Typing narrowing fails depending on the order of if clauses
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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
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- 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