Understanding __get__ and __set__ and Python descriptors

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The descriptor is how Python’s property type is implemented. A descriptor simply implements __get__, __set__, etc. and is then added to another class in its definition (as you did above with the Temperature class). For example:

temp=Temperature()
temp.celsius #calls celsius.__get__

Accessing the property you assigned the descriptor to (celsius in the above example) calls the appropriate descriptor method.

instance in __get__ is the instance of the class (so above, __get__ would receive temp, while owner is the class with the descriptor (so it would be Temperature).

You need to use a descriptor class to encapsulate the logic that powers it. That way, if the descriptor is used to cache some expensive operation (for example), it could store the value on itself and not its class.

An article about descriptors can be found here.

EDIT: As jchl pointed out in the comments, if you simply try Temperature.celsius, instance will be None.

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