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    we (pytype) are currently working on increasing strictness for things like wrong type assignment or reassignment to explicitly annotated variables, and default function parameters of None without a corresponding Optional type (e.g. f(x: int = None)). so we will still accept the following:

    x = [1]
    x.append('2')
    

    but this will be an error:

    x: List[int] = [1]
    x.append('2')
    

    since these changes are potentially very disruptive (because a lot of code that typechecked earlier suddenly does not), we have been gating the new behaviour behind temporary flags; the git history of config.py is some indication of just how hard it is to turn on a stricter check when users have been relying on the lax behaviour.