Automate the boring stuff with Python: Comma Code

Question

Currently working my way through this beginners book and have completed one of the practice projects \'Comma Code\' which asks the user to construct a program which:

Answer 1

Use str.join() to join a sequence of strings with a delimiter. If you do so for all words except for the last, you can insert ' and ' there instead:

def list_thing(words):
    if len(words) == 1:
        return words[0]
    return '{}, and {}'.format(', '.join(words[:-1]), words[-1])

Breaking this down:

• words[-1] takes the last element of a list. words[:-1] slices the list to produce a new list with all words except the last one.

• ', '.join() produces a new string, with all strings of the argument to str.join() joined with ', '. If there is just one element in the input list, that one element is returned, unjoined.

• '{}, and {}'.format() inserts the comma-joined words and the last word into a template (complete with Oxford comma).

If you pass in an empty list, the above function will raise an IndexError exception; you could specifically test for that case in the function if you feel an empty list is a valid use-case for the function.

So the above joins all words except the last with ', ', then adds the last word to the result with ' and '.

Note that if there is just one word, you get that one word; there is nothing to join in that case. If there are two, you get 'word1 and word 2'. More words produces 'word1, word2, ... and lastword'.

Demo:

>>> def list_thing(words):
...     if len(words) == 1:
...         return words[0]
...     return '{}, and {}'.format(', '.join(words[:-1]), words[-1])
...
>>> spam = ['apples', 'bananas', 'tofu', 'cats']
>>> list_thing(spam[:1])
'apples'
>>> list_thing(spam[:2])
'apples, and bananas'
>>> list_thing(spam[:3])
'apples, bananas, and tofu'
>>> list_thing(spam)
'apples, bananas, tofu, and cats'