Want to merge two dictionaries into a new third dictionary without modifying either of the originals? Easy:

>>> dict3 = dict(dict1, **dict2)

Keys in dict2 would override matching keys in dict1. And, as usual, mutable values would be preserved by reference...

Personally I've always wished that dicts would define __add__ to do this... something like:

class MyDict(dict):
    def __add__(self, other):
        return MyDict(self, **other)

if __name__ == '__main__':
    d1 = MyDict(foo=1)
    d2 = MyDict(bar=2, baz=3)
    d3 = MyDict(qux=4)
    d4 = d1 + d2 + d3
    import pprint
    # What if keys collide? Last one wins.
    d5 = d4 + {'foo': 99} + {'foo': 999}