1. Using Modules

2. Today's Goal Introduce important concepts: Functions Classes Objects
Methods
Modules

3. Python Standard Library
Python consists of two parts:
Language
Standard Library
The Python Standard Library contains code you can use in your programs to do things like:
Compute mathematical functions
Perform network communication
Access files
Draw with turtles

4. Standard Library Organization
Code in the Standard Library is organized into Modules
A module is a collection of
Variables
Functions
Classes
Python Standard Library
math pi e
sin()
cos()
sqrt()
random
randrange()
turtle
class Turtle
class Screen

5. Functions
A function is a named list of statements that performs a task.
You've used functions like:
print()
randrange() [in the random module]
The library contains many functions.
Today, you'll start learning to write your own.
def add(x, y): sum = x + y return sum

6. Classes and Data Types Python has a few built-in data types.
Can you recall some?
These data types are defined by classes in the Python Standard Library.
A class is Python's mechanism for defining data types.
The classes in the Standard Library define data types that you can use in your programs.

7. Standard Library Documentation
docs.python.org describes contents of Standard Library
Access the correct version
Navigate to Library Reference
We'll explore it today.

8. Using Modules
Use the import statement to make the contents of a module available to your program.
After importing a module, use its functions and variables by prefixing them with the name of the module.
import math radius = 3.5 area = math.pi * (radius ** 2) sqrtOf4 = math.sqrt(4)

9. Functions

10. Using Functions To use a function, you write a statement like this:
print("Hello")
or this:
x = math.sqrt(3)
We call this a function invocation or function call
To execute this statement, the Python interpreter
Locates the definition of the function
Executes the statements in the function

11. Using Functions A function is like a miniature program:
It needs input to operate on
It performs some calculation using the input
It produces a result

12. Function Input and Output
Functions usually get their information using parameters supplied by your program
A parameter is an expression used in a function call
Parameters are also called arguments
Functions often return a result that you
store in a variable
use as a parameter to another function
Example: round() is a built-in function that rounds a float to the nearest int
y = 3.4
x = round(y + .3)
output
stored in x
input parameter

13. Function Evaluation To execute x = round(y + .3), Python
Evaluates parameter expression y + .3
Invokes round() function, passing in value 3.7
round() executes, producing result (4)
Stores the result in x
y + .3
3.7
round() 4 x

14. Aside: print() is an atypical function
It sends its output to the screen (most functions don't)
It does not return a value to be used in an expression

15. Built-in Functions
The Python Standard Library contains several "built-in" functions
You don't have to import a module to use these:

16. Let's Practice Meet the len() function: Suppose we have a variable:
name = "Fred"
Write a statement to invoke the len() function to determine the length of name

17. Let's Survey Some Modules
math module
os module
Variables pi e
Functions
fabs()
sqrt()
sin()
Activity: Survey Library Documentation
Variables
name
Functions
getlogin()
getcwd()

18. Function Documentation
Function documentation specifies
Required and optional parameters
Return value

19. Caution About Googling
Python has been around a long time.
Many Internet pages present techniques for Python 2.
We're using Python 3.
Some things that work for Python 2 don't work in Python 3.

20. Classes and Objects

21. Classes, continued Data values in Python are called objects.
The makeup and behavior of objects are defined by classes. We say that an object is an instance of a class.
Each object can perform operations and methods defined by its class.
A class defines
attributes - one or more data values that each object contains
methods - functions that the objects perform

22. Classes in Turtle Module
The turtle module contains two classes: Turtle and Screen.
Turtle attributes:
Location
Heading
Shape
Whether pen is up or down
Pen color
Turtle methods:
You know these...
Screen attributes:
State of each pixel on the screen
Screen methods:
clear()
screensize()

23. Using the Turtle Module
import turtle # makes contents of turtle module available
wn = turtle.Screen() # creates instance of Screen class in turtle module
wn.title("Hello") # sets window title of Screen wn
alex = turtle.Turtle() # creates instance of Turtle class in turtle module
alex.forward(150) # invokes forward() method on alex instance

24. Creating Objects
Create some objects, like numbers and strings, by writing literal values
x = 10
s = "Hello"
Create other objects by writing module-name.class-name()
wn = turtle.Screen()
alex = turtle.Turtle()

25. Using Objects Use objects by passing them to functions:
y = math.fabs(x)
size = len(s)
And invoking their methods:
wn.title("Hello")
alex.forward(10)
Activity: Explore using the Turtle methods in the Standard Library
x = 10
s = "Hello"
wn = turtle.Screen()
alex = turtle.Turtle()

26. Instances and State
The state of an object is the combined values of its attributes.
Each object of a given class has its own distinct state.
alex = turtle.Turtle()
alex.forward(150)
gerta = turtle.Turtle()
gerta.forward(10)