Same Exceptions with different specific message

Question

I tried to write a code that can distinguish the following four different errors.

1. TypeError: The first parameter is not an integer;
2. TypeError: The second parameter is not a string;
3. ValueError: The value of the first parameter is not in the range of 1 to 13; or
4. ValueError: The value of the second parameter is not one of the strings in the set {'s', 'h', 'c', 'd'}.

However, I only can get the first one to work but not the other three errors. I tried different ways to make it work, but still can't figure out what's wrong.

class Card: # One object of class Card represents a playing card

    rank = ['','Ace','Two','Three','Four','Five','Six','Seven','Eight','Nine','Ten','Jack','Queen','King']
    suit = {'d':'Diamonds', 'c':'Clubs', 'h':'Hearts', 's':'Spades'}

    def __init__(self, rank=2, suit=0): # Card constructor, executed every time a new Card object is created
        if type(rank) != int:
            raise TypeError()
        if type(suit) != str:
            raise TypeError()
        if rank != self.rank:
            raise ValueError()
        if suit != 'd' or 'c' or 'h' or 's':
            raise ValueError()
        self.rank = rank
        self.suit = suit

    def getRank(self): # Obtain the rank of the card
        return self.rank

    def getSuit(self): # Obtain the suit of the card
        return Card.suit[self.suit]

    def bjValue(self): # Obtain the Blackjack value of a card
        return min(self.rank, 10)

    def __str__(self): # Generate the name of a card in a string
        return "%s of %s" % (Card.rank[int(self.rank)], Card.suit[self.suit])

if __name__ == "__main__": # Test the class Card above and will be skipped if it is imported into separate file to test
    try:
        c1 = Card(19,13)
    except TypeError:
        print ("The first parameter is not an integer")
    except TypeError:
           print ("The second parameter is not a string")
    except ValueError:
        print ("The value of first parameter is not in the range of 1 to 13")
    except ValueError:
        print ("The value of second parameter is not one of the strings in the set {'s','h','c','d'}")

    print(c1)

I know maybe it is due to that I have same TypeError and ValueError. Therefore, Python can't distinguish the second TypeError which I hit c1 = Card(13,13) is different from the first TypeError. So, I only get the message that "The first parameter is not an integer" when I have c1 = Card(13,13).

Answer 1

You are trying to distinguish between the error sources in completely the wrong place. By the time the error gets out of Card.__init__, there is no way to tell why e.g. a TypeError was thrown. For each error class (TypeError, ValueError) only the first except will ever be triggered:

try:
    ...
except TypeError:
    # all TypeErrors end up here
except TypeError:
    # this is *never* reached
except ValueError:
    # all ValueErrors end up here
except ValueError:
    # this is *never* reached

Instead, you should provide the specific error messages inside Card.__init__, when you actually raise the error and already know what the reason is:

if not isinstance(rank, int):  # better than comparing to type
    raise TypeError("The first parameter is not an integer")

Then you can handle them much more simply:

try:
    c1 = Card(19,13)
except (TypeError, ValueError) as err:  # assign the error to the name 'err'
    print(err)  # whichever we catch, show the user the message
else:
    print(c1)  # only print the Card if there were no errors

---

If you have a particular need to distinguish between different errors of the same class, you can either explicitly check the message:

except TypeError as err:
    if err.args[0] == "The first parameter is not an integer":
        # do whatever you need to

or create your own, more specific Exception sub-classes, so you can have separate except blocks:

class FirstParamNotIntError(Exception):
    pass

(this is just an example, they are too specific in your particular case).

---

It's probably worth having a read through the documentation on exceptions and the tutorial on using them.