.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "auto_examples/oop/descriptors.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        Click :ref:`here <sphx_glr_download_auto_examples_oop_descriptors.py>`
        to download the full example code or to run this example in your browser via Binder

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_auto_examples_oop_descriptors.py:


=================
3.12 Descriptors
=================

.. GENERATED FROM PYTHON SOURCE LINES 8-11

Descriptors are another way to control, what happens when a value of an attribute
is set or accessed. We do it while making a class with at least one of
``__get__``, ``__set__`` and ``__del__`` methods.

.. GENERATED FROM PYTHON SOURCE LINES 13-40

.. code-block:: default



    class MyDescriptor(object):
        """
        Basic descriptor to set and get value.
        """

        def __init__(self, initval=None):
            print("__init__ of MyDescriptor called with initial value: ", initval)
            self.__set__(self, initval)

        def __get__(self, instance, owner):
            print(instance, owner)
            print('Getting  self.val: ', self.val)
            return self.val

        def __set__(self, instance, value):
            print('Setting self.val to ', value)
            self.val = value


    class Model(object):
        temp = MyDescriptor(37)  # Descriptor is attached at class definition time


    body = Model()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    __init__ of MyDescriptor called with initial value:  37
    Setting self.val to  37




.. GENERATED FROM PYTHON SOURCE LINES 41-44

When we created an instance of `Model` class, the `MyDescriptor` was initiated. When
`MyDescriptor` was initiated, its ``__init__`` method was called and got
the string printed.

.. GENERATED FROM PYTHON SOURCE LINES 44-47

.. code-block:: default


    print(body.temp)  # a function call from `MyDescriptor` class is hiding here





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    <__main__.Model object at 0x7f136954a7c0> <class '__main__.Model'>
    Getting  self.val:  37
    37




.. GENERATED FROM PYTHON SOURCE LINES 48-50

Above when we ran `body.temp`, the ``__get__`` method of `MyDescriptor`
class for `temp` attribute ran.

.. GENERATED FROM PYTHON SOURCE LINES 50-53

.. code-block:: default


    body.temp = 38  # a function call is hiding here





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Setting self.val to  38




.. GENERATED FROM PYTHON SOURCE LINES 54-57

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    <__main__.Model object at 0x7f136954a7c0> <class '__main__.Model'>
    Getting  self.val:  38
    38




.. GENERATED FROM PYTHON SOURCE LINES 58-61

Thus we see when we get the value of attribute ``temp``, the method ``__get__`` in
`MyDescriptor` gets executed and similarly when we set a value to attribute
`temp`, the method ``__set__`` in `MyDescriptor` gets executed.

.. GENERATED FROM PYTHON SOURCE LINES 63-64

In ``__get__`` method of `MyDescriptor` class, instance is `body` and and owner is `Model`.

.. GENERATED FROM PYTHON SOURCE LINES 66-68

If we want to know what attributes are stored in ``__dict__`` of class and instance,
we can do as following

.. GENERATED FROM PYTHON SOURCE LINES 70-73

.. code-block:: default


    print(body.__dict__)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {}




.. GENERATED FROM PYTHON SOURCE LINES 74-76

The ``__get__`` and ``__set__`` of descriptor can be applied only on those
attributes which are present in ``__dict__`` of owner class i.e. `Model` in this case.

.. GENERATED FROM PYTHON SOURCE LINES 78-85

.. code-block:: default


    # alternative to print(Model.__dict__)


    for key, val in Model.__dict__.items():
        print(key, ': ', val)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    __module__ :  __main__
    temp :  <__main__.MyDescriptor object at 0x7f136957de80>
    __dict__ :  <attribute '__dict__' of 'Model' objects>
    __weakref__ :  <attribute '__weakref__' of 'Model' objects>
    __doc__ :  None




.. GENERATED FROM PYTHON SOURCE LINES 86-87

alternative to print(MyDescriptor.__dict__)

.. GENERATED FROM PYTHON SOURCE LINES 87-91

.. code-block:: default


    for key, val in MyDescriptor.__dict__.items():
        print(key, ': ', val)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    __module__ :  __main__
    __doc__ :  
        Basic descriptor to set and get value.
    
