Built-in Functions and Help

Overview

Teaching: 20 min
Exercises: 15 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.

• We have seen some functions already — now let’s take a closer look.
• An argument is a value passed into a function.
• Any arguments you want to pass into a function must go into the ()
  • print("I am an argument and must go here.")
• You must always use parentheses, because this is how Python knows you are calling a function.
  • You leave them empty if you don’t want or need to pass any arguments in.
• len takes exactly one.
• int, str, and float create a new value from an existing one.
• print takes zero or more.
  • print() prints a blank line.

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.
  • This means that:
    • 'a' is smaller than 'b'
    • 'A' is smaller than 'a'
    • '0' is smaller than 'a'
  • This is useful for ordering alphabetically.

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.

• max and min must be given at least one argument.
• 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.

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

• You’ve seen errors when you try to use a function incorrectly.
  • Can also have errors when you use punctuation incorrectly.
• Python will run the program up until that point, but if the grammar of that line of code has produced an error, then the program will shut down with an error.

# 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 -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.

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 word?

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?

richer = "gold"
poorer = "tin"
print(max(richer, poorer))
print(max(len(richer), len(poorer)))

Solution

tin
4
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

Both functions require an argument to execute

print(max())

TypeError: max expected 1 arguments, got 0

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.