To customize the behavior of the non-equality operator
x != y, override the
__ne__() dunder method in your class definition. Python internally calls
x.__ne__(y) to compare two objects using
x != y. If the
__ne__() method is not defined, Python will use the
t operator per default that checks for two arbitrary objects whether they reside on a different memory address.
To use the not equal to operator on custom objects, define the
__ne__() “dunder” magic method that takes two arguments:
other. You can then use attributes of the custom objects to determine if one is not equal to the other. It should return a Boolean
Let’s have a look at an example next.
In the following code, you check if a Person is not equal to another Person by using the
age attribute as a decision criterion:
class Person: def __init__(self, age): self.age = age def __ne__(self, other): return self.age != other.age alice = Person(18) bob = Person(19) carl = Person(18) print(alice != bob) # True print(alice != carl) # False
Because Alice is 18 years old and Bob is 19 years old, and
18 != 19 is
True, the result of
alice != bob is
True. But the result of
alice != carl evaluates to
False as both have the same age.
Default Implementation of __ne__
Per default, the
__ne__() dunder method is implemented using the
is not identity operator. Identity operators are used to check whether two values or variables reside at a different memory location, i.e., refer to a different object in memory.
Because the fallback identity operator is defined for each object, you can also check non-equality for any two objects.
The following example shows that you can compare custom persons using the non-equality operator
!=, even without defining the
__ne__ method. Internally, Python uses the non-identity operator:
class Person: def __init__(self, age): self.age = age alice = Person(18) bob = Person(19) carl = Person(18) print(alice != bob) # True print(alice != carl) # True print(alice != alice) # False
Background Video Identity Operator
To understand the identity operator, feel free to watch the following background video:
Commutativity of Non-Equality !=
The output of
x != y and
y != x may be different because the former calls
x.__ne__(y) and the latter calls
y have different definitions of the dunder method
__ne__(), the operation becomes non-commutative.
You can see this in the following example:
class Person: def __ne__(self, other): return 42 class Human: def __ne__(self, other): return 0 alice = Person() bob = Human() print(alice != bob) # 42 print(bob != alice) # 0
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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