Descriptors are like ninjas in disguise, hiding in plain sight and waiting for you to discover their true potential. Let’s get cracking with the world of descriptors and see how they work their magic!
To start, what exactly is a descriptor? In simple terms, it’s an object that can customize attribute access or assignment behavior in your classes. Descriptors are defined using special methods like `__get__`, `__set__`, and `__delete__`. These methods allow you to intercept the lookup, set, and delete operations of attributes on a class instance.
Let’s take an example to illustrate this concept. Imagine we have a simple class called “Person” with two properties name and age:
# The following script creates a class called "Person" with two properties: name and age.
# The properties are set to private using "__" before the name, which means they cannot be accessed directly from outside the class.
class Person:
# The "__init__" method is used to initialize the class with the given name and age.
def __init__(self, name, age):
# The "self" parameter refers to the current instance of the class.
# The "__name" and "__age" properties are set to the given values.
self.__name = name
self.__age = age
# The "@property" decorator is used to define a getter method for the "name" property.
# This allows us to access the property as if it were a regular attribute.
@property
def name(self):
# The getter method simply returns the value of the "__name" property.
return self.__name
# The "@name.setter" decorator is used to define a setter method for the "name" property.
# This allows us to set the value of the property using the assignment operator (=).
@name.setter
def name(self, value):
# The "isinstance" function checks if the given value is of type "str".
# If not, a "TypeError" is raised.
if not isinstance(value, str):
raise TypeError("Name must be a string")
# If the value is of type "str", it is assigned to the "__name" property.
self.__name = value
# The "@property" decorator is used to define a getter method for the "age" property.
# This allows us to access the property as if it were a regular attribute.
@property
def age(self):
# The getter method simply returns the value of the "__age" property.
return self.__age
# The "@age.setter" decorator is used to define a setter method for the "age" property.
# This allows us to set the value of the property using the assignment operator (=).
@age.setter
def age(self, value):
# The "isinstance" function checks if the given value is of type "int".
# The "value < 0" condition checks if the value is a non-negative integer.
# If not, a "ValueError" is raised.
if not isinstance(value, int) or value < 0:
raise ValueError("Age must be a non-negative integer")
# If the value is of type "int" and is non-negative, it is assigned to the "__age" property.
self.__age = value
This class has two properties `name` and `age`. The `__init__` method initializes the private attributes `__name` and `__age`, while the property decorators provide getter and setter methods for these attributes. However, this implementation is not perfect. What if we want to add some validation logic or custom behavior when accessing or setting these properties? That’s where descriptors come in!
Let’s rewrite our `Person` class using descriptors:
# Rewritten Person class using descriptors
class Person:
def __init__(self, name, age):
self.__name = name
self.__age = age
# Descriptor for the name attribute
class Name(object):
# Getter method for name attribute
def __get__(self, instance, owner):
# Returns the value of the name attribute from the instance's dictionary
return instance.__dict__[owner].__name
# Setter method for name attribute
def __set__(self, instance, value):
# Checks if the value is a string
if not isinstance(value, str):
# Raises a TypeError if the value is not a string
raise TypeError("Name must be a string")
# Sets the name attribute in the instance's dictionary to the new value
instance.__dict__[owner] = Person._rename_person(instance, value)
# Descriptor for the age attribute
class Age(object):
# Getter method for age attribute
def __get__(self, instance, owner):
# Returns the value of the age attribute from the instance's dictionary
return instance.__dict__[owner].__age
# Setter method for age attribute
def __set__(self, instance, value):
# Checks if the value is an integer and non-negative
if not isinstance(value, int) or value < 0:
# Raises a ValueError if the value is not an integer or is negative
raise ValueError("Age must be a non-negative integer")
# Sets the age attribute in the instance's dictionary to the new value
instance.__dict__[owner] = Person._rename_person(instance, value)
# Private method to rename a person's name
def _rename_person(self, person, new_name):
# Gets the old name from the person's dictionary
old_name = person.__dict__['Name']
# Checks if the new name is valid
if self.is_valid_name(new_name):
# Deletes the old name attribute from the person's dictionary
delattr(person, 'Name')
# Sets the new name attribute in the person's dictionary
setattr(person, 'Name', new_name)
# Returns the new name
return new_name
else:
# Raises a ValueError if the new name is not valid
raise ValueError("Invalid name")
# Method to validate a name
def is_valid_name(self, name):
# TODO: Implement validation logic here
pass
# Creates instances of the Name and Age descriptors for the Person class
Name = Name()
Age = Age()
In this implementation, we’ve created two descriptors `Name` and `Age`. These descriptors are defined as nested classes inside the main class. The `__get__` and `__set__` methods of these descriptors intercept the attribute access or assignment operations for their respective attributes (`name` and `age`) on a class instance.
The `_rename_person` method is used to rename the person object when setting its name using the `Name` descriptor. This method checks if the new name is valid, removes the old name attribute from the person object, sets the new name attribute, and returns the new name.
This implementation has several advantages over our previous one:
1. We can easily add validation logic or custom behavior to any property by defining a descriptor for it. This makes our classes more flexible and powerful.
2. The `Name` and `Age` descriptors are defined as nested classes inside the main class, which keeps them organized and easy to find.
3. By using descriptors instead of properties, we can avoid creating unnecessary getter or setter methods for each property. This reduces code duplication and makes our classes more concise.
4. Descriptors allow us to customize attribute access or assignment behavior at a finer granularity than properties. For example, we could create a descriptor that returns different values depending on the context in which it’s accessed (e.g., inside or outside of a function).
Descriptors are like ninjas in disguise, hiding in plain sight and waiting for you to discover their true potential. They allow objects to customize attribute access or assignment behavior in your classes using special methods like `__get__`, `__set__`, and `__delete__`. By defining descriptors as nested classes inside the main class, we can easily add validation logic or custom behavior to any property without creating unnecessary getter or setter methods. This makes our classes more flexible, powerful, and concise.