How can I get the concatenation of two lists in Python without modifying either one? [duplicate]
In Python, the only way I can find to concatenate two lists is list.extend
, which modifies the first list. Is there开发者_JAVA百科 any concatenation function that returns its result without modifying its arguments?
Yes: list1 + list2
. This gives a new list that is the concatenation of list1
and list2
.
The simplest method is just to use the +
operator, which returns the concatenation of the lists:
concat = first_list + second_list
One disadvantage of this method is that twice the memory is now being used . For very large lists, 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.
concatenated_list = list_1 + list_2
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]
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.
And if you have more than two lists to concatenate:
import operator
from functools import reduce # For Python 3
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
.
Just to let you know:
When you write list1 + list2
, you are calling the __add__
method of list1
, which returns a new list. in this way you can also deal with myobject + list1
by adding the __add__
method to your personal class.
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