Why does Python’s itertools.permutations contain duplicates? (When the original list has duplicates)

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I can’t speak for the designer of itertools.permutations (Raymond Hettinger), but it seems to me that there are a couple of points in favour of the design:

First, if you used a next_permutation-style approach, then you’d be restricted to passing in objects that support a linear ordering. Whereas itertools.permutations provides permutations of any kind of object. Imagine how annoying this would be:

>>> list(itertools.permutations([1+2j, 1-2j, 2+j, 2-j]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: no ordering relation is defined for complex numbers

Second, by not testing for equality on objects, itertools.permutations avoids paying the cost of calling the __eq__ method in the usual case where it’s not necessary.

Basically, itertools.permutations solves the common case reliably and cheaply. There’s certainly an argument to be made that itertools ought to provide a function that avoids duplicate permutations, but such a function should be in addition to itertools.permutations, not instead of it. Why not write such a function and submit a patch?

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