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.

I used the fact that python uses floored division. By diving the negative version and then inverting it I do a ceil division wihtout the call to the math library.

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