Constraints on __sub__, __eq__, etc.

Chris Rebert clp2 at rebertia.com
Fri Feb 19 11:39:59 CET 2010


On Fri, Feb 19, 2010 at 2:30 AM, Roald de Vries <rdv at roalddevries.nl> wrote:
> On Feb 18, 2010, at 5:28 PM, Stephen Hansen wrote:
>> On Thu, Feb 18, 2010 at 8:19 AM, Andrey Fedorov
>> <anfedorov at 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.
>
> Still, it's clear that (for example) '==' is not just a normal function
> call. Look at this example (in ipython):
>
>>>> False == False == False
> True
>>>> True == False == False
> False
>>>> (True == False) == False
> True
>
> Anybody knows how why this is so?

Python is smart enough to recognize chained comparisons and do The
Right Thing (tm).
`X == Y == Z` is equivalent to `X == Y and Y == Z`. Same goes for the
other comparison operators besides == and also possibly for longer
chains.

Cheers,
Chris
--
http://blog.rebertia.com



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