How to Flatten Nested Dictionaries with Compressed Keys in Python?

Flattening Nested Dictionaries with Compressed Keys

Consider a hierarchical dictionary structure like the one below:

{'a': 1,
 'c': {'a': 2,
       'b': {'x': 5,
             'y' : 10}},
 'd': [1, 2, 3]}

To flatten this into a streamlined structure like:

{'a': 1,
 'c_a': 2,
 'c_b_x': 5,
 'c_b_y': 10,
 'd': [1, 2, 3]}

involves flattening both the nested list and the nested dictionaries within.

Approach

This procedure parallels the steps for flattening a nested list. However, it requires additional processing for iterating through the dictionary, generating unique key names for the flattened structure using a separator, and assembling the new dictionary.

Implementation

Using collections.abc.MutableMapping for dictionary type checking, the following code snippet utilizes recursion to traverse the nested structure and construct the compressed representation:

from collections.abc import MutableMapping

def flatten(dictionary, parent_key='', separator='_'):
    items = []
    for key, value in dictionary.items():
        new_key = parent_key + separator + key if parent_key else key
        if isinstance(value, MutableMapping):
            items.extend(flatten(value, new_key, separator=separator).items())
        else:
            items.append((new_key, value))
    return dict(items)

Applying this method to the given example dictionary produces the desired flattened representation:

>>> flatten({'a': 1, 'c': {'a': 2, 'b': {'x': 5, 'y' : 10}}, 'd': [1, 2, 3]})
{'a': 1, 'c_a': 2, 'c_b_x': 5, 'd': [1, 2, 3], 'c_b_y': 10}

This technique efficiently flattens nested dictionaries by compressing the key names using a separator, providing a compact and easily traversable structure.

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