Mypy does not recognize missing method

[cburgdorf]

In the following example, mypy does not catch that self.meh() does not exist. Is this expected behavior because of the static False?

class TestCase:

    def foo(self) -> bool:
        return False and self.meh()

The original bug in our code base was harder to spot than this but it boils down to the same reason so I'm wondering if this is expected behavior.

If I change the example to

class TestCase:

    def foo(self) -> bool:
        return True and self.meh()

then mypy does in fact spot the missing meh method.

mypy_bug.py:5: error: "TestCase" has no attribute "meh"

[JelleZijlstra]

Mypy doesn't type check code it thinks is unreachable. I'm personally not convinced that's the right behavior, because it can lead to hard-to-debug issues, but that's probably not something we can easily change.

[cburgdorf]

Hey @JelleZijlstra thank you for your prompt reply!

I'm personally not convinced that's the right behavior, because it can lead to hard-to-debug issues

Yep, that's really unfortunate. In real code, these things can be far more subtle. The root cause in our case was a missing @property decorator as seen below.

class TestCase():
 
    @property
    def is_running(self) -> bool:
        return not self.is_stopped and self.meh()

    #@property   # <-- if we forget the @property, the missing `meh` goes unnoticed
    def is_stopped(self) -> bool:
        return False

If you add the property decorator, mypy will rightfully catch the missing meh() method but if you forget the property, the missing method just goes unnoticed.

[ilevkivskyi]

One of the possible ways forward is to emit an error/warning when mypy skips some branch because it is unreachable, see #2395

[ilevkivskyi]

Now that we have a dedicated issue #7008, I think we can close this as a duplicate.