A default is useful when there's one common case. Squaring is the common case of exponentiation:

def power(base, exp=2):
    return base ** exp


print(power(5))       # 25   — square, default
print(power(2, 10))   # 1024 — explicit exponent

Two interesting cases: power(5, 0) returns 1 (any number to the power of zero), and power(5, -1) returns 0.2 — a negative exponent turns the expression into a fraction, so the result is a float and not an int. There's no error here.

An important warning to close out this topic: a default value is computed only once, the moment Python reads the def line — not again on every call. With a number or a string this goes unnoticed, but with a list it creates a bug:

def add_item(item, cart=[]):   # don't do this
    cart.append(item)
    return cart


print(add_item("milk"))    # ['milk']
print(add_item("bread"))   # ['milk', 'bread'] — not ['bread']!

The two calls shared the same list. The standard fix: the default is None, and the list is created inside the function:

def add_item(item, cart=None):
    if cart is None:
        cart = []
    cart.append(item)
    return cart

The rule to remember: a default value is always a number, string, True/False, or None — never a list, dictionary, or set.

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Error type: NameError
What name wasn't found: 'name'
Where's the problem: line 3

Lesson completed!