Classes: Bundling Data and Behavior Together

What might confuse you right now

"I already know functions. Why do I need classes?"

Functions are great for processing steps. Classes are great for modeling "object state + behavior."

One-line definition

A class is a template for creating objects. Objects have attributes (data) and methods (behavior).

Real-life analogy

A class is like a "student record template." An object is "one specific student."

Minimal runnable example

class Student:
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def is_pass(self):
        return self.score >= 60

s = Student("Amy", 85)
print(s.is_pass())

Quick quiz (5 min)

1. Define a Book class (title, price).
2. Write an is_expensive() method.
3. Create two objects and test them.

Quiz answer guidelines & grading criteria

• Answer direction: working code that covers core conditions and edge inputs from the prompt.
• Criterion 1 (Correctness): Main flow produces correct results, key branches execute.
• Criterion 2 (Readability): Clear variable names, no excessive nesting.
• Criterion 3 (Robustness): Basic protection against null values, type errors, or unexpected input.

Transfer task (homework)

Convert your "user dictionary processing script" into a User class version.

Acceptance criteria

You can independently:

• Define a class with an init method
• Create objects and call methods
• Distinguish between attribute and method responsibilities

Common errors & debugging steps (beginner edition)

• Can't understand the error: read the last line for the error type (e.g., TypeError, NameError), then trace back to the relevant code line.
• Not sure about a variable's value: temporarily add print(variable, type(variable)) at key points to verify data matches expectations.
• Code changes aren't taking effect: confirm the file is saved, you're running the right file, and your terminal environment (venv) is correct.

Common misconceptions

• Misconception: Classes are just syntactic sugar.
• Reality: Classes solve the modeling problem of binding state and behavior together.