CODEWITHSUNDEEP
python
Fuzzyma
Fuzzyma

Reputation: 8484

How does "in" checks for membership?

I have a multiple instances of a class. I consider two classes equal, when a certain attribute matches. All instances are in an array list = [a, b, c]. I now create a new instance of said class d. When I do d in list it ofc outputs false.

My question is: How is membership checked when using in? Is it normal comparison (which means I can use __eq__ in my class to implement the equality of classes)? If not: How can I achieve that in matches if a certain attribute of a class equals?

Upvotes: 0

Views: 103

Answers (3)

Alex
Alex

Reputation: 1151

class Foo:
    def __init__(self, x):
        self.x = x
    def __eq__(self, other):
        if isinstance(other, Foo):
            return self.x == other.x

a = [1,2,3,Foo(4),Foo(5)]
Foo(5) in a
>>>True
Foo(3) in a
>>>False

Upvotes: 2

Moinuddin Quadri
Moinuddin Quadri

Reputation: 48100

Behavior of in is based on the __contains__() method. Let us see with an example:

class X():
    def __contains__(self, m):
        print 'Hello'

Now when you do in on X()m you can see 'Hello' printed

>>> x = X()
>>> 1 in x
Hello
False

As per the __contains__() document:

For objects that don’t define __contains__(), the membership test first tries iteration via __iter__(), then the old sequence iteration protocol via __getitem__(), see this section in the language reference.

Upvotes: 1

erip
erip

Reputation: 16945

From the docs:

For user-defined classes which define the __contains__() method, x in y is true if and only if y.__contains__(x) is true.

For user-defined classes which do not define __contains__() but do define __iter__(), x in y is true if some value z with x == z is produced while iterating over y. If an exception is raised during the iteration, it is as if in raised that exception.

Lastly, the old-style iteration protocol is tried: if a class defines __getitem__(), x in y is true if and only if there is a non-negative integer index i such that x == y[i], and all lower integer indices do not raise IndexError exception. (If any other exception is raised, it is as if in raised that exception).

Upvotes: 1

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