How to Update Nested Dictionaries in Python Without Overwriting Intermediate Keys?

Updating Nested Dictionaries Without Overwriting Intermediate Keys

When working with nested dictionaries, it's often necessary to update specific keys without affecting the other levels. However, the standard update method in Python overwrites all keys at the same level, making it challenging to achieve this goal.

Recursive Approach

A recursive approach can effectively address this issue. By traversing the dictionaries recursively, we can identify and update keys at the appropriate depths while preserving the existing structure.

For instance, the following recursive function can be used to update the contents of update into dictionary1 without overwriting levelA:

def update(d, u):
    for k, v in u.items():
        if isinstance(v, dict):
            d[k] = update(d.get(k, {}), v)
        else:
            d[k] = v
    return d

In this function, we check if v is a dictionary and recursively call update with the appropriate sub-dictionaries. If v is not a dictionary, we simply update the corresponding key in d. This ensures that intermediate keys are not overwritten.

Example Usage

Consider the following example:

dictionary1 = {
    "level1": {
        "level2": {"levelA": 0, "levelB": 1}
    }
}
update = {
    "level1": {
        "level2": {"levelB": 10}
    }
}

updated_dictionary = update(dictionary1, update)
print(updated_dictionary)

# Output:
# {'level1': {'level2': {'levelA': 0, 'levelB': 10}}}

As you can see, levelA is preserved in the updated dictionary, despite the update containing a value for levelB. This demonstrates the effectiveness of the recursive approach.

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