How to Implement a Bidirectional Hash Table in Python for Multiple Keys with the Same Value?

Bidirectional Hash Table Implementation in Python

Python's built-in dict is a highly useful data structure, providing efficient lookup based on keys. However, situations arise when reverse lookup from values is desired. This article explores the most efficient methods of implementing a bidirectional hash table, offering an example inspired by existing solutions and modified to address specific requirements.

Implementation

The presented solution employs a custom class bidict that extends the standard Python's dict. It maintains an additional inverse dictionary that automatically updates itself whenever the bidict is modified. This inverse dictionary maps values to a list of keys that have that value assigned to them.

Importantly, unlike existing libraries, this implementation allows multiple keys with the same value. This feature is crucial in certain scenarios.

Usage

The bidict class employs the following key methods:

1. bidict(args, kwargs): Constructor, accepting arguments and keyword arguments that are stored in the standard dict and inverse dictionaries.
2. bidict.__setitem__(key, value): Sets the key to the value. If key is already in the bidict, it removes key from the list of keys associated with its current value in the inverse dictionary. It then sets the key to the value in both dictionaries, updating the list of keys associated with the value in the inverse dictionary.
3. bidict.__delitem__(key): Removes the key from the bidict. It finds the associated value of key and updates the inverse dictionary accordingly, removing key from the list of keys associated with value. If the list of keys for value becomes empty, value is removed from the inverse dictionary.

Example Usage

<code class="python">bd = bidict({'a': 1, 'b': 2})  
print(bd.inverse)  # {1: ['a'], 2: ['b']}
bd['c'] = 1 
print(bd.inverse)  # {1: ['a', 'c'], 2: ['b']}
del bd['c']
print(bd.inverse)  # {1: ['a'], 2: ['b']}</code>

The above is the detailed content of How to Implement a Bidirectional Hash Table in Python for Multiple Keys with the Same Value?. For more information, please follow other related articles on the PHP Chinese website!