Then I absolutely understand you :)

How common it is 100 % depends on the code base and what practices are preferred. In Python code bases where I have a word in decisions, all Boolean checks should be x is True or x is False if x should be a Boolean. In that sense, if I read if x or if not x, it’s an indicator that x does not need to be a Boolean.

In that sense, I could say that my preference is to flip it (in Python): Explicitly indicate/check for a Boolean if you expect/need a Boolean, otherwise use a “truethiness” check.