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Learn to program with Python

Equality comparison between objects #

Let's do a quick equality comparison between two MyStr objects.

sentence1 = MyStr("Hello World!")
sentence2 = MyStr("Hello World!")
print(sentence1 == sentence2) # Output: False

This is not the behavior we want. We want our MyStr objects to return True the same way str objects are equal. For example:

sentence1 = "Hello World"
sentence2 = "Hello World"
print(sentence1 == sentence2) # Output: True

Special method __eq__ #

In the first example in this lesson, the only way equality comparison == will return True is when both objects are the same object.

For example:

sentence1 = MyStr("Hello World!")
print(sentence1 == sentence1) # Output: True

To implement custom equality comparison for our objects, we need to define the special method __eq__ in our class. For example:

class Dog:
    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        return self.name == other.name

dog1 = Dog("Boss")
dog2 = Dog("Boss")
dog3 = Dog("Bingo")
print(dog1 == dog2) # Output: True
print(dog1 == dog3) # Output: False

In the example above, the __eq__ method takes two arguments: the object itself self, and the other object to compare to other. This function is called when == operator is used to compare two objects. The code dog1 == dog2 will call the __eq__ method with dog1 as self, and dog2 as other. If both dogs' names are the same, the method returns True, otherwise it returns False.

Exercise #

  • Add the __eq__ method to our MyStr class to mimick regular strings.

Tests #

  • MyStr("Hello World") == MyStr("Hello World") should equal to True
  • MyStr("Hello World") == MyStr("Python is awesome") should equal to False
Output will be displayed here