How do I concatenate two lists in Python?

You can use the + operator to combine them:

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

joinedlist = listone + listtwo

Output:

>>> joinedlist
[1,2,3,4,5,6]

Python >= 3.5 alternative: [*l1, *l2]

Another alternative has been introduced via the acceptance of PEP 448 which deserves mentioning.

The PEP, titled Additional Unpacking Generalizations, generally reduced some syntactic restrictions when using the starred * expression in Python; with it, joining two lists (applies to any iterable) can now also be done with:

>>> l1 = [1, 2, 3]
>>> l2 = [4, 5, 6]
>>> joined_list = [*l1, *l2]  # unpack both iterables in a list literal
>>> print(joined_list)
[1, 2, 3, 4, 5, 6]

This functionality was defined for Python 3.5 it hasn't been backported to previous versions in the 3.x family. In unsupported versions a SyntaxError is going to be raised.

As with the other approaches, this too creates as shallow copy of the elements in the corresponding lists.

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The upside to this approach is that you really don't need lists in order to perform it, anything that is iterable will do. As stated in the PEP:

This is also useful as a more readable way of summing iterables into a list, such as my_list + list(my_tuple) + list(my_range) which is now equivalent to just [*my_list, *my_tuple, *my_range].

So while addition with + would raise a TypeError due to type mismatch:

l = [1, 2, 3]
r = range(4, 7)
res = l + r

The following won't:

res = [*l, *r]

because it will first unpack the contents of the iterables and then simply create a list from the contents.

It's also possible to create a generator that simply iterates over the items in both lists using itertools.chain(). This allows you to chain lists (or any iterable) together for processing without copying the items to a new list:

import itertools
for item in itertools.chain(listone, listtwo):
    # Do something with each list item