    __init__ :  <function MyDescriptor.__init__ at 0x7f1369535ee0>
    __get__ :  <function MyDescriptor.__get__ at 0x7f1369535790>
    __set__ :  <function MyDescriptor.__set__ at 0x7f1369535dc0>
    __dict__ :  <attribute '__dict__' of 'MyDescriptor' objects>
    __weakref__ :  <attribute '__weakref__' of 'MyDescriptor' objects>




.. GENERATED FROM PYTHON SOURCE LINES 92-94

We can call `get` from class and its instance but `set` can and should only be
called from instance. If we do it from class, this means overriding descriptor.

.. GENERATED FROM PYTHON SOURCE LINES 96-100

.. code-block:: default



    print(Model.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    None <class '__main__.Model'>
    Getting  self.val:  38
    38




.. GENERATED FROM PYTHON SOURCE LINES 101-105

.. code-block:: default


    Model.temp = "useless"
    print(Model.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    useless




.. GENERATED FROM PYTHON SOURCE LINES 106-109

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    useless




.. GENERATED FROM PYTHON SOURCE LINES 110-112

This means we should do some type checking before assigning a value to an attribute.
Consider `descriptor` from another angle below.

.. GENERATED FROM PYTHON SOURCE LINES 115-133

.. code-block:: default


    class LazyDescriptor(object):
        def __init__(self, name, inival):
            self._val = inival
            self.name = name

        def __get__(self, instance, owner):
            print('get in descriptor called')
            instance.__dict__[self.name] = self._val
            return self._val


    class Model(object):
        temp = LazyDescriptor("temp", 37)


    body = Model()








.. GENERATED FROM PYTHON SOURCE LINES 134-137

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    get in descriptor called
    37




.. GENERATED FROM PYTHON SOURCE LINES 138-139

Above we we ran `body.temp`, the ``__get__`` method of descriptor was called.

.. GENERATED FROM PYTHON SOURCE LINES 139-142

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    37




.. GENERATED FROM PYTHON SOURCE LINES 143-145

So the first time we referenced temp, it called the descriptor but not the
second time. Let's look at the `__dict__` for better understanding.

.. GENERATED FROM PYTHON SOURCE LINES 147-151

.. code-block:: default


    body = Model()
    print(body.__dict__)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {}




.. GENERATED FROM PYTHON SOURCE LINES 152-155

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    get in descriptor called
    37




.. GENERATED FROM PYTHON SOURCE LINES 156-159

.. code-block:: default


    print(body.__dict__)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {'temp': 37}




.. GENERATED FROM PYTHON SOURCE LINES 160-163

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    37




.. GENERATED FROM PYTHON SOURCE LINES 164-167

.. code-block:: default


    print(body.__dict__)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {'temp': 37}




.. GENERATED FROM PYTHON SOURCE LINES 168-174

So when we tried to access the value of `x` for the first time, the `key` was
not in ``object.__dict__`` so the descriptor's ``__get__`` was called but when it is
already present, the ``__get__`` from descriptor was not called. This is
because of order in which python looks for attributes of objects. For complete
sequence of rules `see this link <http://simeonfranklin.com/talk/descriptors.html#slide-42>`_.
We can achieve exactly same by another way as well.

.. GENERATED FROM PYTHON SOURCE LINES 177-199

.. code-block:: default



    class LazyProperty(object):
        def __init__(self, val):
            self._val = val
            self.name = val.__name__

        def __get__(self, instance, owner):
            print("get in descriptor called")
            result = self._val(instance)
            instance.__dict__[self.name] = result
            return result


    class Model(object):
        @LazyProperty
        def temp(self):
            return 42


    body = Model()








.. GENERATED FROM PYTHON SOURCE LINES 200-203

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    get in descriptor called
    42




.. GENERATED FROM PYTHON SOURCE LINES 204-207

.. code-block:: default


    print(body.temp)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    42




.. GENERATED FROM PYTHON SOURCE LINES 208-212

Usage cases
-------------
Suppose we define a class which takes the `name`, `weight` and `height` as input/for
initiation and has a method to calculate body mass index i.e. `bmi`.

