Module 14: Type Classes

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Learning outcomes

• distinguish values, types, and classes;
• use type() and isinstance() to inspect values;
• explain why Python classes are themselves objects of the type class;
• connect type classes with shared behavior and reusable code.

Prerequisites

Complete Module 13 first. You should be comfortable with step-by-step reasoning about code behavior.

The idea in one sentence

Type classes describe shared operations that different types can support. In the Python source material, this module focuses on the relationship between values, types, and classes, then connects that object model to interfaces and shared behavior.

Everyday analogy

Think of a library. A book is a value, its category is like a type, and the cataloging system is like a class that describes how such values are organized and used.

Syntax

value = 42

print(type(value))
print(type(int))
print(isinstance(value, int))

Worked example

class Student:
    pass

student = Student()

print(type(student) is Student)
print(type(Student) is type)
print(isinstance(student, Student))

Step by step:

1. student is a value, specifically an instance of Student.
2. type(student) returns the class that created the value.
3. Student is itself an object whose type is type.
4. isinstance() checks whether a value belongs to a type or class.

Why this matters in practice

• In college notes, this module clarifies the Python object model.
• In real code, it helps you inspect, validate, and safely process values.
• Shared operations can then be expressed through protocols, ABCs, or other interface patterns.

Important concepts

Values, types, and classes

Every runtime value has a type. Classes define instances, and classes themselves are objects created by the type class.

number = 10
print(type(number))       # <class 'int'>
print(type(int))          # <class 'type'>
print(isinstance(number, int))

Shared behavior

Type classes are a general way to describe operations shared by multiple types. Python commonly expresses this idea with duck typing, Protocol, or an abstract base class.

class HasArea(Protocol):
    def area(self) -> float:
        ...

Protocol and ABC

An abstract base class or protocol can make the expected shared behavior explicit for readers and static-analysis tools.

Technical meaning

The Python object model relates values to types and types to classes. Type classes provide the broader idea of describing shared operations, while Python implements practical versions through duck typing, Protocol, and ABC.

Memory rule

• value = runtime data
• type = classification of a value
• class = object definition and a runtime object
• isinstance() = membership check against a type or class

Quick check

• What is the difference between a value, a type, and a class?
• What does type(value) return?
• Why is type(int) itself type?
• How do protocols and ABCs express shared behavior?

Short exam answer

Type classes describe shared operations across different types. In Python, values have types, classes define values, and classes are themselves objects of type; Protocol, duck typing, and ABC provide practical forms of shared behavior.

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