x = {'a': [1, 2, 3], 'b': [4, 5, 6]}

x = {'a': [1, 2, 3], 'b': [4, 5, 6]}

n = 3

output = [k for k, v in x.items() if n in v]

print(output[0])

output = next(key for key, value in x.items() if n in value)

x_reversed = {num: key for key, value in x.items() for num in value}