pythonpython-3.xdictionarydictview

python3: sum (union) of dictionaries with "+" operand raises exception


I'd like to avoid the update() method and I read that is possible to merge two dictionaries together into a third dictionary using the "+" operand, but what happens in my shell is this:

>>> {'a':1, 'b':2}.items() + {'x':98, 'y':99}.items()
Traceback (most recent call last):
  File "<pyshell#84>", line 1, in <module>
    {'a':1, 'b':2}.items() + {'x':98, 'y':99}.items()
TypeError: unsupported operand type(s) for +: 'dict_items' and 'dict_items'
>>> {'a':1, 'b':2} + {'x':98, 'y':99}
Traceback (most recent call last):
  File "<pyshell#85>", line 1, in <module>
    {'a':1, 'b':2} + {'x':98, 'y':99}
TypeError: unsupported operand type(s) for +: 'dict' and 'dict'

How can I get this to work?


Solution

  • dicts = {'a':1, 'b':2}, {'x':98, 'y':99}
    new_dict = dict(sum(list(d.items()) for d in dicts, []))
    

    or

    new_dict = list({'a':1, 'b':2}.items()) + list({'x':98, 'y':99}.items())
    

    On Python 3, items doesn't return a list like in Python 2, but a dict view. If you want to use +, you need to convert them to lists.

    You're better off using update with or without copy:

    # doesn't change the original dicts
    new_dict = {'a':1, 'b':2}.copy()
    new_dict.update({'x':98, 'y':99})