Python descriptors

February 07, 2023

I only learned today about a neat Python feature: descriptors. I came across that feature twice in the same day: once at work on a custom Django field for translations and a second time in this great article.

From the article, I dived a bit deeper and landed on the amazing “Descriptor How To Guide” from the Python documentation. The example of logging is a simple, yet practical one, that helped me “get” it:

class LoggedAgeAccess:
    """Descriptor class."""

    def __get__(self, obj, objtype=None):
        value = obj._age
        print(f'Accessing age={value}')
        return value

    def __set__(self, obj, value):
        print(f'Updating age to {value=}')
        obj._age = value


class Person:
    """Class using the descriptor."""

    # Descriptor instance
    age = LoggedAgeAccess()

    def __init__(self, name, age):
        self.name = name
        # Calls __set__()
        self.age = age

    def birthday(self):
        # Calls __get__() then __set__()
        self.age += 1

The guide expands on more advanced usages. I love how it starts simple, gets more complicated as you go, before expanding on the more formal definitions. A great resource from Raymond Hettinger.

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