[Python-ideas] add a always turn on "assert"

[语言破碎处]

"assert" is good, but it is used as a guard frequently.
We can make such usage legal by adding a new syntax:
    assert bool_expr, ExceptionType, True

suggest reasons:
1) even "__debug__" turn off, assert is working
    assertion as guard.

2) to avoid boilerplate code
    I write code like this:
        if pred(....) or pred(....):
            raise ValueError('pred(....) or pred(....)')

    Simplifed:
        assert pred(...), ValueError, True
        # the above line will be printed when error.
        # I need not to copy the condition!

3) future: "assert bool_expr, ET, False"
    To aid static tool, like Proof System.
    Better document.
        For complicate algorithm,
        I actually add a invariant outline comment
        above every statement.



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