Override Python's 'in' operator?

Customizing membership testing with contains

Use the __contains__ method to customize membership testing according to your class's purpose. Here's an example where we use it to check if a custom object is in a collection:

class PortfolioItem:
    def __init__(self, ticker):
        self.ticker = ticker

class Portfolio:
    def __init__(self, *args):
        # Best method to diversify? Have more tickers!
        self._holdings = list(args)
    
    def __contains__(self, item):
        # We don't care about the price, just the ticker!
        return any(item.ticker == obj.ticker for obj in self._holdings)

# Usage:
portfolio = Portfolio(PortfolioItem('AAPL'), PortfolioItem('GOOG'), PortfolioItem('TSLA'))
print(PortfolioItem('AAPL') in portfolio)  # True, AAPL joined the party
print(PortfolioItem('MSFT') in portfolio)  # False, MSFT wasn't invited
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When contains isn't enough

If __contains__ isn't defined, Python defaults to __iter__, using it to check for membership. This is particularly useful for large data structures, but may not be the most efficient method for all data types.

Complex membership conditions

The __contains__ method can handle complex conditions beyond checking for simple membership:

class FancyContainer:
    def __init__(self):
        self._data = set()

    def __contains__(self, item):
        # Complex condition: Is the item a member of _data? Or does it meet some other condition?
        return (item in self._data or item_meets_some_other_condition(item))

    def add(self, item):
        self._data.add(item)
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Don't forget to test!

After defining __contains__, test various cases to confirm your in operator behaves correctly.

Keeping it Pythonic

When crafting your __contains__ method, remember the Zen of Python. Your __contains__ should leave little room for interpretation and should behave predictably.