How can I get the concatenation of two lists in Py

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• How to concatenate two lists in Python? 30 answers

In Python, the only way I can find to concatenate two lists is list.extend, which modifies the first list. Is there any concatenation function that returns its result without modifying its arguments?

Answer 1

Depending on how you're going to use it once it's created itertools.chain might be your best bet:

>>> import itertools
>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = itertools.chain(a, b)

This creates a generator for the items in the combined list, which has the advantage that no new list needs to be created, but you can still use c as though it were the concatenation of the two lists:

>>> for i in c:
...     print i
1
2
3
4
5
6

If your lists are large and efficiency is a concern then this and other methods from the itertools module are very handy to know.

Note that this example uses up the items in c, so you'd need to reinitialise it before you can reuse it. Of course you can just use list(c) to create the full list, but that will create a new list in memory.

Answer 2

concatenated_list = list_1 + list_2

Answer 3

Yes: list1+list2. This gives a new list that is the concatenation of list1 and list2.

Answer 4

And if you have more than two lists to concatenate:

import operator
list1, list2, list3 = [1,2,3], ['a','b','c'], [7,8,9]
reduce(operator.add, [list1, list2, list3])

# or with an existing list
all_lists = [list1, list2, list3]
reduce(operator.add, all_lists)

It doesn't actually save you any time (intermediate lists are still created) but nice if you have a variable number of lists to flatten, e.g., *args.

Answer 5

You can also use sum, if you give it a start argument:

>>> list1, list2, list3 = [1,2,3], ['a','b','c'], [7,8,9]
>>> all_lists = sum([list1, list2, list3], [])
>>> all_lists
[1, 2, 3, 'a', 'b', 'c', 7, 8, 9]

This works in general for anything that has the + operator:

>>> sum([(1,2), (1,), ()], ())
(1, 2, 1)

>>> sum([Counter('123'), Counter('234'), Counter('345')], Counter())
Counter({'1':1, '2':2, '3':3, '4':2, '5':1})

>>> sum([True, True, False], False)
2

With the notable exception of strings:

>>> sum(['123', '345', '567'], '')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]

Answer 6

you could always create a new list which is a result of adding two lists.

>>> k = [1,2,3] + [4,7,9]
>>> k
[1, 2, 3, 4, 7, 9]

Lists are mutable sequences so I guess it makes sense to modify the original lists by extend or append.