Today I’d like to show you how to copy a list in three different ways. We’ll be focusing on shallow copies to keep things simple.
If you want, you can watch the video first:
First let’s create a list:
>>> a = [1, 2, 3]Now let’s create another list, such that our a list is an element of it:
>>> b = ['a', 'b', a]
>>> b
['a', 'b', [1, 2, 3]]
Learn how to make beautiful GUI apps
in Python using the Kivy framework.
Comprehensive, for Kivy beginners, easy to follow.
Get the book here (PDF) or on Amazon:
ebook / paperback (black and white) / paperback (full color)
If we now change an element in a, it’ll also change inside the b list:
>>> a[1] = 100
>>> a
[1, 100, 3]
>>> b
['a', 'b', [1, 100, 3]]
This is because of shared references. Both a and b reference the same object in memory, which is a mutable list. So, if we change an element of the list, the change will be made in place and now the last element of the b list will reference the same changed list.
And now let’s get back to our main topic. If you don’t want a change in a list to propagate to other places where you reference it, you can use a copy of the list, not the list itself.
Copy a List by Slicing
How to make a copy? Well, there are a couple of ways we can do it. The first method consists in slicing the whole list, so using a slice operator without the start index and the past index:
>>> a = [1, 2, 3]
>>> b = ['a', 'b', a[:]]
>>> b
['a', 'b', [1, 2, 3]]
Make Awesome Games and Other 3D Apps
with Panda3D and Blender using Python.
Cool stuff, easy to follow articles.
Get the magazine here (PDF).
Now we have two different lists but with the same values. We can check it using the == and is operators. The former states that the values are identical, the latter states that a is not the same object as b.
>>> a == b[2], a is b[2]
(True, False)
Anyway, if we now change an element in the original list, the copy will stay untouched:
>>> a[1] = 100
>>> a
[1, 100, 3]
>>> b
['a', 'b', [1, 2, 3]]
Learn the basics of Python, including OOP.
with lots of exercises, easy to follow
The course is available on Udemy.
The copy Method
Another approach is to use the copy method on the list. The result is the same:
>>> a = [1, 2, 3]
>>> b = ['a', 'b', a.copy()]
>>> b
['a', 'b', [1, 2, 3]]
>>> a[1] = 100
>>> a
[1, 100, 3]
>>> b
['a', 'b', [1, 2, 3]]
Learn the basics of 3D modeling in Blender.
step-by-step, easy to follow, visually rich
The course is available on Udemy and on Skillshare.
The List Method
And one more method: we can pass our a list as an argument to the list method. The list method is usually used to convert objects of different types to lists, but actually, it’s a constructor method which creates a new list from the object passed as an argument. The result will be just like with the slice operator or the copy method:
>>> a = [1, 2, 3]
>>> b = ['a', 'b', list(a)]
>>> b
['a', 'b', [1, 2, 3]]
>>> a[1] = 100
>>> a
[1, 100, 3]
>>> b
['a', 'b', [1, 2, 3]]