lesson 72 Advanced Functions & Recursion · intermediate

Options that must be named

Parameters after a bare `*` can only be passed by name. Calls like `convert(100, rate=0.92)` stay readable, and a mixed-up argument order becomes an error instead of a wrong answer.

You'll be able to

Write def f(data, *, option) so option must be passed by name
Combine keyword-only parameters with defaults
Read the TypeError a positional call produces

The idea

Compare export(rows, True, False) with export(rows, header=True, overwrite=False). The first call is a puzzle: which True is which? And if someone swaps them, the program silently does the wrong thing.

A bare * in the parameter list says: everything after this must be passed by name. It collects nothing; it only draws the line.

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

example-1.py · A rate passed by position

def convert(amount, *, rate):
    return amount * rate

print(convert(100, 0.92))

example-2.py · Currency conversion with a named rate

def convert(amount, *, rate, fee=0):
    return amount * rate - fee

print(convert(100, rate=0.92))
print(convert(100, rate=0.92, fee=1.5))
print(convert(100, fee=1.5, rate=0.92))

Keyword-only parameters suit settings: numbers that would be easy to mix up when they sit side by side. A retry schedule is a good example. After a failed request, a client waits base, then twice that, then four times that, never longer than cap. Three numbers in a row would be easy to swap; named, they are self-explaining.

example-3.py · Retry delays, every setting named

def backoff_delays(*, attempts, base, cap):
    return [min(base * 2 ** i, cap) for i in range(attempts)]

print(backoff_delays(attempts=5, base=1, cap=10))
print(backoff_delays(attempts=3, base=0.5, cap=60))

When to use it: flags (dry_run=True), and settings of the same type that could be confused (attempts, base, cap). Keep the main data positional: convert(100, rate=…) reads naturally. Mistakes to avoid: writing *args when you only wanted the dividing line (that would also swallow extra positional arguments silently), and putting a keyword-only parameter without a default where most calls want the same value.

Review

Parameters after a bare * must be passed by name; a positional call raises TypeError.
Use keyword-only parameters for flags and for settings that could be mixed up.
Keyword-only parameters can have defaults, and named arguments can come in any order.
Next: lambda, a short function written right where it is passed.

Practise this

How the arguments are collected