This rule raises an issue when operators ``++in++`` and ``++not in++`` are called with a right operand not supporting membership protocol.

== Why is this an issue?

Operators ``++in++`` and ``++not in++``, also called https://docs.python.org/3/reference/expressions.html#membership-test-operations["membership test operators"], require that the right operand supports the membership protocol.


In order to support the membership protocol, a user-defined class should implement at least one of the following methods: ``++__contains__++``, ``++__iter__++``, ``++__getitem__++``.

If none of these methods is implemented, a `TypeError` will be raised when performing a membership test.

== How to fix it

== Code examples

=== Noncompliant code example

[source,python]
----
myint = 42

if 42 in myint:  # Noncompliant: integers don't support membership protocol
    ...

class A:
    def __init__(self, values):
        self._values = values

if "mystring" in A(["mystring"]):  # Noncompliant: class A doesn't support membership protocol
    ...
----


=== Compliant solution

[source,python]
----
mylist = [42]

if 42 in mylist:
    ...

class MyContains:
    def __init__(self, values):
        self._values = values

    def __contains__(self, value):
        return value in self._values

if "mystring" in MyContains(["mystring"]):
    ...

# OR

class MyIterable:
    def __init__(self, values):
        self._values = values

    def __iter__(self):
        return iter(self._values)

if "mystring" in MyIterable(["mystring"]):
    ...

# OR

class MyGetItem:
    def __init__(self, values):
        self._values = values

    def __getitem__(self, key):
        return self._values[key]

if "mystring" in MyGetItem(["mystring"]):
    ...
----


== Resources

=== Documentation

* Python Documentation - https://docs.python.org/3/reference/expressions.html#membership-test-operations[Membership test operations]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Change the type of X; type Y does not support membership protocol.


For an expression like "a in X" where X has type Y


=== Highlighting

Primary: The "in" or "not in" operator

Secondary: The operator's right operand


endif::env-github,rspecator-view[]