Rule Definition
Since Python version 2.3 the "is not" keyword pair is an operator in its own right. The expression "x is not y" is equivalent to "not (x is y)" and because of operator precedence rules it can be also expressed without parenthesis as "not x is y". The latter form can be a source of unexpected logical behavior as it appears reversed with respect to the natural English construction "is not". Thus it is recommended to use the "x is not y" form.

Remediation
Highlight operator precedence by adding parenthesis or even better rewrite the identity comparison with the "is not" operator.

Violation Code Sample

>>> if not x is y:
>>>     ... 

Fixed Code Sample

>>> if not (x is y):
>>>     ...
>>>
>>> # Or preferably
>>>
>>> if x is not y:
>>>     ...