exactly where.