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
Personclass with one attribute name of typeName. - You define the
Nameclass 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 typeName. - Python translate the expression
alice.name = 'Alice Python'to__set__(alice.name, 'Alice Python'). - In this magic method call, you set the
name.valueattribute 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?
Enough theory. Let’s get some practice!
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