Difference between the in keyword and __contains__ in Python

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I was wondering if some one could explain the difference between the "in" keyword of Python and the contains method

I was working with a sample list and found this behavior. When are the two supposed to be used? Is there some efficiency that can be achieved if I use one over the other.

    >>> my_list = ["a", "b", "c"]
    >>> my_list.__contains__("a")
    >>> "a" in my_list

From the docs:

For the list and tuple types, x in y is true if and only if there exists an index i such that x == y[i] is true.

string types, x in y is true if and only if x is a substring of y. An equivalent test is y.find(x) != -1.

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.

Functionality of Python `in` vs. `__contains__`, import dis >>> class test(object): def __contains__(self, other): return 6 CALL_FUNCTION 0 (0 positional, 0 keyword pair) 9 COMPARE_OP 6 (in) so if the comparison operator is used in a Boolean context (e.g., in the� There are total 33 keywords in Python 3.6. To get the keywords list on your operating system, open command prompt (terminal on Mac OS) and type “Python” and hit enter. After that type help() and hit enter. Type keywords to get the list of the keywords for the current python version running on your operating system. Chaitanyas-MacBook-Pro

The __contains__() method of an an object is called when you use the in statement.

For lists this is pre-defined, but you can also define your own class, add a __contains__ method and use in on the instances of that class.

You should be using in and not call __contains__() directly.

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Like most magic methods, the __contains__ method is not meant to be called directly. The reason __contains__ exists is precisely so that you can write obj in container instead of having to use method-call syntax. So you should use obj in container.

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Doing "a" in my_list actually calls __contains__ method of my_list if defined.

If __contains__ is not defined then __getitem__ is used.

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