Question

Working my way through Learn Python the Hard Way ex.25, and I just can’t wrap my head around something. Here’s the script:

def break_words(stuff):
    """this function will break waords up for us."""
    words = stuff.split(' ')
    return words

def sort_words(words):
    """Sorts the words."""
    return sorted(words)

def print_first_word(words):
    """Prints the first word after popping it off."""
    word = words.pop(0)
    print word

def print_last_word(words):
    """Prints the last word after popping it off."""
    word = words.pop(-1)
    print word

def sort_sentence(sentence):
    """Takes in a full sentence and returns the sorted words."""
    words = break_words(sentence)
    return sort_words(words)

def print_first_and_last(sentence):
    """Prints the first and last words of the sentence."""
    words = break_words(sentence)
    print_first_word(words)
    print_last_word(words)

def print_first_and_last_sorted(sentence):
    """Sorts the words, then prints the first and last ones."""
    words = sort_sentence(sentence)
    print_first_word(words)
    print_last_word(words)

When running the script, break_words will use any argument I create if I use the command break_words(**). So I can type

sentence = "My balogna has a first name, it's O-S-C-A-R"

and then run break_words(sentence) and end up with a parsed “‘My’ ‘balogna’ ‘has’ (…).

But other functions (like sort_words) will only accept a function with the name “words.” I must type
words = break_words(sentence)

or something for sort_words to work.

Why can I pass any argument in the parentheses for break_words, but only arguments that are actually attributed to “sentence” and “words” specifically for sort_words, print_first_and_last, etc.? I feel like this is something fundamental that I should understand before I move on in the book, and I just can’t get my head around it.

Answer 1

It’s about the type of value that each function accepts as its parameter.

break_words returns a list. sort_words uses the built-in function sorted(), which expects to be passed a list. This means that the parameter you pass to sort_words should be a list.

Maybe the following example illustrates this:

>>> sort_words(break_words(sentence))
['My', 'O-S-C-A-R', 'a', 'balogna', 'first', 'has', "it's", 'name,']

Note that python defaults to being helpful even though this can at times be confusing. So if you pass a string to sorted(), it will treat it as a list of characters.

>>> sorted("foo bar wibble")
[' ', ' ', 'a', 'b', 'b', 'b', 'e', 'f', 'i', 'l', 'o', 'o', 'r', 'w']
>>> sorted(["foo", "bar", "wibble"])
['bar', 'foo', 'wibble']