python / python/mypy

Typing narrowing fails depending on the order of if clauses

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bug
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Python
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Description

Bug Report

Type narrowing fails in the code below.

To Reproduce

def takes_one_str(string: str) -> str:
    return string


# just the problematic case -> failing
def takes_two_optional_str0a(string: str | None, whatever: bool) -> str:
    if not (whatever and (string is None)) and whatever:
        return takes_one_str(string)
    return ""


# reformulated by reversing the order of if clauses -> working
def takes_two_optional_str0b(string: str | None, whatever: bool) -> str:
    if whatever and not (whatever and (string is None)):
        return takes_one_str(string)
    return ""


# what was I actually trying to achieve -> failing
def takes_two_optional_str1(string_a: str | None, string_b: str | None) -> str:
    if (string_a is None) and (string_b is None):
        return takes_one_str("")
    if string_a is None:
        # false-positive arg-type (same without return, with elif)
        # string_b cannot be None
        return takes_one_str(string_b)
    if string_b is None:
        return takes_one_str(string_a)
    return takes_one_str(string_a + string_b)


# workaround variant -> working
def takes_two_optional_str2(string_a: str | None, string_b: str | None) -> str:
    if (string_a is not None) and (string_b is not None):
        return takes_one_str(string_a + string_b)
    if string_a is not None:
        # false-positive arg-type (same without return, with elif)
        return takes_one_str(string_a)
    if string_b is not None:
        return takes_one_str(string_b)
    return takes_one_str("")

https://mypy-play.net/?mypy=latest&python=3.11&gist=3f3ce6b3021c1a71a4ecf31d35f62a98

Expected Behavior

No errors

Actual Behavior

bug.py:8: error: Argument 1 to "takes_one_str" has incompatible type "str | None"; expected "str" [arg-type]
bug.py:26: error: Argument 1 to "takes_one_str" has incompatible type "str | None"; expected "str" [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: mypy 1.4.1 (compiled: yes)
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.11.4

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the reproduction in bug.py with mypy 1.4.1 or the linked mypy-play example, comparing the failing and working clause orders. Trace the type narrowing for string, string_a, and string_b through the if statements. Done means the reported arg-type errors are eliminated without changing the intended behavior.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
42/100

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