NotImplemented is a special singleton class in python, it will be returned when comparison doesn’t know how to deal with current compared objects, then a futher operation can be taken by invoker.
License: (CC 3.0) BY-NC-SA
Here is some source code from openstack oslo-incubator:
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class Resource(object):
def __eq__(self, other):
if not isinstance(other, Resource):
return NotImplemented
# two resources of different types are not equal
if not isinstance(other, self.__class__):
return False
if hasattr(self, 'id') and hasattr(other, 'id'):
return self.id == other.id
return self._info == other._info
At first glance, I thought it is a bug, because line 6 will never reach. Then I realized that __class__ can be sub class of Resource, so there must be some unordinary things inside. After google and run this test, I found the root cause:
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#! /usr/bin/env python
class A(object):
def __eq__(self, other):
if not isinstance(other, A):
return NotImplemented
if not isinstance(other, self.__class__):
print 'result returned by class A: ',
return False
else:
return True
class B(object):
def __eq__(self, other):
if not isinstance(other, B):
print 'result returned by class B: ',
return False
return True
class C(A):
pass
print 'A() == B(), ', A() == B()
print 'A() == C(), ', A() == C()
The console output is
A() == B(), result returned by class B: False
A() == C(), result returned by class A: False
According to the test, if object of class A can not recognize the type of right operand, then it will return a singleton object NotImplemented, then the right operand will invoke it’s comparison method.
It is common behaviour in all object rich comparison, which means __eq__, __lt__, and etc. This is designed for sequence operation, for i.e., sort, filter, map, and etc. If we raise NotImplementedError exception, then operation will be interrupted. The NotImplemented object will notify the invoker that the current object cannot decide what to do with the right operand, and the invoker can do futher action to manipulate comprison.
ref: