Object Oriented Programming with Python - Full Course for Beginners

The idea

Key points

• Several variables with the same prefix (item1, item1_price, item1_quantity) only look related to a human; Python sees independent variables. ▶ 2:05
• type() shows each value's class: 'Phone' is a str, and 100, 5 and 500 are int. ▶ 2:31
• In Python every value is an object instantiated from a class. ▶ 2:35
• A class lets you define your own data type, so one item can hold its name, price and quantity together. ▶ 3:00
• Grouping related data into one object is the main reason to write object oriented code. ▶ 3:15

Words to know

variable

A name that points to a value, like item1_price = 100.

naming convention

A habit in how we name things so people can read the code. Python does not check it or act on it.

class

A blueprint that defines a kind of data, like str or int. Later you will make your own.

object (instance)

A value created from a class. 'Phone' is an object of the str class.

type()

A built-in function that tells you which class a value belongs to.

Code from the screen

item1 = 'Phone'
item1_price = 100
item1_quantity = 5
item1_price_total = item1_price * item1_quantity

print(type(item1)) # str
print(type(item1_price)) # int
print(type(item1_quantity)) # int
print(type(item1_price_total)) # int

Build along in the practice terminal

1. Create a new project folder and a file called main.py in your editor.

   $ python main.py
   <class 'str'>
   <class 'int'>
   <class 'int'>
   <class 'int'>

   You should see: Four lines, each showing a class: str for the name and int for the three numbers.
2. Type the four variables for one item, each with the same prefix item1. You should see: Your file has item1, item1_price, item1_quantity and item1_price_total.
3. Print the type of each variable and run the file to confirm the classes.

Break it on purpose

$ python main.py
NameError: name 'item1' is not defined

Watch out

• Matching prefixes do not make Python treat variables as a group. The link exists only in your head and in the naming.
• A type printed as <class 'int'> means the value is an object of the int class, not a plain number with no type.

Practice

item1 = 'Laptop'
# add the other three variables here

item1 = 'Laptop'
item1_price = 250
item1_quantity = 3
item1_price_total = item1_price * item1_quantity

print(type(item1))
print(type(item1_price))
print(type(item1_quantity))
print(type(item1_price_total))

On your scratchpad

Quiz

1. What does type(item1) print when item1 = 'Phone'?

   1. <class 'str'>
   2. <class 'int'>
   3. <class 'item'>
   4. <class 'Phone'>
   Show the answer

   A. <class 'str'> · 'Phone' is a string, so its class is str.

   ▶ 2:17 watch the moment

2. Why are item1, item1_price and item1_quantity not treated as related by Python?

   1. Python only sees separate variables; the shared prefix is just a naming convention
   2. Python links every variable that starts with the same prefix
   3. Python requires a class before any variable can exist
   4. Python stores all variables with the same prefix in one list
   Show the answer

   A. Python only sees separate variables; the shared prefix is just a naming convention · The prefix is a convention for humans. Python sees independent variables.

   ▶ 2:00 watch the moment

3. What does a class let you do that separate variables do not?

   1. Group related data, such as an item's name, price and quantity, into one object
   2. Make programs run faster than functions
   3. Avoid using variables altogether
   4. Remove the need for types in Python
   Show the answer

   A. Group related data, such as an item's name, price and quantity, into one object · A class lets you define your own data type that holds related data together.

   ▶ 3:00 watch the moment