Copy list

A list references a region of memory. When assigned, the list reference itself is copied. In Python we can instead copy elements into a new list.

Syntax notes

We sometimes need instead to copy all the elements of a list into a new list. The new list then can be independently modified.

Slice example

We use the slice syntax to copy a list. When the start and end numbers are omitted, we consider the slice to equal the entire list.

Tip This is the shortest syntax form. It requires only a list reference and 3 characters.

# A list:
list1 = [10, 20, 30]

# Copy list1 with slice syntax.
list2 = list1[:]

# Modify list2.
list2[0] = 0

print(list1)
print(list2)[10, 20, 30]
[0, 20, 30]

Extend

Next, we use the extend method to copy a list. We first need to allocate a new, empty list. Then we extend that empty list (list2) with the original list.

Note This approach requires two lines. It is also less efficient. Please see the benchmark section.

list1 = [10, 20, 30]

# Create empty list, then extend it.
list2 = []
list2.extend(list1)

# The lists are separate.
list2[0] = 0

print(list1)
print(list2)[10, 20, 30]
[0, 20, 30]

Performance

Here we consider performance of list copying. The list contains five numbers. We use while-loops to repeatedly copy those five elements.

Version 1 This Python code tests the slice syntax. An entire list is copied with by using an unspecified start and end.

Version 2 This code uses the extend method (after creating an empty list). We repeat these operations ten million times.

Result Taking a slice to copy a list is faster. It required just 4.7 seconds, versus 5.3 for the extend method version.

import time

list1 = [100, 200, 300, 400, 500]

print(time.time())

# Version 1: slice
i = 0
while < 10000000:
    list2 = list1[:]
    i += 1

print(time.time())

# Version 2: extend
i = 0
while i < 10000000:
    list2 = []
    list2.extend(list1)
    i += 1

print(time.time())1395252155.968703
1395252160.704971   Slice  = 4.736 s
1395252166.045275   Extend = 5.340 s

Summary

A full-list slice is a good way to copy a list. It requires the least amount of syntax. And it performs faster than the alternative approach that uses the extend method.