.. GENERATED FROM PYTHON SOURCE LINES 214-229

.. code-block:: default



    class Insan:
        def __init__(self, name, weight, height):
            self.name = name
            self.weight = weight  # in kg
            self.height = height  # in meters

        def bmi(self):
            return self.weight / self.height ** 2


    ali = Insan('ali', 78, 1.7)
    ali.bmi()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    26.989619377162633



.. GENERATED FROM PYTHON SOURCE LINES 230-231

The problem with the above code is that one can assign negative values to weight.

.. GENERATED FROM PYTHON SOURCE LINES 233-237

.. code-block:: default


    ali.weight = -10
    ali.bmi()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    -3.4602076124567476



.. GENERATED FROM PYTHON SOURCE LINES 238-240

Definitely it is wrong and we should perform some checks before setting the new value.
We can do it by using `property`

.. GENERATED FROM PYTHON SOURCE LINES 243-270

.. code-block:: default


    class Insan:
        def __init__(self, name, weight, height):
            self.name = name
            self._weight = weight  # in kg
            self.height = height  # in meters

        @property
        def weight(self):
            return self._weight

        @weight.setter
        def weight(self, value):
            if value < 0:
                raise ValueError('weight cannot be negative.')
            self._weight = value

        def bmi(self):
            return self.weight / self.height ** 2


    ali = Insan('ali', 78, 1.7)

    # uncomment following line
    # ali.weight = -80  # ValueError
    ali.bmi()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    26.989619377162633



.. GENERATED FROM PYTHON SOURCE LINES 271-272

Thus upon negative weight, it threw error. But `height` can still be assigned a negative value.

.. GENERATED FROM PYTHON SOURCE LINES 272-277

.. code-block:: default


    ali = Insan('ali', 78, 1.7)
    ali.height = -1.8
    print(ali.height)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    -1.8




.. GENERATED FROM PYTHON SOURCE LINES 278-279

Let's make use of `property` once more.

.. GENERATED FROM PYTHON SOURCE LINES 282-315

.. code-block:: default


    class Insan:
        def __init__(self, name, weight, height):
            self.name = name
            self._weight = weight  # in kg
            self._height = height  # in meters

        @property
        def weight(self):
            return self._weight

        @weight.setter
        def weight(self, value):
            if value < 0:
                raise ValueError('weight cannot be negative.')
            self._weight = value

        @property
        def height(self):
            return self._height

        @height.setter
        def height(self, value):
            if value < 0:
                raise ValueError('height cannot be negative.')
            self._height = value


    ali = Insan('Ali', 78, 1.7)

    # uncomment following line
    # ali.weight = -80  # ValueError








.. GENERATED FROM PYTHON SOURCE LINES 316-324

But we are repeating our code. Both the properties are essentially doing
same thing i.e. throwing errors on negative value assignment, so remember
our code should be DRY (do not repeat yourself)
To helps us, python has the concept of `descriptors`. We can define a
descriptor which can have ``set``, ``get`` and ``del`` methods. The following
code defines the descriptor `NonNegative`. Then inside class `Insan`, we
define class attributes and bind them with the descriptor thus making sure
that these attributes will always be non-negative otherwise an error will be thrown.

.. GENERATED FROM PYTHON SOURCE LINES 326-366

.. code-block:: default


    class NonNegative:
        def __init__(self, name):
            # the name attribute is needed because when the NonNegative object is
            # created , the assignment to attribute named weight/height hasn't
            # happen yet. Thus, we need to explicitly pass the name weight/height to the
            # initializer of the object to use as the key for the instance's __dict__.
            self.name = name

        def __get__(self, instance, owner):
            # we need to reach into the __dict__ object directly, because the
            # builtins would be intercepted
            #  by the descriptor protocols too and cause the RecursionError.
            return instance.__dict__[self.name]  # getattr(instance, self._name)

