Catching the same expection in every method of a class

Question

I, a beginner, am working on a simple card-based GUI. written in Python. There is a base class that, among other things, consists a vocabulary of all the cards, like _cards = {'card1_ID': card1, 'card2_ID': card2}. The cards on the GUI are referenced by their unique IDs.

As I plan to make the code avabile for other beginners, I want to explicitly inform them if they gave a card ID that does not exists (instead of throwing a KeyError). Now I have a ton of repetitive try-expect clause that makes me suspicious:

Part of the code, one-line methods without try-catching:

def shift(self, card_ID, amount):
        """Moves the card by the given amount of pixels.
            :param amount: the horizontal and vertical amount of shifting in pixels; tuple"""

        try:
            self._cards[card_ID].shift(amount)
        except KeyError:
            raise ValueError("Invaild card ID")


    def align(self, card_ID, horizontal, vertical):
        """Aligns the card to the given position."""

        try:
            card = self._cards[card_ID]
            card.align(horizontal, vertical)
        except KeyError:
            raise ValueError("Invaild card ID") 


    def invert(self, card_ID):
        """Inverts the card's colour"""

        try:
            self._cards[card_ID].invert()
        except KeyError:
            raise ValueError("Invaild card ID")

Is this an accepted practice? Are there any better way to catch this KeyError in every method of the class?

Answer 1

Extract the actual getting of the card from the id into a separate method, with a try/except there, and call that method from everywhere else.

def get_card(self, card_id):
    try:
        return self._cards[card_ID]
    except KeyError:
        raise ValueError("Invaild card ID")

def invert(self, card_id):
    return self.get_card(card_id).invert()

...

Answer 2

You could use a decorator to remove some of that repetitive boiler plate.

from functools import wraps

def replace_keyerror(func):
    """Catches KeyError and replaces it with ValueError"""

    @wraps(func)
    def inner(*args, **kwargs):
        try:
            func(*args, **kwargs)
        except KeyError:
            raise ValueError("Invaild card ID")
    return inner

Then you would use it like this:

@replace_keyerror
def align(self, card_ID, horizontal, vertical):
"""Aligns the card to the given position."""
    card = self._cards[card_ID]
    card.align(horizontal, vertical)

@replace_keyerror
def invert(self, card_ID):
    """Inverts the card's colour"""
    self._cards[card_ID].invert()