join8.py

from collections import defaultdict
from pprint import pp

left = [{1: ["10", "11"]}, {2: ["20", "21"]}]
right = [
    {"10": ["a", "b"]},
    {"11": ["a", "b"]},
    {"20": ["x", "y"]},
    {"21": ["x", "z"]}, # <== here is a difference!
]
"""
1) join tables 'left' and 'right'. 
2) Find the differences between both tables:
Expected output: {2: ["20", "21"]}
"""
# pp(left)
dd = defaultdict(list)
for e in left:
    print(e)
    for row in right:
        dd.update(row)

pp(dd)