In this example the assert is less explanatory than a comment. Why do

    if (condition):
        ...
    else:
        assert(!condition)
        ...

When you could instead if (condition): ... else: #if (condition) ...

Or otherwise explicitly comment the condition that was checked earlier. At least then if the comment is unchanged, whoever is touching your code will assume the comment is outdated, rather than assuming that you're asserting that x should be some undocumented magic number.