python merge dictionaries

dict_1 = {1: 'a', 2: 'b'}
dict_2 = {2: 'c', 4: 'd'}

dict_3 = dict_2.copy()
dict_3.update(dict_1)

print(dict_3)

# Python 3.9
z = x | y

# Python 3.5
z = {**x, **y}

# Python <= 3.4
def merge_two_dicts(x, y):
    z = x.copy()   # start with keys and values of x
    z.update(y)    # modifies z with keys and values of y
    return z

z = merge_two_dicts(x, y)

# Example usage:
# Say you have the following dictionaries and want to merge dict_b into dict_a
dict_a = {"ABC": {"DE": {"F": "G"}}}
dict_b = {"ABC": {"DE": {"H": "I"}, "JKL": "M"}} 

def merge_nested_dictionaries(dict_a, dict_b, path=None):
    """
    recursive function for merging dict_b into dict_a
    """
    if path is None:
        path = []
    for key in dict_b:
        if key in dict_a:
            if isinstance(dict_a[key], dict) and isinstance(dict_b[key], dict):
                merge_nested_dictionaries(dict_a[key], dict_b[key], path + [str(key)])
            # if the b dictionary matches the a dictionary from here on, skip adding it
            elif dict_a[key] == dict_b[key]:
                pass
            # if the same series of keys lead to different terminal values in
            # each dictionary, the dictionaries can't be merged unambiguously
            else:
                raise Exception('Conflict at %s' % '.'.join(path + [str(key)]))
        # if the key isn't in a, add the rest of the b dictionary to a at this point
        else:
            dict_a[key] = dict_b[key]
    return dict_a

# running:
merge_nested_dictionaries(dict_a, dict_b)
# returns:
{'ABC': {'DE': {'F': 'G', 'H': 'I'}, 'JKL': 'M'}}