How to make a shallow copy of a list in Python

Question

I am trying to implement an algorithm in Python to generate all Permutations of a list. But I In my for loop I wish to keep the original prefix and rest lists intact, and therefore I am trying to make a copy of those lists using newprefix and newrest, however on printing the variable rest at each iteration, I see that even the variable rest is getting modified! How can I make a shallow copy of the list in Python? Or is there another issue with my attempted logic?

def perm(prefix, rest):     if len(rest) == 0:         print prefix      for i in range(len(rest)):         #prints in the for loop are just for debugging         print "rest:", rest         print "i=", i         newprefix = prefix         newprefix.append(rest[i])         newrest = rest         newrest.pop(i)         print "old pre : ", prefix         print "newpre=", newprefix         print "newrest=", newrest         perm(newprefix, newrest)   perm([], ['a','b','c']) 

Answer 1

To make a shallow copy, you can slice the list:

newprefix = prefix[:] 

Or pass it into the list constructor:

newprefix = list(prefix) 

Also, I think you can simplify your code a little:

def perm(prefix, rest):     print prefix, rest      for i in range(len(rest)):         perm(prefix + [rest[i]], rest[:i] + rest[i + 1:])  perm([], ['a','b','c']) 

Answer 2

import copy  a = [somestuff] b = copy.copy(a) # Shallow copy here. c = copy.deepcopy(a) # Deep copy here. 

The copy module is worth knowing about. https://docs.python.org/3/library/copy.html

(Python 2) http://docs.python.org/2/library/copy.html