        def __set__(self, instance, value):
            if value < 0:
                raise ValueError("{} Cannot be negative.".format(self.name))
            # instead of using builtin function getattr and setattr, we need to reach
            # into the __dict__ object directly, because the builtins would be intercepted
            #  by the descriptor protocols too and cause the RecursionError.
            instance.__dict__[self.name] = value  # setattr(instance, self._name, value)


    class Insan:
        weight = NonNegative('weight')
        height = NonNegative('height')

        def __init__(self, name, weight, height):
            self.name = name
            self.weight = weight  # in kg
            self.height = height  # in meters

        def bmi(self):
            return self.weight / self.height ** 2


    ali = Insan('Ali', 78, 1.7)
    ali.bmi()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    26.989619377162633



.. GENERATED FROM PYTHON SOURCE LINES 367-368

Now we can not assign negative values to attributes ``weight`` and ``height`` of class ``Insan``.

.. GENERATED FROM PYTHON SOURCE LINES 370-374

.. code-block:: default


    # uncomment following line
    # ali.weight = -80 # ValueError: Cannot be negative








.. GENERATED FROM PYTHON SOURCE LINES 375-379

.. code-block:: default


    # uncomment following line
    # ali.height = -1.8  # ValueError: Cannot be negative








.. GENERATED FROM PYTHON SOURCE LINES 380-386

.. code-block:: default


    ### In python 3.6+

    # The `descriptor` definition in python 3.6+ is more flexible.









.. GENERATED FROM PYTHON SOURCE LINES 387-420

.. code-block:: default


    class NonNegative:
        def __get__(self, instance, owner):
            return instance.__dict__[self.name]

        def __set__(self, instance, value):
            if value < 0:
                raise ValueError('{} Cannot be negative.'.format(self.name))
            instance.__dict__[self.name] = value

        # __set_name__ is called at the time the owning class owner is created.
        # The descriptor has been assigned to name. With this protocol, we can now
        # remove the __init__  and bind the attribute name to the descriptor
        def __set_name__(self, owner, name):
            self.name = name


    class Insan:
        weight = NonNegative()
        height = NonNegative()

        def __init__(self, name, weight, height):
            self.name = name
            self.weight = weight  # in kg
            self.height = height  # in meters

        def bmi(self):
            return self.weight / self.height ** 2


    ali = Insan('Ali', 78, 1.7)
    ali.bmi()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    26.989619377162633



.. GENERATED FROM PYTHON SOURCE LINES 421-425

.. code-block:: default


    # uncomment following line
    # ali.weight = -80  # ValueError: Cannot be negative








.. GENERATED FROM PYTHON SOURCE LINES 426-430

.. code-block:: default


    # uncomment following line
    # ali.height = -1.8  # ValueError: Cannot be negative








.. GENERATED FROM PYTHON SOURCE LINES 431-434

Let's say, we want to calculate a new quantity say `bmi` which is multiplication of
`BMI` with `temperature` in Celsius. We can define a property to convert the
temperature into Celsius, in case the temperature is provided in Fahrenheit.

.. GENERATED FROM PYTHON SOURCE LINES 436-476

.. code-block:: default


    class NonNegative:

        def __get__(self, instance, owner):
            return instance.__dict__[self.name]

        def __set__(self, instance, value):
            if value < 0:
                raise ValueError('{} Cannot be negative.'.format(self.name))
            instance.__dict__[self.name] = value

        def __set_name__(self, owner, name):
            self.name = name


    class Insan:
        weight = NonNegative()
        height = NonNegative()

        def __init__(self, name, weight, height, temp_f):
            self.name = name
            self.weight = weight  # in kg
            self.height = height  # in meters
            self.fahrenheit = temp_f

        @property
        def celsius(self):
            return 5 * (self.fahrenheit - 32) / 9.0

        @celsius.setter
        def celsius(self, val):
            self.fahrenheit = 32 + 9 * val / 5.0

        def bmit(self):
            return self.weight / self.height ** 2 * self.celsius


    ali = Insan('Ali', 78, 1.7, 98.2)
    ali.bmit()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    992.6182237600924



.. GENERATED FROM PYTHON SOURCE LINES 477-480

.. code-block:: default


    print(ali.celsius)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    36.77777777777778




.. GENERATED FROM PYTHON SOURCE LINES 481-483

But we can also define it as `descriptor` as follows. Furthermore we are also
performing non-negative check in this descriptor as well.

