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']) ```

Accepted Answer

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'])
```

How to make a shallow copy of a list in Python

Solution

Problem