[Python] How to tackle "Statement is unreachable [unreachable]" with mypy when setting...

Problem description


Suppose a following test

class Foo:

def __init__(self):
self.value: int | None = None

def set_value(self, value: int | None):
self.value = value


def test_foo():

foo = Foo()
assert foo.value is None
foo.set_value(1)
assert isinstance(foo.value, int)
assert foo.value == 1 # unreachable


The test:

• First, checks that foo.value is something
• Then, sets the value using a method.
• Then it checks that the foo.value has changed.

When running the test with mypy version 1.9.0 (latest at the time of writing), and having warn_unreachable set to True, one gets:

(venv) niko@niko-ubuntu-home:~/code/myproj$ python -m mypy tests/test_foo.py
tests/test_foo.py:16: error: Statement is unreachable [unreachable]
Found 1 error in 1 file (checked 1 source file)

What I have found

• There is an open issue in the mypy GitHub: https://github.com/python/mypy/issues/11969 One comment said to use safe-assert, but after rewriting the test as

from safe_assert import safe_assert

def test_foo():

foo = Foo()
safe_assert(foo.value is None)
foo.set_value(1)
safe_assert(isinstance(foo.value, int))
assert foo.value == 1


the problem persists (safe-assert 0.4.0)[1]. This time, both mypy and VS Code Pylance think that foo.set_value(1) two lines above is not reachable.

Question


How can I say to mypy that the foo.value has changed to int and that it should continue checking also everything under the assert isinstance(foo.value, int) line?


[1] UPDATE: The safe_assert v. 0.5.0 has fixed the issue

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