How do I concatenate two lists in Python?

Question

How do I concatenate two lists in Python?

Example:

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

Expected outcome:

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Answer 1

list(set(listone) | set(listtwo))

The above code, does not preserve order, removes duplicate from each list (but not from the concatenated list)

Answer 2

With Python 3.3+ you can use yield from:

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]

Answer 3

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']

Answer 4

You could also use the list.extend() method in order to add a list to the end of another one:

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

listone.extend(listtwo)

If you want to keep the original list intact, you can create a new list object, and extend both lists to it:

mergedlist = []
mergedlist.extend(listone)
mergedlist.extend(listtwo)

Answer 5

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

numpy.concatenate([a,b])

Answer 6

import itertools

A = list(zip([1,3,5,7,9],[2,4,6,8,10]))
B = [1,3,5,7,9]+[2,4,6,8,10]
C = list(set([1,3,5,7,9] + [2,4,6,8,10]))

D = [1,3,5,7,9]
D.append([2,4,6,8,10])

E = [1,3,5,7,9]
E.extend([2,4,6,8,10])

F = []
for a in itertools.chain([1,3,5,7,9], [2,4,6,8,10]):
    F.append(a)


print ("A: " + str(A))
print ("B: " + str(B))
print ("C: " + str(C))
print ("D: " + str(D))
print ("E: " + str(E))
print ("F: " + str(F))

Output:

A: [(1, 2), (3, 4), (5, 6), (7, 8), (9, 10)]
B: [1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
C: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
D: [1, 3, 5, 7, 9, [2, 4, 6, 8, 10]]
E: [1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
F: [1, 3, 5, 7, 9, 2, 4, 6, 8, 10]