My bad, I read len(group) instead of len(set(group)). Sorry about that. Still that makes it a bit puzzling to read. set(group) == set(range(1, 10)) is shorter, pythonic and more readable (in my humble opinion).

They could also pass strings, booleans, bananas and unicorns to that function. In any case, the description says that it is guaranteed that the input is correct and has only valid values.

It will return False. Look closer... It tests the len(set(group)) == 9. So, len(set([2,2,3,3,5,6,7,8,9])) will be 7.

So, the method is perfectly safe.

What will valid([2,2,3,3,5,6,7,8,9]) return?
I think this test method is not safe.

But we are not told in the description that such values may be passed, only that 0-9 appear in the solutions to validate.

I think your valid(group) would accept sets with negative numbers/ number outside of range(1,10), such as (0,2,3,4,5,6,7,8, 10) which is definitely not valid