Merging two Python dictionaries by deep-updating

Sunday, May 7th, 2017

Say we have two Python dictionaries:

{
    'name': 'Ferry',
    'hobbies': ['programming', 'sci-fi']
}

and

{
    'hobbies': ['gaming']
}

What if we want to merge these two dictionaries such that “gaming” is added to the “hobbies” key of the first dictionary? I couldn’t find anything online that did this already, so I wrote the following function for it:

# Copyright Ferry Boender, released under the MIT license.
def deepupdate(target, src):
    """Deep update target dict with src
    For each k,v in src: if k doesn't exist in target, it is deep copied from
    src to target. Otherwise, if v is a list, target[k] is extended with
    src[k]. If v is a set, target[k] is updated with v, If v is a dict,
    recursively deep-update it.

    Examples:
    >>> t = {'name': 'Ferry', 'hobbies': ['programming', 'sci-fi']}
    >>> deepupdate(t, {'hobbies': ['gaming']})
    >>> print t
    {'name': 'Ferry', 'hobbies': ['programming', 'sci-fi', 'gaming']}
    """
    for k, v in src.items():
        if type(v) == list:
            if not k in target:
                target[k] = copy.deepcopy(v)
            else:
                target[k].extend(v)
        elif type(v) == dict:
            if not k in target:
                target[k] = copy.deepcopy(v)
            else:
                deepupdate(target[k], v)
        elif type(v) == set:
            if not k in target:
                target[k] = v.copy()
            else:
                target[k].update(v.copy())
        else:
            target[k] = copy.copy(v)

It uses a combination of deepcopy(), updating and self recursion to perform a complete merger of the two dictionaries.

As mentioned in the comment, the above function is released under the MIT license, so feel free to use it any of your programs.