'Missing return statement' error occurring in the match statement in Python 3.10. #14704

Supertonian · 2023-02-15T07:59:50Z

Bug Report

(A clear and concise description of what the bug is.)

To Reproduce

# Ideally, a small sample program that demonstrates the problem.
# Or even better, a reproducible playground link https://mypy-play.net/ (use the "Gist" button)

def to_bytes(value: Any) -> bytes:
    match value:
        case s if isinstance(s, str):
            return s.encode("utf-8")
        case b if isinstance(b, bytes):
            return b
        case _:
            raise ValueError('value cannot be expressed in bytes')

Expected Behavior

It should not raise error 'Missing return statement'

Actual Behavior

Raises error 'Missing return statement'.

Even changing raise inside case_ to return b"" still produces 'Missing return' error.

Your Environment

Mypy version used: 1.0.0
Mypy command-line flags: mypy
Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
disallow_any_generics = true
disallow_untyped_defs = true
exclude = ['pb2\.py$', 'pb2_grpc\.py$']
files = ["src", "tests"]
follow_imports = "silent"
ignore_missing_imports = true
python_version = "3.10"

Python version used: 3.10.9

The text was updated successfully, but these errors were encountered:

henryiii · 2023-02-22T17:06:29Z

It's unhappy with the guard:

def f(x: object) -> bool:
    match x:
        case {"a": a} if a:
            return True
        case _:
            return False

$ mypy --strict tmp.py
tmp.py:1: error: Missing return statement  [return]
Found 1 error in 1 file (checked 1 source file)

Mypy works if you remove the if a, but claims there is a missing return statement when that if a is present. Guards just allow fall through, so the final case does always trigger if the one above doesn't, there's no missing return statement.

Supertonian · 2023-02-27T10:12:26Z

Is there a way to perform type checking using the match statement, without resorting to using the isinstance function inside the case statement? @henryiii

henryiii · 2023-02-27T14:22:31Z

Not sure what that has to do with the missing return statement bug, but yes - match MyClass() (notice the parentheses) replaces isinstance. That was the motivating reason to introduce pattern matching, as it could replace most isinstance uses. (I guess knowing this does help avoid this bug if you are using isinstance in a guard, which shouldn't be necessary?)

ilmari · 2023-07-11T14:02:23Z

This looks like a duplicate of #12534

henryiii · 2023-07-11T14:42:29Z

For completeness, here is a working version (and better version, match statements are supposed to avoid isinstance) of the code at the top that avoids #12534:

def to_bytes(value: Any) -> bytes:
    match value:
        case str(s):
            return s.encode("utf-8")
        case bytes(b):
            return b
        case _:
            raise ValueError('value cannot be expressed in bytes')

Supertonian added the bug mypy got something wrong label Feb 15, 2023

hauntsaninja added topic-match-statement Python 3.10's match statement topic-reachability Detecting unreachable code labels Feb 15, 2023

JelleZijlstra closed this as not planned Won't fix, can't repro, duplicate, stale Jul 11, 2023

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'Missing return statement' error occurring in the match statement in Python 3.10. #14704

'Missing return statement' error occurring in the match statement in Python 3.10. #14704

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'Missing return statement' error occurring in the match statement in Python 3.10. #14704

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