[Python-Dev] Rich comparison of lists and tuples

[Guido van Rossum]

[Subject fixed]

[Tim shows there's a lot left to the imagination when trying to glean
the meaning of list1==list2 using rich comparisons.]

I would like to break this down by defining the mapping between cmp()
and rich comparisons.

I propose:

- If cmp() is requested but not defined, and rich comparisons are
  defined, try ==, <, > in order; if all three yield false, act as if
  rich comparisons were not defined, and use the fallback comparison
  (i.e. by address).

- If a rich comparison is requested but not defined, use cmp() and use
  the obvious mapping.

- Continue to define the comparison of unequal sequences in terms of
  cmp().

- Testing == or != for sequences takes these shortcuts:

  1. if the lengths differ, the sequences differ

  2. compare the elements using == until a false return is found

Note that this defines 'x!=y' as 'not x==y' for sequences.  We could
easily go the extra mile and define != to use only != on the items;
but is this worth the extra complexity?

--Guido van Rossum (home page: http://www.python.org/~guido/)