Python __getitem__ Method

Last modified April 8, 2025

This comprehensive guide explores Python's __getitem__ method, the special method that enables object indexing and slicing. We'll cover basic usage, sequence emulation, custom containers, and practical examples.

Basic Definitions

The __getitem__ method allows objects to implement the subscript operator []. It's called when an instance is accessed with square bracket notation, like obj[key].

Key characteristics: it accepts the instance as first argument (self), the key as second argument, and should return the corresponding value or raise IndexError/KeyError for invalid keys.

Basic __getitem__ Implementation

Here's a simple implementation showing how __getitem__ enables indexing behavior for custom objects. This example creates a sequence-like class.

class MySequence:
 def __init__(self, data):
 self.data = data
 
 def __getitem__(self, index):
 return self.data[index]
seq = MySequence([10, 20, 30, 40, 50])
print(seq[1]) # 20
print(seq[-1]) # 50
print(seq[1:4]) # [20, 30, 40]

This example demonstrates basic indexing and slicing. The __getitem__ method delegates to the underlying list's indexing functionality.

Python automatically handles negative indices and slices, passing them directly to __getitem__. The method doesn't need special slice handling.

Implementing a Custom Dictionary

__getitem__ is essential for creating dictionary-like objects. This example shows a case-insensitive dictionary implementation.

class CaseInsensitiveDict:
 def __init__(self):
 self._data = {}
 
 def __getitem__(self, key):
 return self._data[key.lower()]
 
 def __setitem__(self, key, value):
 self._data[key.lower()] = value
 
 def __contains__(self, key):
 return key.lower() in self._data
d = CaseInsensitiveDict()
d['Name'] = 'John'
print(d['NAME']) # John
print('name' in d) # True

This dictionary treats keys case-insensitively by converting them to lowercase before storage and lookup. __getitem__ handles the lookup part.

The class also implements __setitem__ and __contains__ for complete dictionary behavior. This pattern is common for custom mappings.

Handling Slices Explicitly

While Python handles basic slicing automatically, you can process slices differently by checking the key type in __getitem__.

class SliceProcessor:
 def __getitem__(self, key):
 if isinstance(key, slice):
 start = key.start if key.start is not None else 0
 stop = key.stop if key.stop is not None else 10
 step = key.step if key.step is not None else 1
 return list(range(start, stop, step))
 elif isinstance(key, int):
 return key * 10
 else:
 raise TypeError("Invalid key type")
sp = SliceProcessor()
print(sp[5]) # 50 (integer handling)
print(sp[1:5]) # [1, 2, 3, 4] (slice handling)
print(sp[1:10:2]) # [1, 3, 5, 7, 9]

This example processes integers and slices differently. For slices, it generates a range, while integers are multiplied by 10.

The isinstance(key, slice) check is crucial for distinguishing between index and slice operations. This allows custom slice behavior.

Implementing a Virtual Sequence

__getitem__ can generate values dynamically rather than storing them. This example creates an infinite sequence of squares.

class Squares:
 def __getitem__(self, key):
 if isinstance(key, slice):
 start = key.start if key.start is not None else 0
 stop = key.stop if key.stop is not None else float('inf')
 step = key.step if key.step is not None else 1
 return [i**2 for i in range(start, stop, step)]
 elif isinstance(key, int):
 return key**2
 else:
 raise TypeError("Invalid key type")
sq = Squares()
print(sq[5]) # 25
print(sq[1:6]) # [1, 4, 9, 16, 25]
print(sq[1:10:2]) # [1, 9, 25, 49, 81]

This sequence generates square numbers on demand without storing them. Both individual indices and slices work, demonstrating __getitem__'s flexibility.

The slice handling includes default values for start, stop, and step to make the sequence work like built-in sequences.

Multi-dimensional Indexing

__getitem__ can handle complex keys like tuples for multi-dimensional indexing. This example implements a simple matrix class.

class Matrix:
 def __init__(self, rows, cols):
 self.rows = rows
 self.cols = cols
 self.data = [[0]*cols for _ in range(rows)]
 
 def __getitem__(self, key):
 if isinstance(key, tuple) and len(key) == 2:
 row, col = key
 return self.data[row][col]
 elif isinstance(key, int):
 return self.data[key]
 else:
 raise TypeError("Invalid key type")
m = Matrix(3, 3)
m.data = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(m[1, 2]) # 6 (row 1, column 2)
print(m[2]) # [7, 8, 9] (entire row 2)

This matrix supports both single-index (entire row) and double-index (specific cell) access patterns. The tuple unpacking in __getitem__ enables the multi-dimensional syntax.

The example shows how __getitem__ can support multiple access patterns in a single implementation by examining the key type.

Best Practices

• Handle expected key types: Check for valid key types and raise TypeError for invalid ones
• Implement error handling: Raise IndexError/KeyError for out-of-bounds keys
• Consider performance: __getitem__ is called frequently, so optimize its implementation
• Support slicing: Handle slice objects for sequence-like behavior
• Document behavior: Clearly document supported key types and return values

Source References

• Python __getitem__ Documentation
• Python Container Emulation Docs

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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