Python __set_name__() Magic Method

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Python’s magic method obj.__set_name__(self, owner, name) method is created on an attribute obj when the class owner holding the attribute is created.

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Syntax and Minimal Example

object.__set_name__(self, owner, name)

Here’s a minimal example:

class Attribute: 
    pass

class My_Class:
    x = Attribute()  # Python calls: x.__set_name__(My_Class, 'x')

The expression x = Attribute() internally causes Python to call x.__set_name__(My_Class, 'x').

Overriding __set_name__() Example

Let’s override the __set_name__ magic method on the Attribute‘s class:

class Attribute: 
    def __set_name__(self, owner, name):
        print('Python is great!')

class My_Class:
    x = Attribute()

In fact, the magic method is called which results in the output:

Python is great!

More Practical Example

The purpose of the magic method __set_name__(), however, is not to print dummy strings to the Python shell—as you may have guessed already.

💡 Note: The __set_name__() method is called automatically by Python for every single attribute held by the owner class object when initializing an object—in our previous example an object of type My_Class. More details here.

However, the __set_name__() method is not called when assigning attributes to an object dynamically—later in the code.

To see what I mean, have a look at this code snippet:

class Attribute: 
    def __set_name__(self, owner, name):
        print('Python is great!')

class My_Class:
    pass


My_Class.x = Attribute()   # x.__set_name__() is NOT called!!
# <No Output>

Now, you can manually call the __set_name__() method to run the same routine you’d have run if you’d initialized the attribute in the class definition right away:

class Attribute: 
    def __set_name__(self, owner, name):
        print('Python is great!')

class My_Class:
    pass


My_Class.x = Attribute()
# <No Output>

My_Class.x.__set_name__(My_Class, 'x')
# Python is great!

References:

Where to Go From Here?

Enough theory. Let’s get some practice!

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