[python] How to unnest a nested list

Possible Duplicate:
Making a flat list out of list of lists in Python

I am trying to find an easy way to turn a multi dimensional (nested) python list into a single list, that contains all the elements of sub lists.

For example:

A = [[1,2,3,4,5]]

turns to

A = [1,2,3,4,5]

or

A = [[1,2], [3,4]]

turns to

A = [1,2,3,4]

Use itertools.chain:

itertools.chain(*iterables):

Make an iterator that returns elements from the first iterable until it is exhausted, then proceeds to the next iterable, until all of the iterables are exhausted. Used for treating consecutive sequences as a single sequence.

Example:

from itertools import chain

A = [[1,2], [3,4]]

print list(chain(*A))
# or better: (available since Python 2.6)
print list(chain.from_iterable(A))

The output is:

[1, 2, 3, 4]
[1, 2, 3, 4]

Use reduce function

reduce(lambda x, y: x + y, A, [])

Or sum

sum(A, [])

itertools provides the chain function for that:

From http://docs.python.org/library/itertools.html#recipes:

def flatten(listOfLists):
    "Flatten one level of nesting"
    return chain.from_iterable(listOfLists)

Note that the result is an iterable, so you may need list(flatten(...)).

the first case can also be easily done as:

A=A[0]