False positive with warn-unreachable and no-strict-optional (canonical)

[snmishra]

Bug Report

Tests with re.match or re.search give incorrect unreachable error when using mypy 1.8.0

To Reproduce

import re

test_str = "XYZ"

if re.match("ABC", test_str):
    print("match")
else:
    print("no match")

Expected Behavior

No mypy errors

Actual Behavior

> mypy test.py
test.py:10: error: Statement is unreachable  [unreachable]
        print("no match")
        ^~~~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):

python_version = "3.12"
check_untyped_defs = true
# We don't have stubs for all libraries
ignore_missing_imports = true
# We need some ignores for Pylance
follow_imports = "normal"
ignore_errors = false
show_error_codes = true
pretty = true
# Strictness
allow_redefinition = false
implicit_reexport = true
local_partial_types = true
no_implicit_optional = true
strict_equality = true
strict_optional = false
# Warnings
warn_no_return = true
warn_redundant_casts = true
warn_unreachable = true
warn_unused_configs = true
warn_unused_ignores = true

• Python version used: 3.12

[hauntsaninja]

This is expected given that you're using strict_optional = false. Don't use it, it is evil

[snmishra]

Thanks! strict_optional = False indeed is evil and doing a lot more than I had expected from the name. The documentation probably should have a fat warning that this is evil.

[HansAarneLiblik]

I don't think this should be closed. What change is the reason behind this new failure?

For us the failing code is strictly typed

def func(inp: str) -> str | None:
  if re.match(MY_REGEX, inp):
    return inp
    
  return None  # This errors as "unreachable"

We have a pretty big codebase, where A LOT of code is still using strict_optional = false, as the migration to strict_optional=true takes time.

[hauntsaninja]

Oh you didn't mention this was a regression. Looks like the relevant change is #16566, so mypy correctly sees that Match is final and cannot have a falsey subclass.

I don't think there's any change to be made here. strict_optional = false tells mypy to not think about None, so mypy will not think about None.

If this is a problem for your existing codebase, I recommend using strict_optional = false only in the modules that need it, and disabling warn_unreachable = true in those modules. This can be accomplished in your mypy config file or by using comments at the top of those files.

[HansAarneLiblik]

Match may be final, but re.match returns Match[str] | None and we're relying on the | None part

We're already using strict_optional=false ONLY on manually defined list of pre-existing files (migrating to true is in the works, but as I said this takes time)

We can ignore this error on the impacted functions, but IMO this is a regression not a bug in our "outdated" codebase