Copying Lists – Deep Copies in Python

In the previous article we were talking about shallow copies of lists. We know that only the reference to a sublist is copied if there are nested lists. If we need a full copy, we need a deep copy. Deep copies are the subject of this article.

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So, if we want a full copy of a list, along with all the sublists, we need a deep copy. There is a deepcopy method in the module copy, which we have to import:

>>> from copy import deepcopy
>>> sports = ["soccer", "tennis", ["hammer throw", "javelin throw"]]

Now we’ll make a deep copy using the imported method:

>>> sports_copy = deepcopy(sports)

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The deepcopy method seems to have worked:

>>> sports
['soccer', 'tennis', ['hammer throw', 'javelin throw']]

>>> sports_copy
['soccer', 'tennis', ['hammer throw', 'javelin throw']]

But let’s check it out. First, let’s change a simple element in the copy. The change should be visible only in the copy:

>>> sports_copy[0] = "football"

>>> sports
['soccer', 'tennis', ['hammer throw', 'javelin throw']]

>>> sports_copy
['football', 'tennis', ['hammer throw', 'javelin throw']]

Works! Now let’s see if the sublist was copied correctly. Let’s change an element in the sublist in the copy:

>>> sports_copy[2][1] = "discus throw"

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Now the sublist in the original should remain untouched whereas in the copy we should see the change in the sublist:

>>> sports
['soccer', 'tennis', ['hammer throw', 'javelin throw']]

>>> sports_copy
['football', 'tennis', ['hammer throw', 'discus throw']]

Works! So, a deep copy is the solution to choose if you need a full copy, along with all the nested lists.

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