Fastest way to swap elements in Python list

Question

Is there any any faster way to swap two list elements in Python than

L[a], L[b] = L[b], L[a] 

or would I have to resort to Cython or Weave or the like?

Answer 1

Looks like the Python compiler optimizes out the temporary tuple with this construct:

code:

import dis  def swap1():   a=5   b=4   a, b = b, a  def swap2():   a=5   b=4   c = a   a = b   b = c  print 'swap1():' dis.dis(swap1) print 'swap2():' dis.dis(swap2) 

output:

swap1():   6           0 LOAD_CONST               1 (5)               3 STORE_FAST               0 (a)    7           6 LOAD_CONST               2 (4)               9 STORE_FAST               1 (b)    8          12 LOAD_FAST                1 (b)              15 LOAD_FAST                0 (a)              18 ROT_TWO                           19 STORE_FAST               0 (a)              22 STORE_FAST               1 (b)              25 LOAD_CONST               0 (None)              28 RETURN_VALUE         swap2():  11           0 LOAD_CONST               1 (5)               3 STORE_FAST               0 (a)   12           6 LOAD_CONST               2 (4)               9 STORE_FAST               1 (b)   13          12 LOAD_FAST                0 (a)              15 STORE_FAST               2 (c)   14          18 LOAD_FAST                1 (b)              21 STORE_FAST               0 (a)   15          24 LOAD_FAST                2 (c)              27 STORE_FAST               1 (b)              30 LOAD_CONST               0 (None)              33 RETURN_VALUE         

Two loads, a ROT_TWO, and two saves, versus three loads and three saves. You are unlikely to find a faster mechanism.

Answer 2

If you could post a representative code sample, we could do a better job of benchmarking your options. FWIW, for the following dumb benchmark, I get about a 3x speedup with Shed Skin and a 10x speedup with PyPy.

from time import time  def swap(L):     for i in xrange(1000000):         for b, a in enumerate(L):             L[a], L[b] = L[b], L[a]  def main():     start = time()     L = list(reversed(range(100)))     swap(L[:])     print time() - start     return L  if __name__ == "__main__":     print len(main())  # for shedskin: # shedskin -b -r -e listswap.py && make # python -c "import listswap; print len(listswap.main())"