Python __set__ Magic Method

Python’s __set__() magic method sets a given attribute on an instance of a class holding the attribute (=owner class) to a new value. When setting the attribute through the owner class, Python dynamically executes the attribute’s __set__() method to override its value on the given instance argument.

object.__set__(self, instance, value)

For example, if you create a class Person with an attribute of type Name (and instances person and name), setting person.name = 42 would result in calling the Name‘s __set__() method to obtain the result.

💡 Terminology: The name attribute of type Name in our example is called a descriptor.

Minimal Example

In the following, you can see a minimal example of a descriptor name attribute defining the __set__ magic method.

class Name:   
    def __set__(self, instance, value):
        self.value = value

class Person:
    name = Name()

   
alice = Person()

# The following calls __set__(alice.name, 'Alice Python'):
alice.name = 'Alice Python'   

print(alice.name.value)
# Alice Python

Let’s go through the code next:

  • You create a Person class with one attribute name of type Name.
  • You define the Name class as a descriptor by overriding the __set__ magic method.
  • The __set__ method takes over responsibility each time you’re about to assign a value to an attribute of type Name.
  • Python translate the expression alice.name = 'Alice Python' to __set__(alice.name, 'Alice Python').
  • In this magic method call, you set the name.value attribute to the string 'Alice Python' passed as an argument. This is the output of this code snippet.

Now, you have an initial intuition. To learn more about how descriptors work, I’d recommend you check out the in-depth official Python tutorial that is a great resource on the topic! 🙂

Where to Go From Here?

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