How Do Python Descriptors\' `__get__` and `__set__` Methods Work?

Understanding Python Descriptors: get and set

In Python, descriptors are a powerful mechanism that allow objects to intercept attribute access and modification. To grasp their functionality, let's delve into the provided code:

class Celsius(object):
    def __init__(self, value=0.0):
        self.value = value
    def __get__(self, instance, owner):
        return self.value
    def __set__(self, instance, value):
        self.value = float(value)

class Temperature(object):
    celsius = Celsius()

Q1: Why do I need a descriptor class?

A1: A descriptor class encapsulates the logic behind intercepting attribute access and modification. By separating this logic from the class using it, you can reuse it and provide a consistent interface for accessing and modifying attributes.

Q2: What are instance and owner in __get__?

A2:

• instance: When accessing an attribute, this parameter represents the instance of the class that owns the descriptor (e.g., temp in temp.celsius).
• owner: This parameter indicates the class that defines the descriptor (e.g., Temperature).

Q3: How would I call/use this example?

A3: You can access and modify the celsius attribute as follows:

temp = Temperature()
temp.celsius  # calls `celsius.__get__` and returns the current Celsius value
temp.celsius = 25.0  # calls `celsius.__set__` to update the Celsius value

Note: If you try to access Temperature.celsius directly, instance will be None.

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