Stop recursive generator and permutations

Question

As an exercise, I tested various ways of generating all list permutations in Python - recursive, non-recursive ... - and comparing performance with itertools.permutations(). But I'm having problems with the generator version of the recursive method, which does not end purely with an exception StopIteration, but instead calls IndexError:

def spawnperms(alist):
    """same algorithm as recursive option, but a generator"""
    if (alist == []):
        yield []
    for perm in spawnperms(alist[:-1]):
        for i in range(len(perm)+1):
            yield perm[:i] + [alist[-1]] + perm[i:]

Calling this from the Python interpreter:

>>> for i in spawnperms(range(3)):
...     print i
... 
[2, 1, 0]
[1, 2, 0]
[1, 0, 2]
[2, 0, 1]
[0, 2, 1]
[0, 1, 2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in spawnperms
  File "<stdin>", line 5, in spawnperms
  File "<stdin>", line 5, in spawnperms
  File "<stdin>", line 7, in spawnperms
IndexError: list index out of range

Uch. I tried to go through it with the help pdbthat almost created a stack overflow in my brain, but I realized that recursion "gets" to an empty list, and then the external (I think) loop forfrom the indices is executed .

How can I fix my code?

: , . else if, , , , . !

Answer 1

else:

if (alist == []):
    yield []
else:
    for ...

, yield , return. yield, .