How do I merge two dictionaries in a single expression?

Question

I want to merge two dictionaries into a new dictionary.

x = {'a': 1, 'b': 2}
y = {'b': 3, 'c': 4}
z = merge(x, y)

>>> z
{'a': 1, 'b': 3, 'c': 4}

Whenever a key k is present in both dictionaries, only the value y[k] should be kept.

Answer 1

A method is deep merging. Making use of the | operator in 3.9+ for the use case of dict new being a set of default settings, and dict existing being a set of existing settings in use. My goal was to merge in any added settings from new without over writing existing settings in existing. I believe this recursive implementation will allow one to upgrade a dict with new values from another dict.

def merge_dict_recursive(new: dict, existing: dict):
    merged = new | existing

    for k, v in merged.items():
        if isinstance(v, dict):
            if k not in existing:
                # The key is not in existing dict at all, so add entire value
                existing[k] = new[k]

            merged[k] = merge_dict_recursive(new[k], existing[k])
    return merged