This rule raises an issue when at least one operand of an identity operator is a new object which has been created just for this check.

Why is this an issue?

Identity operators is and is not check if the same object is on both sides, i.e. a is b returns True if id(a) == id(b).

When a new object is created, it will have its own identity. Thus, if an object is created and used only in an identity check, it is not possible for the other operand to be the same object. The comparison is always False or always True depending on the operator used, is or is not.

This rule raises an issue when at least one operand of an identity operator is a new object which has been created just for this check, i.e.:

How to fix it

Whenever using a newly created object in a comparison, the identity operator should be replaced with the equality operator (== or !=), which will use __eq__ or __ne__ methods under the hood.

Code examples

Noncompliant code example

def func(param):
    param is {1: 2}  # Noncompliant: always False
    param is not {1, 2, 3}  # Noncompliant: always True
    param is [1, 2, 3]  # Noncompliant: always False

    param is dict(a=1)  # Noncompliant: always False

    mylist = []  # mylist is assigned a new object
    param is mylist  # Noncompliant: always False

Compliant solution

def func(param):
    param == {1: 2}
    param != {1, 2, 3}
    param == [1, 2, 3]

    param == dict(a=1)

    mylist = []
    param == mylist

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