How to Implement len Method in Python: Simple Guide

To implement the len method in Python for your own class, define the special method __len__(self) that returns an integer representing the length. This allows you to use the built-in len() function on instances of your class.

Syntax

To enable the len() function on your custom class, define the __len__ method inside it. This method must return an integer representing the length.

• def __len__(self): — method signature
• Return an integer value indicating the length or size

python

class MyCollection:
    def __len__(self):
        # return the length as an integer
        return 0

Example

This example shows a simple class that stores items in a list and implements __len__ to return the number of items. Using len() on an instance calls this method.

python

class Bag:
    def __init__(self):
        self.items = []

    def add(self, item):
        self.items.append(item)

    def __len__(self):
        return len(self.items)

bag = Bag()
bag.add('apple')
bag.add('banana')
print(len(bag))

Output

2

Common Pitfalls

Common mistakes when implementing __len__ include:

• Not returning an integer (e.g., returning None or a string)
• Forgetting to define __len__ so len() raises an error
• Returning negative numbers or floats, which are invalid

Always ensure __len__ returns a non-negative integer.

python

class WrongLen:
    def __len__(self):
        return 'five'  # Wrong: should return int

class CorrectLen:
    def __len__(self):
        return 5  # Correct

obj_wrong = WrongLen()
obj_correct = CorrectLen()

# print(len(obj_wrong))  # This will raise a TypeError
print(len(obj_correct))  # Outputs: 5

Output

5

Quick Reference

Summary tips for implementing __len__:

• Define __len__(self) in your class
• Return a non-negative integer
• Use len() on your object to get this value
• Raise no exceptions inside __len__ for smooth usage

Key Takeaways

Implement the __len__(self) method to enable len() on your class.

__len__ must return a non-negative integer representing length.

Using len(your_object) calls your __len__ method automatically.

Avoid returning non-integers or negative values in __len__.

Defining __len__ improves your class's compatibility with Python features.