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Copying data structures in Python can be deceiving and tricky. You might try to copy a list into a new variable as such:
a = [1, 2, 3]
b = aAt the surface, you may think that a and b are 2 separate lists. But when you go to append to b:
b.append(4)
print(a) # [1, 2, 3, 4]
print(b) # [1, 2, 3, 4]But you never appended to a! What’s going on here?
Assignments are not copies
Remember that the = operator is an assignment. It means that the variable b now points to the same object as a.
» Pointers is how the code is able to identify where something lives in memory.
So, when you do an assignment operation, you’re writing code to say “point to this specific object in memory”:
Creating a shallow copy
In order to avoid this, you’re going to want to use a Python built-in library: copy, which allows you to create copies in one line of code:
import copy
a = [1, 2, 3]
b = copy.copy(a)
b.append(4)
print(a) # [1, 2, 3]
print(b) # [1, 2, 3, 4]If we were to diagram it, it would look like this:
Perfect! This seems to work for a single-layered list. If we were to continue testing, what would happen if I have a nested list?
import copy
a = [[1, 2], [3, 4]]
b = copy.copy(a)
# append to the list and the first element
b.append([5, 6])
b[0].append(99)
print(a) # [[1, 2, 99], [3, 4]]
print(b) # [[1, 2, 99], [3, 4], [5, 6]]Our initial solution of using copy does work, but not entirely. If you want a true clone that’s independent of your original variable, leverage deepcopy instead:
import copy
a = [[1, 2], [3, 4]]
b = copy.deepcopy(a) # <-- This changed to `deepcopy`
# append to the list and the first element
b.append([5, 6])
b[0].append(99)
print(a) # [[1, 2], [3, 4]]
print(b) # [[1, 2, 99], [3, 4], [5, 6]]Do we need to deepcopy everything?
Thankfully, we don’t. There’s some criteria that should be met if you need to use deep copy. You’ll want to use deepcopy when:
Your object is nested (such as above)
The inner objects are mutable, such as lists, dicts, and sets.
You don’t need to deepcopy when:
The object is immutable (ints, floats, strings, tuples of immutables, frozensets)
The data structure has only one level (such as the example above).
Happy coding!
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