.. GENERATED FROM PYTHON SOURCE LINES 486-534

.. code-block:: default


    class Celsius:

        def __get__(self, instance, owner):
            return 5 * (instance.fahrenheit - 32) / 9

        def __set__(self, instance, value):
            if value < 0:
                raise ValueError('{} Cannot be negative.'.format(self.name))
            instance.fahrenheit = 32 + 9 * value / 5

        def __set_name__(self, owner, name):
            self.name = name


    class NonNegative:

        def __get__(self, instance, owner):
            return instance.__dict__[self.name]

        def __set__(self, instance, value):
            if value < 0:
                raise ValueError('{} Cannot be negative.'.format(self.name))
            instance.__dict__[self.name] = value

        def __set_name__(self, owner, name):
            self.name = name


    class Insan:
        weight = NonNegative()
        height = NonNegative()

        celsius = Celsius()

        def __init__(self, name, weight, height, temp_f):
            self.name = name
            self.weight = weight  # in kg
            self.height = height  # in meters
            self.fahrenheit = temp_f  # temperature in fahrenheit

        def bmit(self):
            return self.weight / self.height ** 2 * self.celsius


    ali = Insan('Ali', 78, 1.7, 98.2)
    ali.bmit()





.. rst-class:: sphx-glr-script-out

 .. code-block:: none


    992.6182237600924



.. GENERATED FROM PYTHON SOURCE LINES 535-538

.. code-block:: default


    print(ali.fahrenheit)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    98.2




.. GENERATED FROM PYTHON SOURCE LINES 539-542

.. code-block:: default


    print(ali.celsius)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    36.77777777777778




.. GENERATED FROM PYTHON SOURCE LINES 543-547

.. code-block:: default


    # uncomment following line
    # ali.celsius = -30  # ValueError








.. GENERATED FROM PYTHON SOURCE LINES 548-552

Caveat
--------
Because the descriptors are linked with class and not with instance, so when we create
a new instance, the values get overridden by new instance if they are not linked with instance.

.. GENERATED FROM PYTHON SOURCE LINES 554-593

.. code-block:: default



    class Descriptor:
        def __init__(self):
            self.__temp = 0

        def __get__(self, instance, owner):
            return self.__temp

        def __set__(self, instance, value):
            if isinstance(float(value), float):
                print(value)
            else:
                raise TypeError("Body Temperature must be float or integer")

            if value < 20:
                raise ValueError("Body Temperature can never be less than 20")

            self.__temp = value

        def __set_name__(self, owner, name):
            self.name = name


    class Model:
        temp = Descriptor()

        def __init__(self, name, weight, temp):
            self._name = name
            self.weight = weight
            self.temp = temp

        def __str__(self):
            return "{0} with weight {1} has body temperature {2} Celsius.".format(self._name, self.weight, self.temp)


    body1 = Model("Ali", 80, 40)
    print(body1)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    40
    Ali with weight 80 has body temperature 40 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 594-597

.. code-block:: default


    print(body1.__dict__)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {'_name': 'Ali', 'weight': 80}




.. GENERATED FROM PYTHON SOURCE LINES 598-602

.. code-block:: default


    body2 = Model("Hasan", 75, 37)
    print(body2)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    37
    Hasan with weight 75 has body temperature 37 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 603-606

.. code-block:: default


    print(body1)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Ali with weight 80 has body temperature 37 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 607-608

The solution is to bind the attribute with instance in descriptor as shown below.

