There are two functions involved in this program.
The cube function is straight forward enough, given a
return number ** 3
by_three requires some decision making, and an
if number % 3: return False
But wait, there is no
else! Because we don't need one when the
if case has a return statement. Flow essentially falls through the
if and on to the final return statement.
Were there not a return statement in the
if block, then
else would be an absolute requirement. This is why return is such a great tool.