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:

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:

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

def foo():
    my_list = ["spam", "eggs"]
    x = my_list["spam"]  # Noncompliant: a str is not a valid index

Compliant solution

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]