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.

Use comments to add documentation to programs.

# This sentence isn't executed by Python.
name = 'Library Carpentry'   # Neither is this comment - anything after '#' is ignored.

A function may take zero or more arguments.

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

after

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

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

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

print(max(1, 'a'))
TypeError: unorderable types: str() > int()

Functions may have default values for some arguments.

round(3.712)
4
round(3.712, 1)
3.7

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

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.

Python reports a syntax error when grammar rules (that’s Python grammar, not English grammar) have been violated.

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

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

Every function returns something.

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?
word = 'blah '
word = max(min(word * 2 + 'blur ', 'aaah '), 'ping')
print(word)

Solution

ping

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)))

Solution

tin
4

Why Not?

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

  • Use comments to add documentation to programs.

  • 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.

  • 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.