Creating Your First Python Class

From Plan to Object

Imagine designing a system for bank accounts. Each account needs information such as an account number, an owner's name, and a balance. It also needs actions such as depositing, withdrawing, and checking the balance. You could keep these pieces as separate variables and functions, but they would be scattered across the program. A class gives you a way to organize the related information and actions together.

A class is a blueprint. It describes the data an object will hold and the actions it can perform. An object is an instance created from that blueprint. Multiple objects can come from one class, and each object can have its own values while following the same overall plan.

Data and Behavior Together

An object bundles data and behavior into one organized unit. The data is represented by attributes. The behavior is represented by methods. This bundling means that instead of juggling unrelated variables and functions, you work with one self-contained object.

The important distinction is data versus action. A balance is information about a bank account, so it is an attribute. Depositing money is something the account can do, so it is represented by a method. A method can use an attribute or change an attribute as part of its action.

The Class Structure

The class keyword tells Python that a blueprint is beginning. It is followed by a class name, a colon, and an indented block. Inside that indented block, the class groups the attributes and methods that belong to objects created from it.

class BankAccount:
    account number
    owner name
    balance
    deposit
    withdraw
    check balance

The organizational benefit is practical. When you need to understand or extend a bank account, the related data and behavior are in the BankAccount class. This makes code easier to understand, easier to maintain, easier to debug, and easier to reuse.

Writing and Running Code

Before a class can help you organize a program, you need to know where Python code is written and how it runs. In this course, you write Python in a built-in editor in your browser. You do not need to install Python on your computer. The editor is connected to a Python interpreter.

Writing code and running code are separate actions. When you type a statement into the editor, it is only written code. It has not produced a result yet. When you run the code, the interpreter reads it, executes the instructions, and displays the result in the output area.

print("Hello, World!")

Hello, World!

This statement uses the built-in print function to display text. The parentheses contain the value given to print, and the quotation marks identify Hello, World! as a string. When you run the statement, the interpreter recognizes print as a function call and the text appears in the output area.

A Complete First Run

This small run demonstrates the same general cycle you will use while learning classes: write instructions, run them, inspect the result, and revise the code when necessary. A class organizes related data and behavior, while the interpreter executes the Python statements you write.

print(42)
print(3.14)
print(10 + 5)

42
3.14
15

Python executes statements sequentially from top to bottom. The first statement prints 42, the second prints 3.14, and the third evaluates 10 + 5 before printing 15. The string "42" and the number 42 may look similar when displayed, but Python treats text and numbers as different kinds of values.

Mistakes to Catch Early

• Treating a class and an object as the same thing

  A class is the blueprint, while an object is an instance created from that blueprint.

  Fix: Use the class to describe the common structure and use objects to represent separate accounts with their own attribute values.

• Confusing attributes with methods

  Attributes store information. Methods perform actions.

  Fix: Classify account number, owner name, and balance as attributes; classify deposit, withdraw, and check balance as methods.

• Expecting written code to produce output immediately

  Writing and running are separate actions. The interpreter reads the code only when execution is triggered.

  Fix: Run the code after writing it, then inspect the output area.

• Treating text and numbers as identical values

  Quotation marks identify text as a string, while an unquoted number is treated as a number.

  Fix: Use quotation marks for text and remember that Python treats strings and numbers differently.

• Scattering related data and actions instead of grouping them

  The related pieces become harder to understand, maintain, debug, and reuse.

  Fix: Use a class to group related attributes and methods into a coherent unit.

Your Turn in the Editor

What to Remember

1. A class is a blueprint, and an object is an instance created from that blueprint.
2. Attributes store an object's information, while methods perform actions involving that object's data.
3. The class keyword begins a blueprint and groups related attributes and methods into one organized unit.
4. In this course, you write Python in the browser-based editor and run it through the connected Python interpreter.
5. Writing code places instructions in the editor; running code causes the interpreter to execute them and display output or an error.