join list of lists in python

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

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

result = sum(x, [])

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']

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

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)

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

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

This works recursively for infinitely nested elements:

def iterFlatten(root):
    if isinstance(root, (list, tuple)):
        for element in root:
            for e in iterFlatten(element):
                yield e
    else:
        yield root

Result:

>>> b = [["a", ("b", "c")], "d"]
>>> list(iterFlatten(b))
['a', 'b', 'c', 'd']

Late to the party but ...

I'm new to python and come from a lisp background. This is what I came up with (check out the var names for lulz):

def flatten(lst):
    if lst:
        car,*cdr=lst
        if isinstance(car,(list,tuple)):
            if cdr: return flatten(car) + flatten(cdr)
            return flatten(car)
        if cdr: return [car] + flatten(cdr)
        return [car]

Seems to work. Test:

flatten((1,2,3,(4,5,6,(7,8,(((1,2)))))))

returns:

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

What you're describing is known as flattening a list, and with this new knowledge you'll be able to find many solutions to this on Google (there is no built-in flatten method). Here is one of them, from http://www.daniel-lemire.com/blog/archives/2006/05/10/flattening-lists-in-python/:

def flatten(x):
    flat = True
    ans = []
    for i in x:
        if ( i.__class__ is list):
            ans = flatten(i)
        else:
            ans.append(i)
    return ans

There's always reduce (being deprecated to functools):

>>> x = [ [ 'a', 'b'], ['c'] ]
>>> for el in reduce(lambda a,b: a+b, x, []):
...  print el
...
__main__:1: DeprecationWarning: reduce() not supported in 3.x; use functools.reduce()
a
b
c
>>> import functools
>>> for el in functools.reduce(lambda a,b: a+b, x, []):
...   print el
...
a
b
c
>>>

Unfortunately the plus operator for list concatenation can't be used as a function -- or fortunate, if you prefer lambdas to be ugly for improved visibility.

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

Sadly, Python doesn't have a simple way to flatten lists. Try this:

def flatten(some_list):
    for element in some_list:
        if type(element) in (tuple, list):
            for item in flatten(element):
                yield item
        else:
            yield element

Which will recursively flatten a list; you can then do

result = []
[ result.extend(el) for el in x] 

for el in flatten(result):
      print el