How to Update a Nested Dictionary in Python While Preserving Specific Values at Different Depths?

Nested Dictionary Update with Depth Preservation

In Python, updating a nested dictionary can be tricky, especially when you want to merge its contents with another dictionary while preserving specific values at different depths. Consider the following scenario:

Given two dictionaries, dictionary1 and update, how can we update dictionary1 with the contents of update without overwriting a particular value at the deepest level (e.g., "levelA") while updating all other values?

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

If we simply use the update method, it overwrites the entire "level2" subdictionary, losing the "levelA" value:

dictionary1.update(update)
print(dictionary1)

{
    "level1": {
        "level2": {"levelB": 10}
    }
}

To tackle this challenge, we need a recursive solution that considers the variable depth of the dictionaries and merges values appropriately.

Recursive Solution:

The following code implements a recursive "partial update" function that updates dictionary values without overwriting specified keys:

import copy

def partial_update(d, u, preserve):
    for k, v in u.items():
        if isinstance(v, dict):
            if d.get(k, None) is None:
                d[k] = copy.deepcopy(v)
            else:
                partial_update(d[k], v, preserve)
        elif k in preserve:
            continue
        else:
            d[k] = copy.deepcopy(v)
    return d

The partial_update function takes three arguments:

• d: The dictionary to update
• u: The dictionary containing the updates
• preserve: A list of keys to preserve from overwriting

To preserve the "levelA" value in our example, we can use the following code:

partial_update(dictionary1, update, ["levelA"])
print(dictionary1)

{
    "level1": {
        "level2": {"levelA": 0, "levelB": 10}
    }
}

In this solution, we make a copy of the dictionaries before updating to prevent unexpected modifications. This ensures that the original dictionaries remain intact even after the partial update.

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