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.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Oct 22, 2021 (image).
