Iterating over multiple indices with i > j ( > k) in a pythonic way

Question

i need to iterate over a tuple of indices. all indices must be in the range [0, N) with the condition i > j. The toy example I present here deal

Answer 1

a somewhat 'hacky' attempt using eval (just adding this for completeness. there are nicer answers here!).

the idea is to construct a string like

'((a, b, c) for a in range(5) for b in range(a) for c in range(b))'

and return the eval of that:

def ijk_eval(n, depth):
    '''
    construct a string representation of the genexpr and return eval of it...
    '''

    var = string.ascii_lowercase
    assert len(var) >= depth > 1  # returns int and not tuple if depth=1

    for_str = ('for {} in range({}) '.format(var[0], n) +
               ' '.join('for {} in range({})'.format(nxt, cur)
                        for cur, nxt in zip(var[:depth-1], var[1:depth])))
    return eval('(({}) {})'.format(', '.join(var[:depth]), for_str))

can be used this way and produces the right results.

for i, j in ijk_eval(n=5, depth=2):
    print(i, j)

the construction is not very nice - but the result is: it is a regular genexpr and just as efficient as those are.