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Article:
  Test-Driven Development in Python
Subject:   Why the code failed
Date:   2005-03-30 19:33:11
From:   Gancho
Your code,


def matches(self, date):

    return ((self.year and self.year == date.year or True) and

            (self.month and self.month == date.month or True) and

            (self.day and self.day == date.day or True) and

            (self.weekday and self.weekday == date.weekday() or True))

failed the testMatchesFalse test because == binds more tightly than "or."


>>> c=1

>>> c and c == 2 or True

True

>>> c and c == (2 or True)

False


I agree that the explicit code is cleaner.


I'm just learning Python, and it's interesting that laziness is one of the three virtues in the Perl world yet is decried in Python circles.
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  • Why the code failed
    2005-05-14 17:14:16  cpgray [Reply | View]

    But "c and c == (2 or True)" is no longer of the form "a and b or c" as a simulation of "a ? b : c".

    The reason the code fails is that the simulation isn't exact. If a is True, "a ? b : c" returns b.
    If a is True, "a and b or c" evaluates b. If b evaluates to True, evaluation stops and b is returned. If b evaluates to False (or None or 0 or [], etc.), evaluation continues and c is evaluated and returned. In other words, you'll never return b if b evaluates to False.

    For example "1 == 1 and 2 or 3" returns 2 as expected, but "1 == 1 and 0 or 3" returns 3.