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.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
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