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Python Language Searching in custom classes: __contains__ and __iter__


To allow the use of in for custom classes the class must either provide the magic method __contains__ or, failing that, an __iter__-method.

Suppose you have a class containing a list of lists:

class ListList:
    def __init__(self, value):
        self.value = value
        # Create a set of all values for fast access
        self.setofvalues = set(item for sublist in self.value for item in sublist)
    def __iter__(self):
        print('Using __iter__.')
        # A generator over all sublist elements
        return (item for sublist in self.value for item in sublist)
    def __contains__(self, value):
        print('Using __contains__.')
        # Just lookup if the value is in the set
        return value in self.setofvalues

        # Even without the set you could use the iter method for the contains-check:
        # return any(item == value for item in iter(self))

Using membership testing is possible using in:

a = ListList([[1,1,1],[0,1,1],[1,5,1]])
10 in a    # False
# Prints: Using __contains__.
5 in a     # True
# Prints: Using __contains__.

even after deleting the __contains__ method:

del ListList.__contains__
5 in a     # True
# Prints: Using __iter__.

Note: The looping in (as in for i in a) will always use __iter__ even if the class implements a __contains__ method.