Understanding __get__ and __set__ and Python descriptors

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.

Why do I need the descriptor class?

It gives you extra control over how attributes work. If you're used to getters and setters in Java, for example, then it's Python's way of doing that. One advantage is that it looks to users just like an attribute (there's no change in syntax). So you can start with an ordinary attribute and then, when you need to do something fancy, switch to a descriptor.

An attribute is just a mutable value. A descriptor lets you execute arbitrary code when reading or setting (or deleting) a value. So you could imagine using it to map an attribute to a field in a database, for example – a kind of ORM.

Another use might be refusing to accept a new value by throwing an exception in __set__ – effectively making the "attribute" read only.

What is instance and owner here? (in __get__). What is the purpose of these parameters?

This is pretty subtle (and the reason I am writing a new answer here - I found this question while wondering the same thing and didn't find the existing answer that great).

A descriptor is defined on a class, but is typically called from an instance. When it's called from an instance both instance and owner are set (and you can work out owner from instance so it seems kinda pointless). But when called from a class, only owner is set – which is why it's there.

This is only needed for __get__ because it's the only one that can be called on a class. If you set the class value you set the descriptor itself. Similarly for deletion. Which is why the owner isn't needed there.

How would I call/use this example?

Well, here's a cool trick using similar classes:

class Celsius:    def __get__(self, instance, owner):        return 5 * (instance.fahrenheit - 32) / 9    def __set__(self, instance, value):        instance.fahrenheit = 32 + 9 * value / 5class Temperature:    celsius = Celsius()    def __init__(self, initial_f):        self.fahrenheit = initial_ft = Temperature(212)print(t.celsius)t.celsius = 0print(t.fahrenheit)

(I'm using Python 3; for python 2 you need to make sure those divisions are / 5.0 and / 9.0). That gives:

100.032.0

Now there are other, arguably better ways to achieve the same effect in python (e.g. if celsius were a property, which is the same basic mechanism but places all the source inside the Temperature class), but that shows what can be done...

I am trying to understand what Python's descriptors are and what they can be useful for.

Descriptors are objects in a class namespace that manage instance attributes (like slots, properties, or methods). For example:

class HasDescriptors:    __slots__ = 'a_slot' # creates a descriptor        def a_method(self):  # creates a descriptor        "a regular method"        @staticmethod        # creates a descriptor    def a_static_method():        "a static method"        @classmethod         # creates a descriptor    def a_class_method(cls):        "a class method"        @property            # creates a descriptor    def a_property(self):        "a property"# even a regular function:def a_function(some_obj_or_self):      # creates a descriptor    "create a function suitable for monkey patching"HasDescriptors.a_function = a_function     # (but we usually don't do this)

Pedantically, descriptors are objects with any of the following special methods, which may be known as "descriptor methods":

• __get__: non-data descriptor method, for example on a method/function
• __set__: data descriptor method, for example on a property instance or slot
• __delete__: data descriptor method, again used by properties or slots

These descriptor objects are attributes in other object class namespaces. That is, they live in the __dict__ of the class object.

Descriptor objects programmatically manage the results of a dotted lookup (e.g. foo.descriptor) in a normal expression, an assignment, or a deletion.

Functions/methods, bound methods, property, classmethod, and staticmethod all use these special methods to control how they are accessed via the dotted lookup.

A data descriptor, like property, can allow for lazy evaluation of attributes based on a simpler state of the object, allowing instances to use less memory than if you precomputed each possible attribute.

Another data descriptor, a member_descriptor created by __slots__, allows memory savings (and faster lookups) by having the class store data in a mutable tuple-like datastructure instead of the more flexible but space-consuming __dict__.

Non-data descriptors, instance and class methods, get their implicit first arguments (usually named self and cls, respectively) from their non-data descriptor method, __get__ - and this is how static methods know not to have an implicit first argument.

Most users of Python need to learn only the high-level usage of descriptors, and have no need to learn or understand the implementation of descriptors further.

But understanding how descriptors work can give one greater confidence in one's mastery of Python.

In Depth: What Are Descriptors?

A descriptor is an object with any of the following methods (__get__, __set__, or __delete__), intended to be used via dotted-lookup as if it were a typical attribute of an instance. For an owner-object, obj_instance, with a descriptor object:

• obj_instance.descriptor invokes
  descriptor.__get__(self, obj_instance, owner_class) returning a value
  This is how all methods and the get on a property work.

• obj_instance.descriptor = value invokes
  descriptor.__set__(self, obj_instance, value) returning None
  This is how the setter on a property works.

