This rule raises an issue when an object used as a sequence index does not define an ``++__index__++`` method.

== Why is this an issue?

Objects can be used as sequence indexes to access a specific element from the sequence, through the following syntax:

[source,python]
----
my_list = [1, 2, 3, 4]
x = 1
print(my_list[x])  # This will print 2
----

Whenever an object is used as a sequence index, the Python interpreter calls its ``++__index__++`` method to compute the index that needs to be accessed from the sequence.

Any object can be used as sequence index, as long as it defines an ``++__index__++`` method that returns an `int`. Most commonly, sequence indexes are simply integers.

Similarly, sequences can be sliced through the following syntax:

[source,python]
----
my_list = [1, 2, 3, 4]
x = 1
print(my_list[1:3]) # This will print [2, 3]
----

If an invalid object is used as a sequence index, a `TypeError` will be raised.

== How to fix it

Make sure to use an object that defines an ``++__index__++`` method as sequence indexes.

=== Code examples

==== Noncompliant code example

[source,text,diff-id=1,diff-type=noncompliant]
----
def foo():
    my_list = ["spam", "eggs"]
    x = my_list["spam"]  # Noncompliant: a str is not a valid index
----

==== Compliant solution

[source,text,diff-id=1,diff-type=compliant]
----
def foo():
    my_list = ["spam", "eggs"]
    x = my_list[0]
----


== Resources

=== Documentation

Python Documentation - https://docs.python.org/3/library/operator.html#operator.__index__[__index__ method]