Assigning a variable from either of two optional variables

Bug Report

consider the following example:

from typing import Optional


def test_something(x: Optional[str] = None, y: Optional[str] = None) -> str:
    if not (x or y):
        return "early"

    other: str = x or y  # <<< mypy complains that other should be Optional[str]

    return other

am i not able to assume that other would be of type str given that if neither x nor y is given then the function would return early and the assignment of other would never happen? in other words, at the assignment of other, surely one or both of x or y would be of type str, therefore other would be of type str?

to further the argument, if i modify the example like so:

#!/usr/bin/env python3

from typing import Optional


def test_something(x: Optional[str] = None, y: Optional[str] = None) -> str:
    if not (x or y):
        return "early"

    other: str = x or y

    print(f"x = {x}")
    print(f"y = {y}")
    print(f"other class: {type(other)}")
    print()

    return other

test_something()
test_something(x="hello")
test_something(y="goodbye")
test_something(x="hello", y="goodbye")

and run it i get:

x = hello
y = None
other class: <class 'str'>

x = None
y = goodbye
other class: <class 'str'>

x = hello
y = goodbye
other class: <class 'str'>

To Reproduce

run mypy on the example above

Expected Behavior

mypy would not complain about typing other as str

Actual Behavior

running mypy on the above example yields:

.../test.py: note: In function "test_something":
.../test.py:8:18: error: Incompatible types in assignment (expression has type "Optional[str]", variable has type "str")  [assignment]
        other: str = x or y
                     ^
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

strict = true
pretty = true
show_column_numbers = true
show_error_codes = true
show_error_context = true

• Python version used: 3.7.9
• Operating system and version: MacOSX 10.15.7 Catalina

提供された Python の再現コードを最初の入口として使用し、早期 return の後にある x or y の代入に対して mypy を実行します。この制御フローがどのように解析されるかを追跡します。完了の条件は、他の箇所で報告される Optional の挙動を弱めることなく、例で other を str として受け入れられることです。