.. GENERATED FROM PYTHON SOURCE LINES 610-649

.. code-block:: default



    class Descriptor:
        def __init__(self):
            self.__temp = 0

        def __get__(self, instance, owner):
            return instance.__dict__[self.name]

        def __set__(self, instance, value):
            if isinstance(float(value), float):
                print(value)
            else:
                raise TypeError("Body Temperature must be float or integer")

            if value < 20:
                raise ValueError("Body Temperature can never be less than 20")

            instance.__dict__[self.name] = value

        def __set_name__(self, owner, name):
            self.name = name


    class Model:
        temp = Descriptor()

        def __init__(self, name, weight, temp):
            self.name = name
            self.weight = weight
            self.temp = temp

        def __str__(self):
            return "{0} with weight {1} has body temperature {2} Celsius.".format(self.name, self.weight, self.temp)


    body1 = Model("Ali", 80, 40)
    print(body1)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    40
    Ali with weight 80 has body temperature 40 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 650-653

.. code-block:: default


    print(body1.__dict__)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {'name': 'Ali', 'weight': 80, 'temp': 40}




.. GENERATED FROM PYTHON SOURCE LINES 654-658

.. code-block:: default


    body2 = Model("Hasan", 75, 37)
    print(body2)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    37
    Hasan with weight 75 has body temperature 37 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 659-662

.. code-block:: default


    print(body1)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Ali with weight 80 has body temperature 40 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 663-667

Using WeakKeyDictionary
-------------------------
Usually the attributes from descriptors are saved in ``WeakKeyDictionary``. The
above code can be implemented using ``WeakKeyDictionary`` as shown below

.. GENERATED FROM PYTHON SOURCE LINES 669-710

.. code-block:: default


    from weakref import WeakKeyDictionary


    class Descriptor:
        def __init__(self):
            self.data = WeakKeyDictionary()

        def __get__(self, instance, owner):
            return self.data[instance]

        def __set__(self, instance, value):
            if isinstance(float(value), float):
                print(value)
            else:
                raise TypeError("Body Temperature must be float or integer")

            if value < 20:
                raise ValueError("Body Temperature can never be less than 20")

            self.data[instance] = value

        def __set_name__(self, owner, name):
            self.name = name


    class Model:
        temp = Descriptor()

        def __init__(self, name, weight, temp):
            self.name = name
            self.weight = weight
            self.temp = temp

        def __str__(self):
            return "{0} with weight {1} has body temperature {2} Celsius.".format(self.name, self.weight, self.temp)


    body1 = Model("Ali", 80, 40)
    print(body1)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    40
    Ali with weight 80 has body temperature 40 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 711-715

.. code-block:: default


    body2 = Model("Hasan", 75, 37)
    print(body2)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    37
    Hasan with weight 75 has body temperature 37 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 716-719

.. code-block:: default


    print(body1)





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    Ali with weight 80 has body temperature 40 Celsius.




.. GENERATED FROM PYTHON SOURCE LINES 722-732

**References:**

The material in this lesson is inspired from following posts

   * `talk on descriptors <http://simeonfranklin.com/talk/descriptors.html>`_
   * `Python course eu website <https://www.python-course.eu/python3_descriptors.php>`_
   * `Encapsulation with descriptors <https://pyvideo.org/pycon-us-2013/encapsulation-with-descriptors.html>`_
   * `Some great answers on stackoverflow <https://stackoverflow.com/questions/3798835/understanding-get-and-set-and-python-descriptors>`_
   * `A post by Daw Ran Liou <https://dev.to/dawranliou/writing-descriptors-in-python-36>`_
   * `DataCamp <https://www.datacamp.com/community/tutorials/python-descriptors>`_


.. rst-class:: sphx-glr-timing

   **Total running time of the script:** ( 0 minutes  0.021 seconds)


.. _sphx_glr_download_auto_examples_oop_descriptors.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example


    .. container:: binder-badge

      .. image:: images/binder_badge_logo.svg
        :target: https://mybinder.org/v2/gh/AtrCheema/python-seekho/master?urlpath=lab/tree/notebooks/auto_examples/oop/descriptors.ipynb
        :alt: Launch binder
        :width: 150 px

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: descriptors.py <descriptors.py>`

    .. container:: sphx-glr-download sphx-glr-download-jupyter

      :download:`Download Jupyter notebook: descriptors.ipynb <descriptors.ipynb>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.github.io>`_