Inside a function, every line should sit at the same level of detail. A function that calls charge(card) and then slices a postcode by hand mixes the plan with its details, and the reader has to keep changing zoom.

Write the top function as the steps of the plan, each one a call to a well-named helper, and push the details down into those helpers. Read from the top, the file then goes from overview to detail.

def make_slug(title):
    words = clean_words(title)
    return "-".join(words)

def clean_words(title):
    words = title.lower().split()
    return [word for word in words
            if word.isalnum()]

print(make_slug("Clean Code in Python"))

make_slug says what happens and clean_words says how. Python looks a helper up when the caller runs, so helpers can sit below the function that uses them.

If a step needs a comment to explain it, it is usually a helper waiting for a name.

Rewrite signup(form) as three calls: clean_email(text) (stripped, lower case, ValueError unless there is text on both sides of an @), clean_name(text) (capitalized words, ValueError if empty) and greeting(name). signup itself must not call string methods, and it returns the same dict as before.

The explanation opens here once every check passes.