How to create and increment a value in a dictionary from a list using dictionary comprehension

Question

I'm creating a simple function that takes a list of numbers and returns a dictionary with the sum of all the odd and even numbers.

I'm able to do so with a traditional for loop, so I was wondering if there's a way to do the same using a dictionary comprehension. I tried, but I can't find a way to increment each value inside the comprehension using +=.

Here's my code with a for loop:

def sum(a):
    results = {"even":0, "odd":0}
    for val in a:
        if val % 2 == 0:
            results["even"] += val
        elif val % 2 != 0:
            results["odd"] += val

This was my attempt using a dictionary comprehension:

def sum(a):
    results = {even:+=x if x % 2 == 0 else "odd" for x in a}

Answer 1

There is no way to one-line this efficiently. It can be one lined but then you are doing two iterations instead of one:

def sum_odd_even(a):
   return {'odd': sum(x for x in a if x % 2), 'even': sum(x for x in a if x % 2 == 0)}

You are better off doing it how you are doing it now. That being said, don't name a function sum. It is a built-in function.

You are better off looping how you are now since it only takes one iteration:

def sum_odd_even(a):
  results = {"even":0, "odd":0}
  for val in a:
    if val % 2 == 0:
      results["even"] += val
    else:
      results["odd"] += val