etuardu etuardu - 2 months ago 31
Python Question

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?

agf agf
Answer
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})