Python Data Model : Overriding special method "contains"

Question

So I am trying to understand python better with the book "Fluent Python". In python data model the author describes how special methods can be defined for classes.

class FrenchDeck:
      ranks = [str(n) for n in range(2,11)] + list('JQKA')
      suits = 'spades diamonds clubs hearts'.split()

      def __init__(self):
          self._cards = [Card(rank,suit) for rank in self.ranks
                                    for suit in self.suits]

      def __len__(self):
          return len(self._cards)

      def __getitem__(self,position):
          return self._cards[position]

      def __contains__(self,*args):
          return "Contains has been overridden"
deck  = FrenchDeck()
print(Card('Q','hearts') in deck)

---

Running the above code gives me the following result, whereas I expect it to say "Contains has been overridden"

True

Why can't I override contains?

Answer 1

I think your situation is the combination of many things:

• .__contains__() is expected to return True or False
• Membership test operations evaluate to True or False
• In truth value testing, "Operations and built-in functions that have a Boolean result always return 0 or False for false and 1 or True for true[…]"

So since you returned a non-empty string, it was evaluated as a True value. If you changed it to an empty string your will have a value of False. Here's an example:

import dataclasses

@dataclasses.dataclass
class Card:
    rank: str
    suit: str 

class FrenchDeck:
    ranks = [str(n) for n in range(2,11)] + list('JQKA')
    suits = 'spades diamonds clubs hearts'.split()

    def __init__(self):
        self._cards = [Card(rank, suit) for rank in self.ranks
                                for suit in self.suits]

    def __len__(self):
        return len(self._cards)

    def __getitem__(self, position):
        return self._cards[position]

    def __contains__(self, item):
        return ""

deck  = FrenchDeck()
print(Card('Q','hearts') in deck)

But don't try to change fundamentally the meaning of a magic method like you did here. It will confuse other people (including your future self)