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.
adjustment = 0.5   # Neither is this - anything after '#' is ignored.

A function may take zero or more arguments.

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

after

Every function returns something.

result = print('example')
print('result of print is', result)
example
result of print is None

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

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

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

print(max(1, 'a'))
TypeError                                 Traceback (most recent call last)
<ipython-input-52-3f049acf3762> in <module>
----> 1 print(max(1, 'a'))

TypeError: '>' not supported between instances of 'str' and 'int'

Functions may have default values for some arguments.

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

Functions attached to objects are called methods

my_string = 'Hello world!'  # creation of a string object 

print(len(my_string))       # function with the string as an argument

print(my_string.__len__())  # method acting upon the string object
12
12
print(my_string.isupper())          # Not all the letters are uppercase
print(my_string.upper())            # This capitalizes all the letters

print(my_string.upper().isupper())  # Now all the letters are uppercase
False
HELLO WORLD
True

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

help(round)
Help on built-in function round in module builtins:

round(number, ndigits=None)
    Round a number to a given precision in decimal digits.
    
    The return value is an integer if ndigits is omitted or None.  Otherwise
    the return value has the same type as the number.  ndigits may be negative.

The Jupyter Notebook has two ways to get help.

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

# Forgot to close the quote marks around the string.
name = 'Feng
  File "<ipython-input-56-f42768451d55>", line 2
    name = 'Feng
                ^
SyntaxError: EOL while scanning string literal
# An extra '=' in the assignment.
age = = 52
  File "<ipython-input-57-ccc3df3cf902>", line 2
    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                                 Traceback (most recent call last)
<ipython-input-59-1214fb6c55fc> in <module>
      1 age = 53
----> 2 remaining = 100 - aege # mis-spelled 'age'

NameError: name 'aege' is not defined

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))

Solution

  1. Order of operations:
    1. 1.1 * radiance = 1.1
    2. 1.1 - 0.5 = 0.6
    3. min(radiance, 0.6) = 0.6
    4. 2.0 + 0.6 = 2.6
    5. max(2.1, 2.6) = 2.6
  2. At the end, radiance = 2.6

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

Solution

print(max(easy_string))
c
print(max(rich, poor))
tin
print(max(len(rich), len(poor)))
4

max(len(rich), poor) throws a TypeError. This turns into max(4, 'tin') and as we discussed earlier a string and integer cannot meaningfully be compared.

TypeError                                 Traceback (most recent call last)
<ipython-input-65-bc82ad05177a> in <module>
----> 1 max(len(rich), poor)

TypeError: '>' not supported between instances of 'str' and 'int'

Why Not?

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

Solution

max and min return TypeErrors in this case because the correct number of parameters was not supplied. If it just returned None, the error would be much harder to trace as it would likely be stored into a variable and used later in the program, only to likely throw a runtime error.

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.

  • 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.