[Tutor] True and 1 [was Re: use of the newer dict types]

[Albert-Jan Roskam]

----- Original Message -----

> From: Steven D'Aprano <steve at pearwood.info>
> To: tutor at python.org
> Cc: 
> Sent: Sunday, July 28, 2013 10:33 AM
> Subject: Re: [Tutor] True and 1 [was Re: use of the newer dict types]
> 
> On 28/07/13 18:07, Alan Gauld wrote:
>>>  Comparison operators haven't depended on cmp() for a long time. You 
> can
>>>  google "rich comparison operators" for more info:
>>> 
>>>  https://duckduckgo.com/?q=rich%20comparison%20operators
>> 
>>  I only read the first two but one question remains:
>>  If cmp() is gone is __cmp__() still supported? I'm assuming it
>>  must be for backward compatibility?
> 
> Not in Python 3, it's gone. In Python 3, you have to define all six the rich 
> comparison methods __eq__ __ne__ __lt__ __gt__ __le__ __ge__ if you wish to 
> support the comparison operators. There's a helper function in functools to 
> help simplify the job:

Wow, too bad __cmp__ is no longer present. Not so much for cmp(), but because it's such a compact way to define all the comparison methods. Btw, my book says "Python will supply the __ne__() (not equal) inequality operator automatically if wel implement __eq__ but don't implement __ne__()". [Programming in Python 3 (2009), M. Summerfield, p.213]

 
> @functools.total_ordering
> class MyClass:
>      ...
> 
> 
> total_ordering will automatically fill in the missing comparison methods using 
> rules like these:
> 
> "<=" is equivalent to "not >"
> ">=" is equivalent to "not <"
> "!=" is equivalent to "not =="
> 
> "<=" is equivalent to "< or =="
> ">=" is equivalent to "> or =="
> 
> 
> 
> -- 
> Steven
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