Python · Functions: arguments and return values

def name(params): defines a function, and a docstring right under the def line documents it. If a function has no return, it returns None.

Arguments can be passed by position or by keyword (f(x=1)), and parameters can have default values.

The classic trap: default values are evaluated once, when the def runs. A mutable default like [] is therefore shared by every call. The fix is to default to None and create the list inside the function.

return a, b returns a single tuple, and the caller unpacks it.

def grade(marks, total=100):
    """Return percentage and letter grade."""
    pct = 100 * marks / total
    letter = "A" if pct >= 85 else "B" if pct >= 70 else "C"
    return pct, letter

p, g = grade(88)
print(p, g)
print(grade(total=50, marks=40))

def bad_append(x, items=[]):
    items.append(x)
    return items

def good_append(x, items=None):
    if items is None:
        items = []
    items.append(x)
    return items

print(bad_append(1), bad_append(2))    # shared list!
print(good_append(1), good_append(2))
print(grade.__doc__)

Output:

88.0 A
(80.0, 'B')
[1, 2] [1, 2]
[1] [2]
Return percentage and letter grade.