Using functions well

What you will be able to do

After this lesson you can

• give a parameter a default value,
• name arguments in a call,
• write a function that answers yes or no, and
• build a function out of other functions.

Default values

A parameter can have a default value. If the call gives no argument for it, Python uses the default. This is useful when one value is usual, and you want to change it only now and then.

One rule: the parameters with defaults come last. def power(exponent=2, base): is a SyntaxError.

Named arguments

You can name the arguments in a call. The names make a call easy to read, and they let you skip a parameter that has a default.

You have seen this form already: print("a", "b", sep="-") and sort(reverse=True) use named arguments.

A default value makes the common case short, and still lets you choose.

Functions that answer a question

A function can return True or False. Look at the second part of the earlier program: is_even(n) returns the result of n % 2 == 0. A name that starts with is_ (or has_) tells the reader that the function answers yes or no. You can then write natural lines: if is_even(n):.

Here is a style tip. A beginner often writes

def is_even(n):
    if n % 2 == 0:
        return True
    else:
        return False

This is correct, but it is longer than needed. The condition n % 2 == 0 is already True or False, so return n % 2 == 0 says the same in one line.

One function can use another

describe calls is_even inside its body. Small functions that do one job each are easy to understand and to test, and they can be put together like building blocks. A big problem is then a list of small ones.

A guide: if you can describe what a function does without using the word "and", it does one job. "It finds the area" is one job. "It finds the area and prints a report and saves a file" is three.

A function should return in every path

If a function has an if that returns, think about what happens when the condition is False. If no return is reached, the function gives back None, and a later sum fails. The usual cure: put a final return at the end for the other case, as in describe.

Common mistakes

• A parameter with a default before one without. A SyntaxError.
• Forgetting the order of the parameters in a call without names.
• if x == True: when x is already a yes/no value. Write if x:.
• A function that does too many jobs. Split it.
• A path with no return. The answer is None.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Yes or no functions. You write two functions that give True or False, and the checker tries each on many values.
2. Build. Open the problem A yes or no function and the problem A default value.
3. Stretch. Open the problem One function inside another, and build a function that uses two others.

Remember these

• def power(base, exponent=2): gives exponent a default. Parameters with defaults come last.
• Name the arguments to make a call clear, and to skip a default.
• A name that starts with is_ tells the reader that the function gives True or False.
• Small functions that do one job can call each other.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is power(3) with a default exponent of 2?

2. Write is_even in one line.

3. How do you call make_tag with only a new school?