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Python3 doc, operator reflection

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  Python3 doc, operator reflection Johannes Bauer <dfnsonfsduifb@gmx.de> - 2013-10-28 13:00 +0100
    Re: Python3 doc, operator reflection Chris Angelico <rosuav@gmail.com> - 2013-10-28 23:23 +1100
      Re: Python3 doc, operator reflection Johannes Bauer <dfnsonfsduifb@gmx.de> - 2013-10-28 13:45 +0100
    Re: Python3 doc, operator reflection random832@fastmail.us - 2013-10-28 14:31 -0400

#57812 — Python3 doc, operator reflection

Hi group,

in http://docs.python.org/3/reference/datamodel.html#customization the
doc reads:

> There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other’s reflection, __le__() and __ge__() are each other’s reflection, and __eq__() and __ne__() are their own reflection.

But shouldn't __lt__ be the reflection or __ge__ and __gt__ the
reflection of __le__?

Best regards,
Johannes

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#57813

On Mon, Oct 28, 2013 at 11:00 PM, Johannes Bauer <dfnsonfsduifb@gmx.de> wrote:
>> There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other’s reflection, __le__() and __ge__() are each other’s reflection, and __eq__() and __ne__() are their own reflection.
>
> But shouldn't __lt__ be the reflection or __ge__ and __gt__ the
> reflection of __le__?

lt is the negation of ge, but it's the reflection of gt. Consider this:

1 < 2
2 > 1

If Python can't ask 1 if it's less than 2, it'll ask 2 if it's greater than 1.

ChrisA

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#57815

Am 28.10.2013 13:23, schrieb Chris Angelico:
> On Mon, Oct 28, 2013 at 11:00 PM, Johannes Bauer <dfnsonfsduifb@gmx.de> wrote:
>>> There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other’s reflection, __le__() and __ge__() are each other’s reflection, and __eq__() and __ne__() are their own reflection.
>>
>> But shouldn't __lt__ be the reflection or __ge__ and __gt__ the
>> reflection of __le__?
> 
> lt is the negation of ge, but it's the reflection of gt. Consider this:
> 
> 1 < 2
> 2 > 1
> 
> If Python can't ask 1 if it's less than 2, it'll ask 2 if it's greater than 1.

Ah, I see. Thanks for clearing that up!

Best regards,
Joe

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#57845

On Mon, Oct 28, 2013, at 8:00, Johannes Bauer wrote:
> Hi group,
> 
> in http://docs.python.org/3/reference/datamodel.html#customization the
> doc reads:
> 
> > There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other’s reflection, __le__() and __ge__() are each other’s reflection, and __eq__() and __ne__() are their own reflection.
> 
> But shouldn't __lt__ be the reflection or __ge__ and __gt__ the
> reflection of __le__?

No... a < b is b > a - you're thinking of the relationship between a < b
and not(a >= b), which is not the argument-swapping it is referring to.

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