2.4 Default and Keyword Arguments | Code Scholars Tutoring

How it works

Defaults are evaluated once when the function is defined, so a mutable default is shared across every call.

Use an immutable sentinel as the default and create the mutable value inside the body.

Common error

A mutable default argument accumulating values between calls, which is one of the most confusing bugs in the language.

Worked example

Python

def greet(name, prefix="Hi"):
    return f"{prefix}, {name}"

print(greet("Ada"))
print(greet(prefix="Hello", name="Lin"))

Output

Hi, Ada
Hello, Lin

Defaults supply omitted arguments. Keyword arguments make the selected parameter explicit and can improve readability when several arguments have similar types.

Exercises

2.4.1 · Explain

Why is def add_item(item, values=[]) risky across calls?

Show solution

The default list is created once and reused. Mutating it can leak state into later calls. Use a None default and allocate a new list when needed.

2.4.2 · Write code

Write collect(item, values=None) that uses a new list when no list is supplied. Show two independent calls.

Show solution

Allocate the default container inside the function.

One valid solution

def collect(item, values=None):
    if values is None:
        values = []
    values.append(item)
    return values

print(collect(1))
print(collect(2))

Expected output

[1]
[2]

Open Python compiler