Python __getattribute__() Magic Method

Python’s magic method __getattribute__() implements the built-in getattr() function that returns the value associated with a given attribute name. If the __getattribute__() error results in an AttributeError due to a non-existent attribute, Python will call the __getattr__() function for resolution.

Thus, the __getattribute__() method takes precedence over the __getattr__() method.

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.__getattribute__(self, attr_name)

Let’s have a look at an example where you override the __getattribute__ magic method of a custom class Person to simply print out the string 'hello world' when calling the getattr() built-in function.

class Person:
    def __getattribute__(self, attr_name):
        print('hello world')
    

alice = Person()
getattr(alice, 'age')
# hello world

__getattribute__ vs __getattr__

The __getattribute__ method is similar to __getattr__. What’s the difference?

Assume a user wants to access an attribute from a given object like so:

my_obj.my_attr

• my_obj.__getattribute__(my_attr) is called first. If it yields an AttributeError, Python will hand it to:
• my_obj.__getattr__(my_attr) which is called second.

You can see that the former takes precedence over the latter in the following code snippet that defines both methods—but __getattribute__() is taken as it doesn’t result in an error.

class Person:
    def __getattribute__(self, attr_name):
        print('hello world')

    def __getattr__(self, attr_name):
        print('hello universe')
    

alice = Person()
getattr(alice, 'age')
# hello world

Here’s what would’ve happened in the same scenario when raising an AttributeError in __getattribute__:

class Person:
    def __getattribute__(self, attr_name):
        raise AttributeError

    def __getattr__(self, attr_name):
        print('hello universe')
    

alice = Person()
getattr(alice, 'age')
# hello universe

Python doesn’t even mention the error but passes the execution flow to the __getattr__() method.

Background getattr()

Python’s built-in getattr(object, string) function returns the value of the object‘s attribute with name string.

If this doesn’t exist, it returns the value provided as an optional third default argument.

If that doesn’t exist either, it raises an AttributeError.

An example is getattr(porsche, 'speed') which is equivalent to porsche.speed.

# Define class with one attribute
class Car:
    def __init__(self, brand, speed):
        self.brand = brand
        self.speed = speed


# Create object
porsche = Car('porsche', 100)
tesla = Car('tesla', 110)

# Two alternatives to get instance attributes:
print(getattr(porsche, 'brand') + " " + str(getattr(porsche, 'speed')))
print(tesla.brand + " " + str(tesla.speed))


# Get an attribute that doesn't exist with default argument:
print(getattr(porsche, 'color', 'red'))

Output:

porsche 100
tesla 110
red

Further Reading:

• Python __delattr__() magic method
• Python setattr() built-in function
• Python getattr() built-in function
• Python __getattr__() vs __getattribute__()
• https://docs.python.org/3/reference/datamodel.html

Where to Go From Here?

Enough theory. Let’s get some practice!

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