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[Tut] Python __eq__ Magic Method

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Python __eq__ Magic Method

To customize the behavior of the equality operator x == y, override the __eq__() dunder method in your class definition. Python internally calls x.__eq__(y) to compare two objects using x == y. If the __eq__() method is not defined, Python will use the is operator per default that checks for two arbitrary objects whether they reside on the same memory address.

Syntax


__equal__(self, other)

To use the equal to operator on custom objects, define the __eq__() “dunder” magic method that takes two arguments: self and other. You can then use attributes of the custom objects to determine if one is equal to the other. It should return a Boolean True or False.

Let’s have a look at an example next.

Example


In the following code, you check if a Person is equal to another Person by using the age attribute as a decision criterion:

class Person: def __init__(self, age): self.age = age def __eq__(self, other): return self.age == other.age alice = Person(18)
bob = Person(19)
carl = Person(18) print(alice == bob)
# False print(alice == carl)
# True

Because Alice is 18 years old and Bob is 19 years old, the result of alice == bob is False. But the result of alice == carl evaluates to True as both have the same age.

Background Video





Default Implementation of __eq__


Per default, the __eq__() dunder method is implemented using the is identity operator. Identity operators are used to check whether two values or variables reside at the same memory location, i.e., refer to the same object in memory.

Because the fallback identity operator is defined for each object, you can also check equality for any two object.

The following example shows that you can compare custom persons using the equality operator ==, even without defining the __eq__ method. Internally, Python uses the identity operator:

class Person: def __init__(self, age): self.age = age alice = Person(18)
bob = Person(19)
carl = Person(18) print(alice == bob)
# False print(alice == carl)
# False

Background Video Identity Operator


To understand the identity operator, feel free to watch the following background video:




Commutativity of Equality ==


The output of x == y and y == x may be different because the former calls x.__eq__(y) and the latter calls y.__eq__(x). If x and y have different definitions of the dunder method __eq__(), the operation becomes non-commutative.

You can see this in the following example:

class Person: def __eq__(self, other): return False class Human: def __eq__(self, other): return True alice = Person()
bob = Human() print(alice == bob)
# False print(bob == alice)
# True

Where to Go From Here?


Enough theory, let’s get some practice!

To become successful in coding, you need to get out there and solve real problems for real people. That’s how you can become a six-figure earner easily. And that’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?

Practice projects is how you sharpen your saw in coding!

Do you want to become a code master by focusing on practical code projects that actually earn you money and solve problems for people?

Then become a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.

Join my free webinar “How to Build Your High-Income Skill Python” and watch how I grew my coding business online and how you can, too—from the comfort of your own home.

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https://www.sickgaming.net/blog/2021/11/...ic-method/
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