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Descriptors in Python

Python descriptors: Here, we are going to learn about python descriptors with its get, set and delete methods for an object (which are generally called as descriptors).
Submitted by Bestha Radha Krishna, on April 10, 2019

In Python, a class that implements a get, set or delete methods for an object is called descriptors. Descriptors are the way to create attributes and add managed attributes to objects. These are used to protect the attributes from changes and any modifications. Descriptors can able to increase the readability of a program and coding skills. They can help to validate the data.

For example, we need only positive integer values for attribute age and string values for attribute string then descriptors provides an efficient solution.

To create a descriptor, we need __get__, __set__ and __delete__ methods.

Descriptor Protocol

To create a descriptor, we need a descriptor protocol. By defining the following methods we can efficiently manage the attributes. They are:

    __get__(self, obj, type=None)
    __set__(self, obj, value)
    __delete__(self, obj)

Here,

  • __get__ : It gets the value from an object and returns it.
  • __set__ : It sets a value to the object and returns none.
  • __delete__ : It deletes the value in the object and return none.

These methods are normally referred to as getter and setter methods. Python doesn't provide private variables and by using descriptors we can achieve them. The descriptor with the only __get__ method is called non-data descriptors and these are created only for a class, not for an instance. A class can have other than these methods if necessary.

Creating and calling Descriptors

We can create a descriptor with many ways:

  1. Create a class and override any of the __get__, __set__ and __delete__ methods and use them.
  2. We can use the property type to create descriptor.
  3. We can create descriptors by combining both property type and python decorators.

Let look over each way of creating descriptor.

Creating descriptors using class

class rk:
	def __init__(self):
		self.value=0
		
	def __get__(self,ob, ty):
		print ("get method")		
		return self.value
		
	def __set__(self, ob, ty):
		self.value = ty
		print("set method",self.value)
	
	def __delete__(self, ob):	
		print("delete method")
		del self.value

class inc:
	r=rk()
	
a=inc()
print(a.r)
a.r=3
del a.r

Output

get method
0
set method 3
delete method

In the above program, the get method will __get__ the value, the __set__ method will set the value to attribute and __delete__ method will delete the attribute.

Creating descriptor using property type

By using property() it is easy to create descriptors.

Syntax for creating a property method is:

    property(fget=None, fset=None, fdel=None, doc=None)

Here,

  • fget : Function to be used for getting an attribute value
  • fset : Function to be used for setting an attribute value
  • fdel : Function to be used for deleting an attribute
  • doc : docstring

Now, the same program can be written using property type,

class rk:
	def __init__(self):
		self.value=0

	def fget(self):
		print("Get method")
		return self.value

	def fset(self, ty):
		print ("Set method")
		self.value = ty

	def fdel(self):
		print("delete method")
		del self.value       

	name = property(fget, fset, fdel, "I'm the property.")

r=rk()
r.name=1
print(r.name)
del r.name

Output

Set method
Get method
1
delete method

Creating descriptors using property type and decorators

Python decorators are callable objects used to modify functions or classes. By using decorators we can modify the attribute methods.

Example to create descriptors using property type and decorators:

class rk(object):
	def __init__(self):
		self.i=0

	@property
	def inc(self):
		print("get method")
		return self.i

	@inc.setter
	def inc(self, ty):
		self.i=ty
		print ("Set method",self.i)

	@inc.deleter
	def inc(self):
		print ("delete method")
		del self.i

r=rk()

print(r.inc)
r.inc=3
del r.inc

Output

get method
0
Set method 3
delete method

Advantages of descriptors

Descriptors can increase the readability of a program and it can validate the data based on our requirements. These are low-level features and by using them we can reuse the code.

Descriptors create a way to implement private variables and manage the data. We can protect the attributes from any modifications and any updations.






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