Question

From a python list of tuples (which is essentially a cartesian product of a list with itself) I want to delete (a,b) if (b,a) is in the list.Only one of (a,b) or (b,a) must be retained. So a list

[(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3)] 

must reduce to

[(1,2),(1,3),(2,3)]

(Although deleting (1,2) and retaining (2,1) is fine)

I tried doing this but I am not sure about deleting from a list while iterating over it. This doesn’t work. (Gives me [(1, 2), (2, 1), (2, 3), (3, 1), (3, 3)])

[pairs.remove((a,b)) for (a,b) in pairs if ((b,a) in pairs)]

Answer 1

Why delete the incorrect ones from the list?

Use itertools.combinations to generate the correct ones instead.

>>> import itertools
>>> list(itertools.combinations((1, 2, 3), 2))
[(1, 2), (1, 3), (2, 3)]