Bad type inference with a partial None type and a for loop. #8637
Description
I believe there's a bug in mypy (from master) which makes it not detect that a variabe used in a loop may be uninitialised (or use the value assigned before the loop) unless you are iterating over a literal []/().
This code:
def test() -> str:
c = None
for c in "":
pass
if c is None:
return None
return "foo"
print(test())
Prints:
$ python3 opt.py
None
But mypy says:
$ mypy opt.py
Success: no issues found in 1 source file
This is with mypy just updated from git and Python 3.7.4 but also happens with mypy 0.720 and Python 2.7:
$ mypy --version
mypy 0.770+dev.d54fc8e78a8dd5be3c97c1aa0da357e313113dec
If you replace "" with a [] or () I get:
$ mypy opt.py
opt.py:10: error: Incompatible return value type (got "None", expected "str")
Found 1 error in 1 file (checked 1 source file)
It's also interesting that, if the loop iterates over a variable, then nothing potentially wrong is detected:
import typing
def test(l : typing.Sequence[str]) -> str:
c = None
for c in l:
pass
if c is None:
return None
return "foo"
print(test([]))
print(test(""))
print(test(["hello", "world"]))