False positive for Optional with if branches

[dedalusj]

Bug report

• Repro case

def foo(a: Dict, b: Optional[str] = None):
    if b is None and 'k' not in a:
        b = '1'
    elif 'k' in a:
        b = '2'
    c: str = b  # Incompatible types error reported here
    print(c)

• What is the actual behavior/output?

Incompatible types error reported on c var assignment

• What is the behavior/output you expect?

No error reported

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

mypy 0.641. The is present in master as well.

• What are the mypy flags you are using? (For example --strict-optional)

None

[gvanrossum]

Looks like you're being too clever here. Mypy doesn't understand that the elif clause rules out b being None. (Honestly I had to do a double take and re-read the code too before I trusted this was correct.)

We don't have plans to add such logic to mypy, so I recommend rewriting your code to be a bit simpler.