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. To avoid this problem the identity operator could be replaced with an equality operator (== or !=), which will
use __eq__ or __ne__ methods under the hood.
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.:
- When it is a dict, list or set literal.
- When it is a call to
dict, set, list or complex built-in functions.
- When such a new object is assigned to only one variable and this variable is used in an identity check.
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
See
