danger of recursive functions

Question

Often people say that it\'s not recommended to use recursive functions in python (recursion depth restrictions, memory consumption, etc)

I took a permutation example

Answer 1

Your timings are completely wrong:

def perms1(str):
  if len(str) <=1:
    yield str
  else:
    for perm in perms1(str[1:]):
        for i in range(len(perm)+1):
            yield perm[:i] + str[0:1] + perm[i:]

def perms2(string):
  perm = [string[0]]
  for e in string[1:]:
    perm_next = []
    for p in perm:
        perm_next.extend(p[:i] + e + p[i:] for i in range(len(p) + 1))
    perm = perm_next

  for p in perm:
    yield p

s = "01235678"
import itertools
perms3 = itertools.permutations

Now test it with timeit:

thc:~$ for i in 1 2 3; do echo "perms$i:"; python -m timeit -s "import permtest as p" "list(p.perms$i(p.s))"; done 
perms1:
10 loops, best of 3: 23.9 msec per loop
perms2:
10 loops, best of 3: 39.1 msec per loop
perms3:
100 loops, best of 3: 5.64 msec per loop

As you can see itertools.permutations is by far the fastest, followed by the recursive version.

But both the pure Python function had no chance to be fast, because they do costly operations such as adding lists ala perm[:i] + str[0:1] + perm[i:]