Python’s magic method __getattr__() implements the built-in getattr() function that returns the value associated with a given attribute name. Additionally, __getattr__() is called if the normal attribute access (e.g., my_object.my_attribute) results in an AttributeError.
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 Example
object.__getattr__(self, attr_name)Let’s have a look at an example where you override the __getattr__ 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 __getattr__(self, attr_name):
print('hello world')
alice = Person()
getattr(alice, 'age')
# hello world
Note that __getattr__() is also called if you try to access an attribute that doesn’t exist and, thus, would yield an AttributeError otherwise.
class Person:
def __getattr__(self, attr_name):
print('hello world')
alice = Person()
alice.age # attribute doesn't exist!
# hello world
Here’s what would’ve happened in the same scenario without defining the __getattr__() magic method:
class Person:
pass
alice = Person()
alice.age # attribute doesn't exist!As the alice.age attribute doesn’t exist, Python raises an AttributeError:
Traceback (most recent call last):
File "C:\Users\xcent\Desktop\code.py", line 6, in <module>
alice.age # attribute doesn't exist!
AttributeError: 'Person' object has no attribute 'age'You can see this scenario next in our recap on the built-in setattr() function.
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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