Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.
This particular exercise requires that you use the raise statement to "throw" ValueErrors if the Queen constructor is passed numbers that are negative, or numbers that are not a valid row or column. The tests will only pass if you both raise the exception and include a message with it. You also should raise a ValueError if both the queens passed to the can_attack() method are on the same location.
To raise a ValueError with a message, write the message as an argument to the exception type:
# if the row parameter is negative
raise ValueError("row not positive")
# if the row parameter is not on the defined board
raise ValueError("row not on board")
# if the column parameter is negative
raise ValueError("column not positive")
# if the column parameter is not on the defined board
raise ValueError("column not on board")
# if both the queens are on the same location
raise ValueError("Invalid queen position: both queens in the same square")