> > It may be intuitive to you, but its not true, written down anywhere, nor > assumed by the language, and the mathematical meaning of the operators > doesn't matter to Python. Python purposefully does not enforce anything for > these methods.
Right, so neither is anything in PEP-8, but it's still considered "good practice". I'm running across examples like you gave (__sub__ having a side-effect on the left-hand operand) in some code, and am trying to find concrete justification for avoiding it. - Andrey On Thu, Feb 18, 2010 at 11:28 AM, Stephen Hansen <apt.shan...@gmail.com>wrote: > On Thu, Feb 18, 2010 at 8:19 AM, Andrey Fedorov <anfedo...@gmail.com>wrote: > >> It seems intuitive to me that the magic methods for overriding the +, -, >> <, ==, >, etc. operators should have no sideffects on their operands. Also, >> that == should be commutative and transitive, that > and < should be >> transitive, and anti-commutative. >> >> Is this intuition written up in a PEP, or assumed to follow from the >> mathematical meanings? >> > > It may be intuitive to you, but its not true, written down anywhere, nor > assumed by the language, and the mathematical meaning of the operators > doesn't matter to Python. Python purposefully does not enforce anything for > these methods. Consider: > > >>> class Test(object): > ... def __init__(self, v): > ... self.v = v > ... def __add__(self, other): > ... self.v = self.v + other > ... return "Ow!" > ... > >>> t = Test(5) > >>> t + 2 > 'Ow!' > >>> t.v > 7 > > It not only alters an operand, but its not even returning a meaningful > result. This can be abused, but is also useful for certain uses. > > --S >
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