Question

How do I concatenate two lists in Python?

Example:

listone = [1,2,3]listtwo = [4,5,6]

Expected outcome:

joinedlist == [1, 2, 3, 4, 5, 6]

what about if listone was [3,2,1]? Will the output change? — Nov 12 '09 at 7:06
Could have found answer in any Python reference or tutorial. Personally I don't mind, but this question looks like asked solely for reputation mining ;) — Feb 8 '14 at 12:46
'merge' as 'create one shallow-copy', 'deep-copy' or 'iterate from'? (@Clergyman, it's not at all that trivial) — Sep 12 '14 at 3:31
Isn’t this operation called concatination rather than merging? I always thought merging means concatination + sorting. — Nov 8 '14 at 2:33
This question's searchability could be improved if the terms 'looping' and 'concatenate' were added. — Sep 9 '15 at 13:22

19.1k 3 24 34 · Accepted Answer · 2009-11-12 07:07:06Z

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Python makes this ridiculously easy.

mergedlist = listone + listtwo

answered

Nov 12 '09 at 7:07

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does this create a deep copy of listone and appends listtwo? –

Apr 19 '12 at 12:34

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@Daniel it will create a new list with a shallow copy of the items in the first list, followed by a shallow copy of the items in the second list. Use copy.deepcopy to get deep copies of lists. –

Apr 19 '12 at 14:51

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another useful detail here: listone += listtwo results in listone == [1, 2, 3, 4, 5, 6] –

Jan 29 '14 at 16:14

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@br1ckb0t will that change what listone is pointing at? So:list3 = listone listone+=listtwo Is list3 changed as well? –

Feb 19 '14 at 5:01

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it does change list3. However, if that isn't a problem, it's simpler more readable to add the two lists instead of creating a new one. –

Feb 20 '14 at 18:55

52.5k 16 89 177 · Answer 2 · 2009-11-12 20:01:50Z

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It's also possible to create a generator that simply iterates over the items in both lists. This allows you to chain lists (or any iterable) together for processing without copying the items to a new list:

import itertoolsfor item in itertools.chain(listone, listtwo):   # do something with each list item

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Nov 12 '09 at 20:01

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This is better way because it also works with numpy array. –

Sep 25 '12 at 9:37

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will this work the same way: mergedList = itertools.chain(listone, listtwo) for item in mergedList: –

Mar 1 '13 at 0:55

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@d.putto: individual item access is very slow for numpy arrays (each access requires to convert the raw memory with a C type to Python object. Vectorized operations such as np.dot() work on the C types directly without the round trip to Python and therefore fast). You could use merged = np.r_[a, b] to get concatenated numpy array. –

Apr 30 '14 at 3:38

101k 11 115 186 · Answer 3 · 2010-09-20 08:43:58Z

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You can use sets to obtain merged list of unique values

mergedlist = list(set(listone + listtwo))

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Sep 20 '10 at 8:43

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this will lose ordering information. –

Sep 20 '10 at 8:45

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True, however, it will also remove duplicates, if that's what you are interested in. List addition along would not do that. –

Aug 21 '12 at 0:28

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Better than listone + [x for x in listtwo if x not in listone] –

Jan 29 '13 at 13:13

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If I had a list of lists, such as this one: [[0, 5], [1, 10], [0, 7], [3, 5]] How would you merge them to avoid duplicates in the key (first value in each sub-list), but if they are duplicates, end up with the sum of the second values? Like so: [[0, 12], [1, 10], [3, 5]] Thanks –

Jul 23 '13 at 12:24

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+1 IMHO this is the correct way to "merge" (union) lists while the "approved" answer describes how to combine/add lists (multiset) –

Apr 27 '14 at 4:07

1,628 9 16 · Answer 4 · 2009-11-12 07:07:37Z

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This is quite simple, I think it was even shown in the :

>>> listone = [1,2,3]>>> listtwo = [4,5,6]>>>>>> listone + listtwo[1, 2, 3, 4, 5, 6]

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Nov 12 '09 at 7:07

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score 26 · Answer 5

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You could also do

listone = [1,2,3]listtwo = [4,5,6]mergedlist = []mergedlist.extend(listone)mergedlist.extend(listtwo)

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community wiki

@MatthewPurdon extend won't take a single item, it expects a list. –

Oct 15 '14 at 21:43

9,214 10 64 98 · Answer 6 · 2013-03-05 09:15:56Z

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You could simply use the + or += operator as follows:

a = [1, 2, 3]b = [4, 5, 6]c = a + b

Or:

c = []a = [1, 2, 3]b = [4, 5, 6]c += (a + b)

Also, if you want the values in the merged list to be unique you can do:

c = list(set(a + b))

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Mar 5 '13 at 9:15

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1,781 2 23 28 · Answer 7 · 2013-02-05 05:01:34Z

