a.py

from typing import Optional 
 
a: Optional[str] = None 
def init(): 
    global a 
    a = 'abc' 
def g() -> str:  # supposed to be called after calling init() 
    return a 
# $ mypy a.py 
# a.py:8: error: Incompatible return value type (got "Optional[str]", expected "str")