lesson 44 Functions · beginner

Default values and local names

A parameter with a default can be left out of the call. Names a function assigns are local: they exist only during the call and never change the names outside.

You'll be able to

Write a parameter with a default (rate=0.08) and override it by position or by name
Explain that a name assigned inside a function is local to that call
Fix an UnboundLocalError by passing the value in and returning the new one

The idea

Most calls to a tax function use the same rate. A default makes that rate optional: def with_tax(price, rate=0.08):. A call that leaves rate out gets 0.08; a call that passes it, by position or as rate=0.2, overrides it. Parameters with defaults go after the ones without.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Tax with a usual rate

def with_tax(price, rate=0.08):
    return price * (1 + rate)

print(f"{with_tax(50):.2f}")
print(f"{with_tax(50, 0.2):.2f}")
print(f"{with_tax(rate=0, price=50):.2f}")

Each call gets its own fresh set of local names: the parameters and anything the body assigns. They disappear when the call ends, and they never touch a name with the same spelling outside. That isolation is what makes a function safe to call from anywhere.

The flip side surprises everyone once. If the body assigns a name anywhere, that name is local for the whole body, so reading it before the assignment fails even if an outer name exists.

example-2.py · Changing an outer counter

visits = 0

def record_visit():
    visits = visits + 1

record_visit()

example-3.py · Pass in, return out

def record_visit(visits, step=1):
    visits = visits + step
    return visits

total = 0
visits = 10
total = record_visit(total)
total = record_visit(total, 5)
print(total, visits)

When to use it: give a default to a parameter that has a value most callers want (a standard rate, a usual threshold, a separator). Keep every working name local, and move data in through parameters and out through return. Mistakes to avoid:

Putting a defaulted parameter before one without a default: that is a SyntaxError.
Expecting an assignment inside a function to change an outer name.
Using a list or dictionary as a default value: the same object is shared by every call. Use a number, text or None as a default.

Review

def f(x, rate=0.08): makes rate optional; a call can override it by position or by name.
Names assigned inside a function are local to the call and never change outer names.
Move data in with parameters and out with return instead of changing outer names.
Next: document what a function promises, and build bigger functions from small ones.

Practise this