Functions, default arguments and *args

Writing functions properly, plus the mutable-default trap that silently corrupts data.

12 min read · Lesson 5 of 6 · Pro

Why functions exist

Not to save typing. To give a chunk of logic a name, so that the rest of the program reads like a description of what it does instead of how.

Python 3

def percentage(marks, total=100):
    """Return marks as a percentage of total, rounded to 2 places."""
    return round(marks / total * 100, 2)

print(percentage(72))          # 72.0
print(percentage(145, 200))    # 72.5

total=100 is a default argument. If the caller does not supply it, 100 is used. Defaults must come after non-default parameters, so def f(a=1, b) is a syntax error.

Positional and keyword arguments

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