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Guard clause prevents match from seeing that all paths return #12534
Description
Bug Report
In a match statement in which all cases include a return statement, mypy 0.942 falsely reports "Missing return statement [return]" if at least one case includes a guard clause.
To Reproduce
def func(e) -> int:
match e:
case x if True:
return x
case _:
return 0
Or even more simply:
def func(e) -> int:
match e:
case _ if True:
return 0
Expected Behavior
This code should type-check.
Actual Behavior
$ mypy --python-version 3.10 --show-error-codes x5.py
x5.py:1: error: Missing return statement [return]
Found 1 error in 1 file (checked 1 source file)
Notes
If we change the first case to something that does not include a guard clause, like this:
def func(e) -> int:
match e:
case (x, _):
return x
case _:
return 0
then mypy returns no errors.
Environment
- Mypy version used: 0.942
- Mypy command-line flags: --python-version 3.10 --show-error-codes
- Mypy configuration options from
mypy.ini(and other config files): (none) - Python version used: 3.10.2
- Operating system and version: Ubuntu 18.04
Activity
Improve match narrowing and reachability analysis
Improve match narrowing and reachability analysis
hauntsaninja commentedon Aug 16, 2023
Opened a PR that should fix this. Are any of the people here encountering this in open source projects? I could use some more match statement coverage in https://github.com/hauntsaninja/mypy_primer
Improve match narrowing and reachability analysis (#15882)
hauntsaninja commentedon Aug 19, 2023
I've merged the fix, but if your project is open source, please let me know!