deepdictmerge.py

# If add is True (default, it will add them, else it merges.
def deepdictmerge(a, b, path=None, add=True):
    if path is None: path = []
    for key in b: # for every key in b..
        if key not in a: # handle simple case where it's new
            a[key] = b[key]
        else: # key must be in a and b
            # both dictionaries.
            if isinstance(a[key], dict) and isinstance(b[key], dict):
                deepdictmerge(a[key], b[key], path + [str(key)]) # recurse down
            # if one is a dict and the other has a key..
            elif isinstance(a[key], dict) and key in b:
                raise Exception("a is a dict, but b has key set and is not a dict")
            # same deal.. switched
            elif key in a and isinstance(b[key], dict):
                raise Exception("b is a dict, but a has key set and is not a dict")
            else: # key is in a and b.. neither are dictionaries
                if add: # if we're adding.. it's all good
                    a[key] += b[key]
                else: # we are doing a regular merge
                    if a[key] != b[key]: # they are both set.. and don't match
                        raise Exception('Conflict at %s' % '.'.join(path + [str(key)]))
                    else: # they are both set.. but match
                        pass
    return a