• del obj_instance.descriptor invokes
  descriptor.__delete__(self, obj_instance) returning None
  This is how the deleter on a property works.

obj_instance is the instance whose class contains the descriptor object's instance. self is the instance of the descriptor (probably just one for the class of the obj_instance)

To define this with code, an object is a descriptor if the set of its attributes intersects with any of the required attributes:

def has_descriptor_attrs(obj):    return set(['__get__', '__set__', '__delete__']).intersection(dir(obj))def is_descriptor(obj):    """obj can be instance of descriptor or the descriptor class"""    return bool(has_descriptor_attrs(obj))

A Data Descriptor has a __set__ and/or __delete__.
A Non-Data-Descriptor has neither __set__ nor __delete__.

def has_data_descriptor_attrs(obj):    return set(['__set__', '__delete__']) & set(dir(obj))def is_data_descriptor(obj):    return bool(has_data_descriptor_attrs(obj))

Builtin Descriptor Object Examples:

• classmethod
• staticmethod
• property
• functions in general

Non-Data Descriptors

We can see that classmethod and staticmethod are Non-Data-Descriptors:

>>> is_descriptor(classmethod), is_data_descriptor(classmethod)(True, False)>>> is_descriptor(staticmethod), is_data_descriptor(staticmethod)(True, False)

Both only have the __get__ method:

>>> has_descriptor_attrs(classmethod), has_descriptor_attrs(staticmethod)(set(['__get__']), set(['__get__']))

Note that all functions are also Non-Data-Descriptors:

>>> def foo(): pass... >>> is_descriptor(foo), is_data_descriptor(foo)(True, False)

Data Descriptor, property

However, property is a Data-Descriptor:

>>> is_data_descriptor(property)True>>> has_descriptor_attrs(property)set(['__set__', '__get__', '__delete__'])

Dotted Lookup Order

These are important distinctions, as they affect the lookup order for a dotted lookup.

obj_instance.attribute

1. First the above looks to see if the attribute is a Data-Descriptor on the class of the instance,
2. If not, it looks to see if the attribute is in the obj_instance's __dict__, then
3. it finally falls back to a Non-Data-Descriptor.

The consequence of this lookup order is that Non-Data-Descriptors like functions/methods can be overridden by instances.

Recap and Next Steps

We have learned that descriptors are objects with any of __get__, __set__, or __delete__. These descriptor objects can be used as attributes on other object class definitions. Now we will look at how they are used, using your code as an example.

Analysis of Code from the Question

Here's your code, followed by your questions and answers to each:

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

1. Why do I need the descriptor class?

Your descriptor ensures you always have a float for this class attribute of Temperature, and that you can't use del to delete the attribute:

>>> t1 = Temperature()>>> del t1.celsiusTraceback (most recent call last):  File "<stdin>", line 1, in <module>AttributeError: __delete__

Otherwise, your descriptors ignore the owner-class and instances of the owner, instead, storing state in the descriptor. You could just as easily share state across all instances with a simple class attribute (so long as you always set it as a float to the class and never delete it, or are comfortable with users of your code doing so):

class Temperature(object):    celsius = 0.0

This gets you exactly the same behavior as your example (see response to question 3 below), but uses a Pythons builtin (property), and would be considered more idiomatic:

class Temperature(object):    _celsius = 0.0    @property    def celsius(self):        return type(self)._celsius    @celsius.setter    def celsius(self, value):        type(self)._celsius = float(value)

2. What is instance and owner here? (in get). What is the purpose of these parameters?

instance is the instance of the owner that is calling the descriptor. The owner is the class in which the descriptor object is used to manage access to the data point. See the descriptions of the special methods that define descriptors next to the first paragraph of this answer for more descriptive variable names.

3. How would I call/use this example?

Here's a demonstration:

>>> t1 = Temperature()>>> t1.celsius0.0>>> t1.celsius = 1>>> >>> t1.celsius1.0>>> t2 = Temperature()>>> t2.celsius1.0

You can't delete the attribute:

>>> del t2.celsiusTraceback (most recent call last):  File "<stdin>", line 1, in <module>AttributeError: __delete__

And you can't assign a variable that can't be converted to a float:

>>> t1.celsius = '0x02'Traceback (most recent call last):  File "<stdin>", line 1, in <module>  File "<stdin>", line 7, in __set__ValueError: invalid literal for float(): 0x02

Otherwise, what you have here is a global state for all instances, that is managed by assigning to any instance.

The expected way that most experienced Python programmers would accomplish this outcome would be to use the property decorator, which makes use of the same descriptors under the hood, but brings the behavior into the implementation of the owner class (again, as defined above):

class Temperature(object):    _celsius = 0.0    @property    def celsius(self):        return type(self)._celsius    @celsius.setter    def celsius(self, value):        type(self)._celsius = float(value)

Which has the exact same expected behavior of the original piece of code:

>>> t1 = Temperature()>>> t2 = Temperature()>>> t1.celsius0.0>>> t1.celsius = 1.0>>> t2.celsius1.0>>> del t1.celsiusTraceback (most recent call last):  File "<stdin>", line 1, in <module>AttributeError: can't delete attribute>>> t1.celsius = '0x02'Traceback (most recent call last):  File "<stdin>", line 1, in <module>  File "<stdin>", line 8, in celsiusValueError: invalid literal for float(): 0x02

Conclusion

We've covered the attributes that define descriptors, the difference between data- and non-data-descriptors, builtin objects that use them, and specific questions about use.

So again, how would you use the question's example? I hope you wouldn't. I hope you would start with my first suggestion (a simple class attribute) and move on to the second suggestion (the property decorator) if you feel it is necessary.