`Callable` used as condition silently disables type checks

[selimb]

Bug Report

If the condition of an if block is a Callable (or not Callable), the body of the else (or if) is not analyzed by mypy.

To Reproduce

Run mypy on:

def func() -> bool:
    return True

if not func:
    a: str = 4
    reveal_type(a)
else:
    b: str = 4
    reveal_type(b)

if func:
    c: str = 4
    reveal_type(c)
else:
    d: str = 4
    reveal_type(d)

Expected Behavior

Either:

test.py:5: error: Incompatible types in assignment (expression has type "int", variable has type "str")
test.py:6: note: Revealed type is "builtins.str"
test.py:8: error: Incompatible types in assignment (expression has type "int", variable has type "str")
test.py:9: note: Revealed type is "builtins.str"
test.py:12: error: Incompatible types in assignment (expression has type "int", variable has type "str")
test.py:13: note: Revealed type is "builtins.str"
test.py:15: error: Incompatible types in assignment (expression has type "int", variable has type "str")
test.py:16: note: Revealed type is "builtins.str"

or maybe statement-unreachable on lines 5 and 15?

Actual Behavior

test.py:8: error: Incompatible types in assignment (expression has type "int", variable has type "str")
test.py:9: note: Revealed type is "builtins.str"
test.py:12: error: Incompatible types in assignment (expression has type "int", variable has type "str")
test.py:13: note: Revealed type is "builtins.str"

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: Just mypy test.py.
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.3
• Operating system and version: Windows 10 Pro 21H1

[sobolevn]

Hi, @selimb! Thanks a lot for raising this!

Forgetting to actually call a callable is a common mistake. In the next version it will be supporting truthy-bool error code, see #10666

For now, you can use --warn-unreachable that kinda helps in this case as well:

[selimb]

Thanks @sobolevn for the quick response!

Ah, I overlooked --warn-unreachable. I also see I'm far from the first one to submit a bug for this; looks like I wasn't using the right keywords XD. Apologies.

Is there any reason not to set --warn-unreachable with --strict?
If so, will the upcoming truthy-bool error be on by default or enabled with --strict?

[ilevkivskyi]

truthy-function is now on by default so the original example is handled safely.