How do I concatenate two lists in Python?

Question

How do I concatenate two lists in Python?

Example:

listone = [1, 2, 3]
listtwo = [4, 5, 6]

Expected outcome:

>&g         


        

Answer 1

As already pointed out by many, itertools.chain() is the way to go if one needs to apply exactly the same treatment to both lists. In my case, I had a label and a flag which were different from one list to the other, so I needed something slightly more complex. As it turns out, behind the scenes itertools.chain() simply does the following:

for it in iterables:
    for element in it:
        yield element

(see https://docs.python.org/2/library/itertools.html), so I took inspiration from here and wrote something along these lines:

for iterable, header, flag in ( (newList, 'New', ''), (modList, 'Modified', '-f')):
    print header + ':'
    for path in iterable:
        [...]
        command = 'cp -r' if os.path.isdir(srcPath) else 'cp'
        print >> SCRIPT , command, flag, srcPath, mergedDirPath
        [...]

The main points to understand here are that lists are just a special case of iterable, which are objects like any other; and that for ... in loops in python can work with tuple variables, so it is simple to loop on multiple variables at the same time.