permutations with unique values

Question

itertools.permutations generates where its elements are treated as unique based on their position, not on their value. So basically I want to avoid duplicates like this:

Answer 1

A naive approach might be to take the set of the permutations:

list(set(it.permutations([1, 1, 1])))
# [(1, 1, 1)]

However, this technique wastefully computes replicate permutations and discards them. A more efficient approach would be more_itertools.distinct_permutations, a third-party tool.

Code

import itertools as it

import more_itertools as mit


list(mit.distinct_permutations([1, 1, 1]))
# [(1, 1, 1)]

Performance

Using a larger iterable, we will compare the performances between the naive and third-party techniques.

iterable = [1, 1, 1, 1, 1, 1]
len(list(it.permutations(iterable)))
# 720

%timeit -n 10000 list(set(it.permutations(iterable)))
# 10000 loops, best of 3: 111 µs per loop

%timeit -n 10000 list(mit.distinct_permutations(iterable))
# 10000 loops, best of 3: 16.7 µs per loop

We see more_itertools.distinct_permutations is an order of magnitude faster.

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Details

From the source, a recursion algorithm (as seen in the accepted answer) is used to compute distinct permutations, thereby obviating wasteful computations. See the source code for more details.