Example of super terse lambda to satisfy defaultdict default values

lambda_defaultdict.py

# solves: https://leetcode.com/problems/divide-array-into-equal-pairs/
# when you want to avoid KeyError for missing keys, you need to supply
# a func to the defaultdict constuctor method; here's an example
from collections import defaultdict

class Solution:
    def divideArray(self, nums: List[int]) -> bool:
        counts = defaultdict(lambda: 0)  # argument is a func to return default for missing key
        for num in nums:
            counts[num] = counts[num] + 1
        for key in counts.keys():
            if counts[key] % 2 != 0:
                return False
        return True