How to Overload [] Operator in Python: Syntax and Example

In Python, you overload the [] operator by defining the __getitem__ method to access items and __setitem__ to assign items in your class. These special methods let you customize how your objects behave when using square brackets.

Syntax

To overload the [] operator, define these methods in your class:

• __getitem__(self, key): Called when accessing an item like obj[key].
• __setitem__(self, key, value): Called when assigning an item like obj[key] = value.

The key can be any type (integer, string, tuple) depending on your use case.

python

class MyClass:
    def __getitem__(self, key):
        # Return value for obj[key]
        pass

    def __setitem__(self, key, value):
        # Set value for obj[key] = value
        pass

Example

This example shows a simple class that stores items in a dictionary and uses __getitem__ and __setitem__ to access and set values with square brackets.

python

class SimpleDict:
    def __init__(self):
        self.data = {}

    def __getitem__(self, key):
        return self.data.get(key, 'Key not found')

    def __setitem__(self, key, value):
        self.data[key] = value

obj = SimpleDict()
obj['apple'] = 10
obj['banana'] = 20
print(obj['apple'])
print(obj['banana'])
print(obj['cherry'])

Output

10
20
Key not found

Common Pitfalls

Common mistakes when overloading [] include:

• Not handling missing keys in __getitem__, which can cause KeyError.
• Forgetting to implement __setitem__ if you want to allow assignment.
• Using mutable default arguments or shared state incorrectly inside these methods.

Always handle errors gracefully and keep your internal data consistent.

python

class BadExample:
    def __getitem__(self, key):
        # This will raise KeyError if key not found
        return self.data[key]

    # Missing __setitem__ means obj[key] = value will fail

# Correct way:
class GoodExample:
    def __init__(self):
        self.data = {}

    def __getitem__(self, key):
        return self.data.get(key, None)  # Returns None if key missing

    def __setitem__(self, key, value):
        self.data[key] = value

Quick Reference

Summary tips for overloading [] operator:

• Use __getitem__(self, key) to define behavior for obj[key].
• Use __setitem__(self, key, value) to define behavior for obj[key] = value.
• Handle missing keys carefully to avoid exceptions.
• Support any key type your use case needs (int, str, tuple, etc.).

Key Takeaways

Overload [] operator by defining __getitem__ and __setitem__ methods in your class.

__getitem__ handles accessing items; __setitem__ handles assigning items.

Always handle missing keys gracefully to avoid errors.

The key parameter can be any type depending on your design.

Implement both methods to fully support bracket access and assignment.