[Python-ideas] Fwd: Extremely weird itertools.permutations

[Tim Peters]

[Oscar Benjamin]
> ...
> Damn, you're right. I obviously didn't think that one through hard
> enough. Okay how about this?
> d = TransformDict(lambda x: (tuple(map(type, x)), tuple(x)))

Oscar, please give this up - it's not going to work.  `x` can be any
object whatsoever, with arbitrarily complex internal structure, and
the user can have an arbitrarily convoluted idea of what "equal"
means.  Did I mention that these lists don't *only* have ints and
floats as elements, but also nested sublists?  Oh ya - they also want
a float and a singleton list containing the same float to be
considered equal ;-)  Etc.

Besides, you're trying to solve a problem I didn't have to begin with
;-)  That is, I don't care much about the cost of building equivalence
classes - it's a startup cost for the generator, not an "inner loop"
cost.  Even if you could bash every case into a different convoluted
hashable tuple, in general it's going to be - in this specific problem
- far easier for the user to define an equal() function they like,
working directly on the two objects.  That doesn't require an endless
sequence of tricks.