join list of lists in python [duplicate]

Answer 1

If you need a list, not a generator, use list():

from itertools import chain
x = [["a","b"], ["c"]]
y = list(chain(*x))

Answer 2

Or a recursive operation:

def flatten(input):
    ret = []
    if not isinstance(input, (list, tuple)):
        return [input]
    for i in input:
        if isinstance(i, (list, tuple)):
            ret.extend(flatten(i))
        else:
            ret.append(i)
    return ret

Answer 3

import itertools
a = [['a','b'], ['c']]
print(list(itertools.chain.from_iterable(a)))

Answer 4

If you're only going one level deep, a nested comprehension will also work:

>>> x = [["a","b"], ["c"]]
>>> [inner
...     for outer in x
...         for inner in outer]
['a', 'b', 'c']

On one line, that becomes:

>>> [j for i in x for j in i]
['a', 'b', 'c']

Answer 5

x = [["a","b"], ["c"]]

result = sum(x, [])

Answer 6

This is known as flattening, and there are a LOT of implementations out there:

• More on python flatten
• Python tricks
• Flattening lists in Python

How about this, although it will only work for 1 level deep nesting:

>>> x = [["a","b"], ["c"]]
>>> for el in sum(x, []):
...     print el
...
a
b
c

From those links, apparently the most complete-fast-elegant-etc implementation is the following:

def flatten(l, ltypes=(list, tuple)):
    ltype = type(l)
    l = list(l)
    i = 0
    while i < len(l):
        while isinstance(l[i], ltypes):
            if not l[i]:
                l.pop(i)
                i -= 1
                break
            else:
                l[i:i + 1] = l[i]
        i += 1
    return ltype(l)