How do I merge a 2D array in Python into one string with List Comprehension?

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

Like so:

[ item for innerlist in outerlist for item in innerlist ]

Turning that directly into a string with separators:

','.join(str(item) for innerlist in outerlist for item in innerlist)

Yes, the order of 'for innerlist in outerlist' and 'for item in innerlist' is correct. Even though the "body" of the loop is at the start of the listcomp, the order of nested loops (and 'if' clauses) is still the same as when you would write the loop out:

for innerlist in outerlist:
    for item in innerlist:
        ...