[Python-ideas] Specify an alternative exception for "assert"

[Giampaolo Rodola']

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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