assert statement gives the possibility to display the text which goes along with the AssertionError exception. Most of the times though, what would be more appropriate is to raise a different exception (e.g. ValueError). My proposal is to be able to specify an exception as a replacement for AssertionError as in:

>>> assert callable(fun), ValueError("object is not a callable")

ValueError: object is not a callable

Specifically, this would be useful at the top of a function or method, where argument types or values are usually checked:

def retry(times=3, timeout=0.1, callback=None):

    assert times >= 1, ValueError("times must be >= 1")

    assert isinstance(timeout, (int, float)), ValueError("invalid timeout")

    assert callable(callback), ValueError("callback is not a callable")

...as opposed to:

def retry(times=3, timeout=0.1, callback=None):

    if not times >= 1:

        raise ValueError("times must be >= 1")

    if not isinstance(timeout, (int, float)):

        raise ValueError("invalid timeout")

    if not callable(callback):

        raise ValueError("callback is not a callable")

Other than saving 1 line for each type/value check, this has the advantage that the assertion logic (e.g. "times >= 1") is shown in the traceback message itself, because it's on the same line, enriching the context and giving more information in case of error.

Thoughts?

--

Giampaolo - http://grodola.blogspot.com

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