[dmurray]

I thought I spotted a bug on the very first day, part 2.

  def day1_2(nums):
    "Find 3 distinct numbers that sum to 2020, and return their product."
    return first(x * y * z 
                 for x, y in combinations(nums, 2) 
                 for z in nums & {2020 - x - y} 
                 if x != z != y)

The last line (if x != z != y) returns true when x and y are equal and z is different.

But x and y are already constrained to be different since nums is a set and itertools combinations picks distinct elements from the iterable. If the test had been x != y != z it would have been a bug.

[norvig]

Good point, dmurray. It was a subtle point here, and probably I should have commented on it.