🔹Dunder Variables in Python

Back 🔹Dunder Variables in Python 26 Jul, 2025

ABHISHEK AGNIHOTRI

In Python, dunder variables (short for "double underscore"), are special variables or methods that start and end with double underscores, like __name__, __init__, __str__, etc.

🔹 1. What are Dunder Variables?

Dunder variables (also called magic or special variables/methods) are used internally by Python to implement certain behaviors or hooks. They are not meant to be defined arbitrarily, unless you know what you're doing.

🔹 2. Common Dunder Variables

Dunder Variable	Description
`__name__`	Tells whether the file is being run directly or imported as a module.
`__file__`	The path of the file being executed.
`__doc__`	Returns the docstring of a module, class, or function.
`__dict__`	Returns a dictionary of the object’s attributes.
`__class__`	Returns the class type of an instance.

🔹 3. Common Dunder Methods

Dunder Method	Description
`__init__()`	Constructor, used to initialize a new object.
`__str__()`	Returns a readable string representation of the object.
`__repr__()`	Returns an official string representation (for developers).
`__len__()`	Defines behavior for `len(obj)`.
`__getitem__()`	Used to access items via indexing.

🔸 Example: `name` Usage

# file: mymodule.py
def greet():
    print("Hello!")

if __name__ == "__main__":
    greet()

When run directly: __name__ == "__main__" → greet() is called.
When imported: __name__ == "mymodule" → greet() is not called.

🔸 Example: Custom `str` Method

class Dog:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return f"This is a dog named {self.name}"

d = Dog("Buddy")
print(d)  # Output: This is a dog named Buddy

🧠 Tip:

Never name your own variables or functions with leading/trailing double underscores. These are reserved for Python's internal use.