Copying a list

What doesn't work

As with changing a list, our first attempt at copying a list may not succeed. The most common thing students try is something like this,

magnitudes = [41, 14, 27, 5, 32]
copy = magnitudes

Reasoning that copy will now contain a copy of magnitudes. Of course a moment's thought about how reference semantics works will show that magnitudes and copy are now both references to the same list object, i.e.

What works (sort of) 1

One way to copy a list is to create an empty one, and then loop through the source list appending elements to the new one,

magnitudes = [41, 14, 27, 5, 32]
copy = []
for m in magnitudes:
    copy.append(m)

this gets us what we wanted,

What works (sort of) 2 and 3

Copying a list is so common that there are some Pythonic shortcuts available. One is to take a slice of the entire list and assign it to copy,

copy = magnitudes[:]

This works because slices create new objects.

Another is to use the class constructor¹ for lists, list, to create a new list from an existing one,

copy = list(magnitude)

What's with the “(sort of)”?

Well sometimes these do all we want, but other times its important to realize they are all creating first level copies of the lists. Recall our Tic-Tac-Toe lists:

g = [['X', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]

Watch what happens when make a copy and then modify the copy,

>>> g = [['X', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>> cc = g[:]
>>> print(cc)
[['X', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>> cc[0][0] = 'A'
>>> print(cc)
[['A', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>> print(g)
[['A', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>>

Do you see it? Changing element cc[0][0] to 'A' also changed element g[0][0]. But if cc is a copy of g that shouldn't have happened! The problem is that cc isn't a complete copy of g. It only copied the first level references in g. Those references still point to the same sublist objects, i.e.

So since cc[0] and g[0] still reference the same list object, changing cc[0][0] changes g[0][0].

Deep copying

As the Tic-Tac-Toe example shows sometimes we have objects containing references to other objects, and in some applications there can be far more than just two levels of reference. What to do if we still need a copy of the complete structure? This comes up often enough that Python provides a standard library function to create the copy for you,

>>> import copy
>>> g = [['X', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>> cc = copy.deepcopy(g)
>>> cc[0][0] = 'A'
>>> print(cc)
[['A', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>> print(g)
[['X', 'O', ''], ['O', 'X', 'O'], ['', '', 'X']]
>>>

Summary

Create a shallow copy using a slice, copy = original[:]

or the list constructor, copy = list(original).

If a deep copy is needed (it often isn't), use the copy module, copy = copy.deepcopy(original).