2.1 Parameters and Arguments | Code Scholars Tutoring

How it works

Python passes references to objects. Rebinding a parameter inside a function does not affect the caller, but mutating a mutable argument does.

Decide for each parameter whether the function may modify it, and document that.

Common error

Mutating a list argument unintentionally, which changes the caller data invisibly.

Worked example

Python

def scale(value, factor):
    return value * factor

print(scale(4, 3))

Output

Arguments in the call supply the values used by local parameter names. Reusing the function with different arguments avoids duplicating the calculation.

Exercises

2.1.1 · Trace

Determine the exact output and explain the state changes.

Python

def change(values):
    values.append(9)

data = [1]
change(data)
print(data)

Show solution

Output

[1, 9]

The local parameter refers to the caller list, so mutation is visible.

2.1.2 · Write code

Write add_tax(price, rate) and show a price of 100 at rate 0.08.

Show solution

Return the price multiplied by one plus the rate.

One valid solution

def add_tax(price, rate):
    return price * (1 + rate)

print(add_tax(100, 0.08))

Expected output

108.0

Open Python compiler