R. David Murray added the comment:

Unless I'm misremembering, it is exactly __lt__ (or __gt__, if __lt__ returns 
NotImplemented) that sorting depends on.  Since I'm sure there is code out 
there that depends on this fact, I wonder if it should be part of the language 
definition.

Also, the comparison documentation 
(https://docs.python.org/3/reference/expressions.html#comparisons) speaks about 
"total ordering" as being the requirement, which has a specific mathematical 
meaning (which sets, for example, do not satisfy, even though they have a 
__lt__ method).  Whether or not the distinction is worth explaining in the 
tutorial is a open question.

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue14050>
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