Non-data descriptor

Learn non-data descriptors with only __get__.

What is a non-data descriptor?

A non-data descriptor implements only __get__; there is no __set__. It intercepts reads but not writes.

This has an important consequence: a same-named entry in the instance dict (obj.__dict__) overrides a non-data descriptor. So writing box.label = "new" puts the value in the instance dict and subsequent reads return that value, not the descriptor. classmethod and staticmethod are non-data descriptors. This flexibility is ideal for simple computations that allow overriding.

Priority order

Python follows this priority for attribute access: first data descriptors, then the instance dict (obj.__dict__), then non-data descriptors, finally the class dict.

This explains why property (a data descriptor) overrides an instance value but classmethod (a non-data descriptor) can be overridden. A data descriptor always wins because it implements __set__ and writes go through it. A non-data descriptor only kicks in at read time and steps aside if the instance dict has a value.

Exercise Computation with a non-data descriptor

Write a Squared non-data descriptor. Store self.base with __init__(self, base). __get__(self, obj, objtype=None) returns self.base ** 2. In a Math class define area = Squared(5). Create an instance and print area.

Show example solution

# Write the Squared descriptor
class Squared:
    def __init__(self, base):
        self.base = base
    def __get__(self, obj, objtype=None):
        return self.base ** 2

class Math:
    area = Squared(5)

m = Math()
print(m.area)

Test yourself

Complete the non-data descriptor definition

class Squared:
    def ______(self, obj, objtype=None):
        return self.base ** 2

Show answer

class Squared:
    def __get(self, obj, objtype=None):
        return self.base ** 2

Match non-data descriptor concepts

__get__

Intercepts read
no __set__

Write is free
instance dict

Overrides descriptor
classmethod

Non-data example
When is a non-data descriptor overridden?
```
class Desc:
    def __get__(self, o, t=None):
        return "a"

class C:
    x = Desc()

c = C()
c.x = "b"
print(c.x)
```
- Never overridden
- When a value is written to the instance dict
- On class access
- Only when __set__ is defined
Show answer

When a value is written to the instance dict — c.x = "b" goes to the instance dict; the non-data descriptor steps aside.

Next lesson: @property

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies