Constraints on __sub__, __eq__, etc.
__peter__ at web.de
Fri Feb 19 12:30:15 CET 2010
Roald de Vries 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 == False == False
> >>> (True == False) == False
> Anybody knows how why this is so?
As Chris said
expr1 <op1> expr2 <op2> expr3 <op3> ...
is resolved as
(expr1 <op1> expr2) and (expr2 <op2> expr3) and (expr3 <op3> ...
where each exprN is evaluated just once.
For this to become the obvious way you have to look at interval checks like
a < b < c
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