How to join list in Python but make the last separator different?

Question

I\'m trying to turn a list into separated strings joined with an ampersand if there are only two items, or commas and an ampersand between the last two e.g.

Answer 1

('{}, '*(len(authors)-2) + '{} & '*(len(authors)>1) + '{}').format(*authors)

This solution can handle a list of authors of length > 0, though it can be modified to handle 0-length lists as well. The idea is to first create a format string that we can format by unpacking list. This solution avoids slicing the list so it should be fairly efficient for large lists of authors.

First we concatenate '{}, ' for every additional author beyond two authors. Then we concatenate '{} & ' if there are two or more authors. Finally we append '{}' for the last author, but this subexpression can be '{}'*(len(authors)>0) instead if we wish to be able to handle an empty list of authors. Finally, we format our completed string by unpacking the elements of the list using the * unpacking syntax.

If you don't need a one-liner, here is the code in an efficient function form.

def format_authors(authors):
    n = len(authors)
    if n > 1:
        return ('{}, '*(n-2) + '{} & {}').format(*authors)
    elif n > 0:
        return authors[0]
    else:
        return ''

This can handle a list of authors of any length.