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.

• 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.

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 quote 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.

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

Exercises on slides

• What happens when
• Use of functions

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.