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

It looks like while I was working on my answer, someone may have beaten me to the punch with a similar one. Here's mine for comparison. Note that this also handles cases of 0, 1, or 2 members in the list.

# Python 3.x, should also work with Python 2.x.
def my_join(my_list):
    x = len(my_list)
    if x > 2:
        s = ', & '.join([', '.join(my_list[:-1]), my_list[-1]])
    elif x == 2:
        s = ' & '.join(my_list)
    elif x == 1:
        s = my_list[0]
    else:
        s = ''
    return s

assert my_join(['Jim', 'Jack', 'John']) == 'Jim, Jack, & John'
assert my_join(['Jim', 'Jack']) == 'Jim & Jack'
assert my_join(['Jim',]) == 'Jim'
assert my_join([]) == ''

Answer 2

('{}, '*(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.

Answer 3

Here is a one line example that handles all the edge cases (empty list, one entry, two entries):

' & '.join(filter(None, [', '.join(my_list[:-1])] + my_list[-1:]))

The filter() function is used to filter out the empty entries that happens when my_list is empty or only has one entry.

Answer 4

Here's a simple one that also works for empty or 1 element lists:

' and '.join([', '.join(mylist[:-1])]+mylist[-1:])

The reason it works is that for empty lists both [:-1] and [-1:] give us an empty list again

Answer 5

You could break this up into two joins. Join all but the last item with ", ". Then join this string and the last item with " & ".

all_but_last = ', '.join(authors[:-1])
last = authors[-1]

' & '.join([all_but_last, last])

Note: This doesn't deal with edge cases, such as when authors is empty or has only one element.

Answer 6

"&".join([",".join(my_list[:-1]),my_list[-1]])

I would think would work

or maybe just

",".join(my_list[:-1]) +"&"+my_list[-1]

to handle edge cases where only 2 items you could

"&".join([",".join(my_list[:-1]),my_list[-1]] if len(my_list) > 2 else my_list)