Starting with Python 3.14, using NotImplemented in boolean contexts raises a TypeError. This change was introduced to
prevent confusion and bugs that could arise from the implicit boolean evaluation of NotImplemented.
Previously, NotImplemented would evaluate to True in boolean contexts, which could lead to unexpected behavior in code
that was meant to check whether an operation was actually implemented. The NotImplemented singleton is specifically designed to be
returned by binary special methods (like __eq__, __lt__, etc.) to indicate that the operation is not supported for the given
operand types, and should be compared explicitly using identity checks.
This breaking change was preceded by a DeprecationWarning since Python 3.9, giving developers time to update their code. The change
ensures that NotImplemented is used correctly and prevents subtle bugs in comparison operations and other contexts where boolean
evaluation might occur unintentionally.
What is the potential impact?
Code using NotImplemented in boolean contexts will raise a TypeError at runtime in Python 3.14 and later versions. This
can cause application crashes and unexpected behavior. The impact is particularly significant for libraries implementing custom comparison operators
or arithmetic operations that return NotImplemented and then check its value incorrectly.
How to fix?
Replace boolean evaluation of NotImplemented with explicit identity checks using is or is not. This clearly
expresses the intent to check whether the operation was implemented.
Non-compliant code example
def __eq__(self, other):
result = NotImplemented
if result: # Noncompliant
return True
return False
Compliant code example
def __eq__(self, other):
result = NotImplemented
if result is not NotImplemented:
return True
return False
Documentation
Standards
