Built-in Functions and Help

Overview

Teaching: 15 min
Exercises: 10 min

Questions

• How can I use built-in functions?

• How can I find out what they do?

• What kind of errors can occur in programs?

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.

• Correctly describe situations in which SyntaxError and NameError occur.

Python has ~70 built in functions

• Built in functions are immediately available for use by the python interpreter
• We have already seen some of these (e.g., print and type)
• Others include min, int, len, round, open, del, and help
• Functions allow us to ‘encapsulate’ some well described behavior, hiding the underlying code so we can just focus on the input and output

A function may take zero or more arguments

• An argument is a value passed into a function, placed between parentheses
• len takes exactly one argument, and returns its length
• int, str, and float create a new value from an existing one
• print takes zero or more arguments
• 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

Use len to find the length of a string

print(len('helium'))

6

• Note: nested functions are evaluated from the inside out, just like in mathematics.

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

• len does not work when passed in a number because the result would be ambiguous. For example, what is the ‘length’ of 10?

print(len(10))

TypeError: object of type 'int' has no len()

• 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 additional 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.

Python reports a syntax error when it can’t understand the source of a program

• Won’t even try to run the program if it can’t be parsed.

# Forgot to close the quotation marks around the string.
name = 'Feng

SyntaxError: EOL while scanning string literal

# An extra '=' in the assignment.
age = = 52

SyntaxError: invalid syntax

• Look more closely at the error message:

print("hello world"

  File "<ipython-input-6-d1cc229bf815>", line 1
    print ("hello world"
                        ^
SyntaxError: unexpected EOF while parsing

• The message indicates a problem on first line of the input (“line 1”).
  • In this case the “ipython-input” section of the file name tells us that we are working with input into IPython, the Python interpreter used by the Jupyter Notebook.
• The -6- part of the filename indicates that the error occurred in cell 6 of our Notebook.
• Next is the problematic line of code, indicating the problem with a ^ pointer.

Python reports a runtime error when something goes wrong while a program is executing

age = 53
remaining = 100 - aege # mis-spelled 'age'

NameError: name 'aege' is not defined

• Fix syntax errors by reading the source and runtime errors by tracing execution.

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?

Solution

Consider that None signifies the successful execution of a function that doesn’t otherwise return a value. Such a response would be nonsensical, because no comparison is possible on an empty set of arguments.

Last Character of a String

If Python starts counting from zero, and len returns the number of characters in a string, what index expression will get the last character in the string name? (Note: we will see a simpler way to do this in a later episode.)

Solution

name[len(name) - 1]

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.

• Python reports a syntax error when it can’t understand the source of a program.

• Python reports a runtime error when something goes wrong while a program is executing.

• Fix syntax errors by reading the source code, and runtime errors by tracing the program’s execution.