How do I merge a 2D array in Python into one strin

Question

List Comprehension for me seems to be like the opaque block of granite that regular expressions are for me. I need pointers.

Say, I have a 2D list:

li = [[0,1,2],[3,4,5],[6,7,8]]

I would like to merge this either into one long list

li2 = [0,1,2,3,4,5,6,7,8]

or into a string with separators:

s = "0,1,2,3,4,5,6,7,8"

Really, I'd like to know how to do both.

Answer 1

Try that:

li=[[0,1,2],[3,4,5],[6,7,8]]
li2 = [ y for x in li for y in x]

You can read it like this:
Give me the list of every ys.
The ys come from the xs.
The xs come from li.

To map that in a string:

','.join(map(str,li2))

Answer 2

To make it a flattened list use either:

1. http://code.activestate.com/recipes/121294/
2. http://code.activestate.com/recipes/363051/

Then, join to make it a string.

Answer 3

My favorite, and the shortest one, is this:

li2 = sum(li, [])

and

s = ','.join(li2)

EDIT: use sum instead of reduce, (thanks Thomas Wouters!)

Answer 4

import itertools
itertools.flatten( li )

Answer 5

There's a couple choices. First, you can just create a new list and add the contents of each list to it:

li2 = []
for sublist in li:
    li2.extend(sublist)

Alternately, you can use the itertools module's chain function, which produces an iterable containing all the items in multiple iterables:

import itertools
li2 = list(itertools.chain(*li))

If you take this approach, you can produce the string without creating an intermediate list:

s = ",".join(itertools.chain(*li))

Answer 6

For the second one, there is a built-in string method to do that :

>>> print ','.join(str(x) for x in li2)
"0,1,2,3,4,5,6,7,8"

For the first one, you can use join within a comprehension list :

>>> print ",".join([",".join(str(x) for x in li])
"0,1,2,3,4,5,6,7,8"

But it's easier to use itertools.flatten :

>>> import itertools
>>> print itertools.flatten(li)
[0,1,2,3,4,5,6,7,8]
>>> print ",".join(str(x) for x in itertools.flatten(li))
"0,1,2,3,4,5,6,7,8"

N.B : itertools is a module that help you to deal with common tasks with iterators such as list, tuples or string... It's handy because it does not store a copy of the structure you're working on but process the items one by one.

EDIT : funny, I am learning plenty of way to do it. Who said that there was only one good way to do it ?