How do I concatenate two lists in Python?

As of 3.9, these are the most popular stdlib methods for concatenating two (or more) lists in Python.

Footnotes

1. This is a slick solution because of its succinctness. But sum performs concatenation in a pairwise fashion, which means this is a quadratic operation as memory has to be allocated for each step. DO NOT USE if your lists are large.

2. See chain and chain.from_iterable from the docs. You will need to from itertools import chain first. Concatenation is linear in memory, so this is the best in terms of performance and version compatibility. chain.from_iterable was introduced in 2.6.

3. This method uses Additional Unpacking Generalizations (PEP 448), but cannot generalize to N lists unless you manually unpack each one yourself.

4. a += b and a.extend(b) are more or less equivalent for all practical purposes. += when called on a list will internally call list.__iadd__, which extends the first list by the second.

Performance

2-List Concatenation¹

There's not much difference between these methods but that makes sense given they all have the same order of complexity (linear). There's no particular reason to prefer one over the other except as a matter of style.

N-List Concatenation

Plots have been generated using the perfplot module. Code, for your reference.

_{1. The iadd (+=) and extend methods operate in-place, so a copy has to be generated each time before testing. To keep things fair, all methods have a pre-copy step for the left-hand list which can be ignored.}

Comments on Other Solutions

• DO NOT USE THE DUNDER METHOD list.__add__ directly in any way, shape or form. In fact, stay clear of dunder methods, and use the operators and operator functions like they were designed for. Python has careful semantics baked into these which are more complicated than just calling the dunder directly. Here is an example. So, to summarise, a.__add__(b) => BAD; a + b => GOOD.

• Some answers here offer reduce(operator.add, [a, b]) for pairwise concatenation -- this is the same as sum([a, b], []) only more wordy.

• Any method that uses set will drop duplicates and lose ordering. Use with caution.

• for i in b: a.append(i) is more wordy, and slower than a.extend(b), which is single function call and more idiomatic. append is slower because of the semantics with which memory is allocated and grown for lists. See here for a similar discussion.

• heapq.merge will work, but its use case is for merging sorted lists in linear time. Using it in any other situation is an anti-pattern.

• yielding list elements from a function is an acceptable method, but chain does this faster and better (it has a code path in C, so it is fast).

• operator.add(a, b) is an acceptable functional equivalent to a + b. It's use cases are mainly for dynamic method dispatch. Otherwise, prefer a + b which is shorter and more readable, in my opinion. YMMV.

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])

If you are using NumPy, you can concatenate two arrays of compatible dimensions with this command:

numpy.concatenate([a,b])

Use a simple list comprehension:

joined_list = [item for list_ in [list_one, list_two] for item in list_]

It has all the advantages of the newest approach of using Additional Unpacking Generalizations - i.e. you can concatenate an arbitrary number of different iterables (for example, lists, tuples, ranges, and generators) that way - and it's not limited to Python 3.5 or later.

Another way:

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

I recommend three methods to concatenate the list, but the first method is most recommended,

# Easiest and least complexity method <= recommended

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

newlist = listone + listtwo
print(newlist)

# Second-easiest method
newlist = listone.copy()
newlist.extend(listtwo)
print(newlist)

In the second method, I assign newlist to a copy of the listone, because I don't want to change listone.

# Third method
newlist = listone.copy()
for j in listtwo:
    newlist.append(j)

print(newlist)

This is not a good way to concatenate lists because we are using a for loop to concatenate the lists. So time complexity is much higher than with the other two methods.

The most common method used to concatenate lists are the plus operator and the built-in method append, for example:

list = [1,2]

list = list + [3]
# list = [1,2,3]

list.append(3)
# list = [1,2,3]

list.append([3,4])
# list = [1,2,[3,4]]

For most of the cases, this will work, but the append function will not extend a list if one was added. Because that is not expected, you can use another method called extend. It should work with structures:

list = [1,2]
list.extend([3,4])
# list = [1,2,3,4]

So there are two easy ways.

1. Using +: It creates a new list from provided lists

Example:

In [1]: a = [1, 2, 3]

In [2]: b = [4, 5, 6]

In [3]: a + b
Out[3]: [1, 2, 3, 4, 5, 6]

In [4]: %timeit a + b
10000000 loops, best of 3: 126 ns per loop

2. Using extend: It appends new list to existing list. That means it does not create a separate list.

Example:

In [1]: a = [1, 2, 3]

In [2]: b = [4, 5, 6]

In [3]: %timeit a.extend(b)
10000000 loops, best of 3: 91.1 ns per loop

Thus we see that out of two of most popular methods, extend is efficient.

You could also just use sum.

>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> sum([a, b], [])
[1, 2, 3, 4, 5, 6]
>>>

This works for any length and any element type of list:

>>> a = ['a', 'b', 'c', 'd']
>>> b = [1, 2, 3, 4]
>>> c = [1, 2]
>>> sum([a, b, c], [])
['a', 'b', 'c', 'd', 1, 2, 3, 4, 1, 2]
>>>

The reason I add [], is because the start argument is set to 0 by default, so it loops through the list and adds to start, but 0 + [1, 2, 3] would give an error, so if we set the start to []. It would add to [], and [] + [1, 2, 3] would work as expected.

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

Output

>>> [1, 2, 3, 4, 5, 6]

In the above code, the "+" operator is used to concatenate the two lists into a single list.

Another solution

 a = [1, 2, 3]
 b = [4, 5, 6]
 c = [] # Empty list in which we are going to append the values of list (a) and (b)

 for i in a:
     c.append(i)
 for j in b:
     c.append(j)

 print(c)

Output

>>> [1, 2, 3, 4, 5, 6]

I assume you want one of the two methods:

Keep duplicate elements

It is very easy. Just concatenate like a string:

def concat_list(l1,l2):
    l3 = l1+l2
    return l3

Next, if you want to eliminate duplicate elements

def concat_list(l1,l2):
   l3 = []
   for i in [l1,l2]:
     for j in i:
       if j not in l3:
         # Check if element exists in final list, if no then add element to list
         l3.append(j)
   return l3

The solutions provided are for a single list. In case there are lists within a list and the merging of corresponding lists is required, the "+" operation through a for loop does the work.

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

b = [[0,1,2], [7,8,9]]

for i in range(len(a)):
    cc.append(a[i] + b[i])

Output: [[1, 2, 3, 0, 1, 2], [4, 5, 6, 7, 8, 9]]

You can use the union() function in Python.

joinedlist = union(listone, listtwo)
print(joinedlist)

Essentially, it’s removing one of every duplicate in the two lists. Since your lists don't have any duplicates it, it just returns the concatenated version of the two lists.