Built-in Functions and Help

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I use built-in functions?

• How can I find out what they do?

Objectives

• Explain the purpose of functions.

• Correctly call built-in Python functions.

• Correctly nest calls to built-in functions.

• Use help to display documentation for built-in functions.

A function may take zero or more arguments.

• We have seen some functions already — now let’s take a closer look.
• An argument is a value passed into a function.
• len takes exactly one.
• int, str, and float create a new value from an existing one.
• print takes zero or more.
• print with no arguments prints a blank line.
  • Must always use parentheses, even if they’re empty, so that Python knows a function is being called.

print('before')
print()
print('after')

before

after

Commonly-used built-in functions include max, min, and round.

• Use max to find the largest value of one or more values.
• Use min to find the smallest.
• Both work on character strings as well as numbers.
  • “Larger” and “smaller” use (0-9, A-Z, a-z) to compare letters.

print(max(1, 2, 3))
print(min('a', 'A', '0'))

3
0

Functions may only work for certain (combinations of) arguments.

• max and min must be given at least one argument.
  • “Largest of the empty set” is a meaningless question.
• And they must be given things that can meaningfully be compared.

print(max(1, 'a'))

TypeError: unorderable types: str() > int()

Functions may have default values for some arguments.

• round will round off a floating-point number.
• By default, rounds to zero decimal places.

round(3.712)

4

• We can specify the number of decimal places we want.

round(3.712, 1)

3.7

Use the built-in function help to get help for a function.

• Every built-in function has online documentation.

help(round)

Help on built-in function round in module builtins:

round(...)
    round(number[, ndigits]) -> number

    Round a number to a given precision in decimal digits (default 0 digits).
    This returns an int when called with one argument, otherwise the
    same type as the number. ndigits may be negative.

The Jupyter Notebook has two ways to get help.

• Place the cursor inside the parenthesis of the function, hold down shift, and press tab.
• Or type a function name with a question mark after it.

Every function returns something.

• Every function call produces some result.
• If the function doesn’t have a useful result to return, it usually returns the special value None.

result = print('example')
print('result of print is', result)

example
result of print is None

What Happens When

1. Explain in simple terms the order of operations in the following program: when does the addition happen, when does the subtraction happen, when is each function called, etc.
2. What is the final value of radiance?

radiance = 1.0
radiance = max(2.1, 2.0 + min(radiance, 1.1 * radiance - 0.5))

Spot the Difference

1. Predict what each of the print statements in the program below will print.
2. Does max(len(rich), poor) run or produce an error message? If it runs, does its result make any sense?

rich = "gold"
poor = "tin"
print(max(rich, poor))
print(max(len(rich), len(poor)))

Why Not?

Why don’t max and min return None when they are given no arguments?

Key Points

• A function may take zero or more arguments.

• Commonly-used built-in functions include max, min, and round.

• Functions may only work for certain (combinations of) arguments.

• Functions may have default values for some arguments.

• Use the built-in function help to get help for a function.

• The Jupyter Notebook has two ways to get help.

• Every function returns something.