What is the best way to copy a list in Python?

Question

lst1 = ['one', 2, 3]

// What is the best way of the following  -- or is there another way?
lst2 = list(lst1)
lst2 = lst1[:]

import copy
lst2 = copy.copy(lst1)

Answer 1

If you want a shallow copy (elements aren't copied) use:

lst2=lst1[:]

If you want to make a deep copy then use the copy module:

import copy
lst2=copy.deepcopy(lst1)

Answer 2

I often use:

lst2 = lst1 * 1

If lst1 it contains other containers (like other lists) you should use deepcopy from the copy lib as shown by Mark.

---

UPDATE: Explaining deepcopy

>>> a = range(5)
>>> b = a*1
>>> a,b
([0, 1, 2, 3, 4], [0, 1, 2, 3, 4])
>>> a[2] = 55 
>>> a,b
([0, 1, 55, 3, 4], [0, 1, 2, 3, 4])

As you may see only a changed... I'll try now with a list of lists

>>> 
>>> a = [range(i,i+3) for i in range(3)]
>>> a
[[0, 1, 2], [1, 2, 3], [2, 3, 4]]
>>> b = a*1
>>> a,b
([[0, 1, 2], [1, 2, 3], [2, 3, 4]], [[0, 1, 2], [1, 2, 3], [2, 3, 4]])

Not so readable, let me print it with a for:

>>> for i in (a,b): print i   
[[0, 1, 2], [1, 2, 3], [2, 3, 4]]
[[0, 1, 2], [1, 2, 3], [2, 3, 4]]
>>> a[1].append('appended')
>>> for i in (a,b): print i

[[0, 1, 2], [1, 2, 3, 'appended'], [2, 3, 4]]
[[0, 1, 2], [1, 2, 3, 'appended'], [2, 3, 4]]

You see that? It appended to the b[1] too, so b[1] and a[1] are the very same object. Now try it with deepcopy

>>> from copy import deepcopy
>>> b = deepcopy(a)
>>> a[0].append('again...')
>>> for i in (a,b): print i

[[0, 1, 2, 'again...'], [1, 2, 3, 'appended'], [2, 3, 4]]
[[0, 1, 2], [1, 2, 3, 'appended'], [2, 3, 4]]

In this case it works with copy too, because there is only one deeper level, but if you have list of lists of list of... or other objects (i took lists just for simplicity) you need deepcopy.

Answer 3

You can also do this:

import copy
list2 = copy.copy(list1)

This should do the same thing as Mark Roddy's shallow copy.

Answer 4

You can also do:

a = [1, 2, 3]
b = list(a)

Answer 5

I like to do:

lst2 = list(lst1)

The advantage over lst1[:] is that the same idiom works for dicts:

dct2 = dict(dct1)