It's worth noting that the itertools.chain function accepts variable number of arguments:

>>> l1 = ['a']; l2 = ['b', 'c']; l3 = ['d', 'e', 'f']>>> [i for i in itertools.chain(l1, l2)]['a', 'b', 'c']>>> [i for i in itertools.chain(l1, l2, l3)]['a', 'b', 'c', 'd', 'e', 'f']

If an iterable (tuple, list, generator, etc.) is the input, the from_iterable class method may be used:

>>> il = [['a'], ['b', 'c'], ['d', 'e', 'f']]>>> [i for i in itertools.chain.from_iterable(il)]['a', 'b', 'c', 'd', 'e', 'f']

user688635 · Answer 8 · 2014-07-21 00:53:24Z

With Python 3.3+ you can use :

listone = [1,2,3]listtwo = [4,5,6]def merge(l1, l2):    yield from l1    yield from l2>>> list(merge(listone, listtwo))[1, 2, 3, 4, 5, 6]

Or, if you want to support an arbitrary number of iterators:

def merge(*iters):    for it in iters:        yield from it>>> list(merge(listone, listtwo, 'abcd', [20, 21, 22]))[1, 2, 3, 4, 5, 6, 'a', 'b', 'c', 'd', 20, 21, 22]

4,466 2 27 43 · Answer 9 · 2015-07-07 13:30:48Z

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If you want to merge the two lists in sorted form, you can use merge function from the heapq library.

from heapq import mergea = [1,2,4]b = [2,4,6,7]print list(merge(a,b))

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9,214 10 64 98 · Answer 10 · 2013-11-07 18:04:54Z

If you need to merge two ordered lists with complicated sorting rules, you might have to roll it yourself like in the following code (using a simple sorting rule for readability :-) ).

list1 = [1,2,5]list2 = [2,3,4]newlist = []while list1 and list2:    if list1[0] == list2[0]:        newlist.append(list1.pop(0))        list2.pop(0)    elif list1[0] < list2[0]:        newlist.append(list1.pop(0))    else:        newlist.append(list2.pop(0))if list1:    newlist.extend(list1)if list2:    newlist.extend(list2)assert(newlist == [1, 2, 3, 4, 5])

41.5k 6 27 55 · Answer 11 · 2015-07-06 20:06:35Z

As a more general way for more lists you can put them within a list and use itertools.chain.from_iterable()¹ function which based on answer is the best way for flatting a nested list :

>>> l=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]>>> import itertools>>> list(itertools.chain.from_iterable(l))[1, 2, 3, 4, 5, 6, 7, 8, 9]

_{1. Note that chain.from_iterable() is available in python =>2.6.In other versions use chain(*l)}

score 3 · Answer 12 · 2014-10-20 09:32:32Z

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If you don't want to or can't use the plus operator (+), you can uses the __add__ function:

listone = [1,2,3]listtwo = [4,5,6]result = list.__add__(listone, listtwo)print(result)>>> [1, 2, 3, 4, 5, 6]

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Oct 20 '14 at 9:32

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412 4 17 · Answer 13 · 2013-07-16 02:02:49Z

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You could use the append() method

mergedlist =[]for elem in listone:    mergedlist.append(elem)for elem in listtwo:    mergedlist.append(elem)

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Jul 16 '13 at 2:02

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just so you know, if this is what you're doing in practice, this is much, much slower than the other proposed methods. see –

Jul 16 '13 at 2:10

155 1 6 · Answer 14 · 2015-08-20 22:12:20Z

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list1 = [1,2,3]list2 = [4,5,6]joinedlist = list1 + list2#output : [1, 2, 3, 4, 5, 6]

Yes, its that simple.

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

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Aug 20 '15 at 22:12

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644 5 7 · Answer 15 · 2015-08-29 18:03:13Z

Joining two lists in Python:

>>> a = [1, 2, 3, 4]>>> b = [1, 4, 6, 7]>>> c = a + b >>> c[1, 2, 3, 4, 1, 4, 6, 7]

If you don't want any duplication :

>>> a = [1, 2, 3, 4, 5, 6]>>> b = [5, 6, 7, 8]>>> c = list(set(a + b)) >>> c[1, 2, 3, 4, 5, 6, 7, 8]

score 1 · Answer 16 · 2015-10-09 23:21:27Z

This question directly asks about joining two lists. However it's pretty high in search even when you are looking for a way of joining many lists (including the case when you joining zero lists). Consider this more generic approach:

a = [[1,2,3], [4,5,6], [7,8,9]]reduce(lambda c, x: c + x, a, [])

Will output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Note, this also works correctly when a is [] or [[1,2,3]].

Update

Consider better alternative suggested by Patrick Collins in the comments:

sum(a, [])

17 Answers