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'

Answer 2

I am a fairly new pythonista. In the question it was asked that the function return the list contents as a string in the format that the other solutions in this forum have 'printed' it. Following is (in my opinion) a cleaner solution to this problem.

This illustrates the comma code solution of chapter 4 [Lists] in Automate The Boring Stuff.

def comma_code(argument):

    argument_in_string = ''
    argument_len = len(argument)
    for i in range(argument_len):
        if i == (argument_len - 1):
            argument_in_string = argument_in_string + 'and ' + argument[i]
            return argument_in_string

        argument_in_string = argument_in_string + argument[i] + ', '

spam = ['apples', 'bananas', 'tofu', 'cats']
return_value = comma_code(spam)
print(return_value)"

Answer 3

As it was not mentioned, here's an answer with f string, for reference:

def list_things(my_list):
    print(f'{", ".join(my_list[:-1])} and {my_list[-1]}.')

An example interpolating a custom message and accepting also a string as argument:

def like(my_animals = None):
    message = 'The animals I like the most are'
    if my_animals == None or my_animals == '' or len(my_animals) == 0:
        return 'I don\'t like any animals.'
    elif len(my_animals) <= 1 or type(my_animals) == str:
        return f'{message} {my_animals if type(my_animals) == str else my_animals[0]}.'
    return f'{message} {", ".join(my_animals[:-1])} and {my_animals[-1]}.'


>>> like()
>>> like('')
>>> like([])
# 'I don't like any animals.'

>>> like('unicorns') 
>>> like(['unicorns']) 
# 'The animals I like the most are unicorns.'

>>> animals = ['unicorns', 'dogs', 'rabbits', 'dragons']
>>> like(animals) 
# 'The animals I like the most are unicorns, dogs, rabbits and dragons.'

Answer 4

This code works no matter what data types are in the lists, boolean, int, string, float, etc.

def commaCode(spam):
    count = 0
    max_count = len(spam) - 1

    for x in range(len(spam)):
        if count < max_count:
            print(str(spam[count]) + ', ', end='')
            count += 1
        else:
            print('and ' + str(spam[max_count]))

spam1 = ['cat', 'bananas', 'tofu', 'cats']
spam2 = [23, '', True, 'cats']
spam3 = []

commaCode(spam1)
commaCode(spam2)
commaCode(spam3)

Answer 5

def listall(lst):               # everything "returned" is class string
    if not lst:                 # equates to if not True. Empty container is always False
        return 'NONE'           # empty list returns string - NONE
    elif len(lst) < 2:          # single value lists
        return str(lst[0])      # return passed value as a string (do it as the element so 
                                #  as not to return [])
    return (', '.join(str(i) for i in lst[:-1])) + ' and ' + str(lst[-1])
        # joins all elements in list sent, up to last element, with (comma, space) 
        # AND coverts all elements to string. 
        # Then inserts "and". lastly adds final element of list as a string.

This isn't designed to answer the original question. It's to show how to define the function addressing all the matters requested by the book without being complex. I think that's acceptable as the original question posts the books "Comma Code" test. Important Note: Something I found confusing that might help others is. "a list value" means a value of type list or "an entire list" it doesn't mean an individual value (or slices) from "type list". Hope that's helpful

Here are the samples I used to test it:

empty = []
ugh = listall(empty)
print(type(ugh))
print(ugh)
test = ['rabbits', 'dogs', 3, 'squirrels', 'numbers', 3]
ughtest = listall(test)
print(type(ughtest))
print(ughtest)
supertest = [['ra', 'zues', 'ares'],
            ['rabbit'],
            ['Who said', 'biscuits', 3, 'or', 16.71]]
one = listall(supertest[0])
print(type(one))
print(one)
two = listall(supertest[1])
print(type(two))
print(two)
last = listall(supertest[2])
print(type(last))
print(last)

Answer 6

Others have given great one-liner solutions, but a good way to improve your actual implementation - and fix the fact that it does not work when elements are repeated - is to use enumerate in the for loop to keep track of the index, rather than using index which always finds the first occurrence of the target.

for counter, element in enumerate(list):
    new_string = new_string + str(element)
    if counter == (len(list)-2):
        ...