Hey guys!

For the last couple of days, I've been fighting a war against

generators and they've beaten the crap out of me several times. What I

want to do is implement one that yields every possible permutation of

a given sequence (I had lists in mind, but I could swear that this

would work on strings too...if it worked at all).

Here's what I have so far:

def perm(seq):

"Reshuffles the elements of seq in every possible way"

if len(seq) == 1:

yield seq

else:

for p in perm(seq[1:]):

for i in range(len(seq)):

yield p.insert(i, seq[0])

Basically, I let the recursion reshuffle the elements of the

subsequence that starts from the second element of the original one,

and then I insert seq[0], the first element of the original sequence,

in every position of the shuffled sublist.

Here's what I get when I try to list the permutations of [1, 2, 3, 4]

(print list(perm([1, 2, 3, 4]))):

Traceback (most recent call last):

File "perm.py", line 15, in <module>

print list(perm([1, 2, 3, 4]))

File "perm.py", line 11, in perm

for p in perm(seq[1:]):

File "perm.py", line 13, in perm

yield p.insert(i, seq[0])

AttributeError: 'NoneType' object has no attribute 'insert'

Could somebody please explain to me why my p variable has no type at

all?

For every call to perm(seq), seq should always be of the same type as

the sequence that was used in the first call. Any ideas as to where

seq loses it's type?

Thanks in advance,

Pablo