Python Object Internals – Understand How Python Objects Work Under the Hood

Introduction – Why Learn Python Object Internals?

Every value in Python—whether it’s an integer, string, function, or class—is an object. Understanding the internal structure of Python objects helps you:

• Write efficient and memory-safe code
• Understand attribute lookups and method resolution
• Debug complex issues in OOP, metaclasses, and descriptors
• Master Python at a deeper, more professional level

Whether you’re working on performance, design patterns, or frameworks—object internals are the foundation of Python programming.

In this guide, you’ll learn:

• What Python objects are made of
• How attributes are stored and accessed
• Special internal attributes (__dict__, __slots__, __class__)
• Memory layout and object identity
• Inheritance and method resolution order (MRO)

Everything Is an Object in Python

print(type(42))            # <class 'int'>
print(type("Hello"))       # <class 'str'>
print(type(len))           # <class 'builtin_function_or_method'>
print(type(object))        # <class 'type'>

Even types and classes are themselves objects in Python. Python follows a consistent object model, with everything derived from object.

Internal Structure of Python Objects

Each Python object has a fixed internal layout. At a high level, this includes:

Using __dict__ to Inspect Attributes

class Person:
    def __init__(self, name):
        self.name = name
        self.age = 30

p = Person("Alice")
print(p.__dict__)

Output:

{'name': 'Alice', 'age': 30}

All instance attributes are stored in the __dict__ unless __slots__ is defined.

Optimize with __slots__

class User:
    __slots__ = ('username', 'email')

    def __init__(self, username, email):
        self.username = username
        self.email = email

Benefits:

• Saves memory (no __dict__)
• Slight speed improvement
• Prevents creation of new attributes

__slots__ is optional and used in performance-critical code.

Understanding __class__

Every object has a reference to its class via __class__:

x = 10
print(x.__class__)  # <class 'int'>

Method Resolution Order (MRO)

MRO defines how Python looks up methods in case of inheritance.

Example:

class A: pass
class B(A): pass
class C(B): pass

print(C.__mro__)

Output:

(<class '__main__.C'>, <class '__main__.B'>, <class '__main__.A'>, <class 'object'>)

Python uses the C3 Linearization Algorithm for MRO.

Identity and Memory with id()

x = "hello"
print(id(x))  # Memory address of object

id() returns the unique identity of an object, used for object comparison.

Interning of Small Objects

Python caches small immutable objects for performance.

a = 100
b = 100
print(a is b)  #  True – same object (cached)

a = 1000
b = 1000
print(a is b)  #  Might be False – not interned

Built-in Tools to Inspect Objects

Real-World Use Case – Dynamic Object Introspection

def dump_object(obj):
    print(f"Class: {obj.__class__.__name__}")
    print("Attributes:", obj.__dict__)
    print("Methods:", [m for m in dir(obj) if callable(getattr(obj, m)) and not m.startswith("__")])

class Product:
    def __init__(self, name):
        self.name = name
    def display(self):
        print("Product:", self.name)

p = Product("Laptop")
dump_object(p)

Useful for debuggers, serializers, and frameworks.

Best Practices

Summary – Recap & Next Steps

Python object internals give you deep insight into how objects store attributes, manage memory, and resolve methods.

Key Takeaways:

• All Python values are objects with class, attributes, and identity
• Instance variables live in __dict__
• Use __slots__ to reduce memory for fixed-attribute classes
• MRO controls how methods are resolved in inheritance
• id(), __class__, and dir() reveal internal structure

Real-World Relevance:
Essential for writing frameworks, building ORMs, debugging tools, and performance optimization.

FAQ – Python Object Internals

What is __dict__?

A dictionary holding an object’s writable attributes.

What is __slots__ used for?

To restrict attribute creation and save memory by removing __dict__.

What’s the difference between is and ==?

• is: checks object identity
• ==: checks value equality

How can I find an object’s class?

Use obj.__class__ or type(obj).

What is the MRO?

MRO (Method Resolution Order) is the order Python uses to search for methods in a class hierarchy.