[Python] Summing instead of overriding values in dictionary comprehension

When the keys in a dictionary comprehension is the same, I want their values to be added up. For example,

>>> dct = {-1: 1, 0: 2, 1: 3}
>>> {k**2: v for k, v in dct.items()}
{1: 3, 0: 2}


However, what I want to get in this case is {1: 4, 0: 2}, because both the square of 1 and -1 is 1, and 1 + 3 = 4.

Clearly, I can do it with a for loop, but is there a shorthand?

Continue reading...