Python Descriptors Unveiled: A Comprehensive Examination


Python is a high-level programming language that offers a wide range of features and tools to developers. One such feature that has gained popularity among Python programmers over the years is descriptors.

Descriptors are a powerful tool in Python that allow programmers to define how attributes are accessed, modified and deleted in their code. In this article, we will take an in-depth look at Python descriptors, what they are, how they work, and why it’s important to understand them as a developer.

Explanation of what Python descriptors are

Descriptors in Python are objects that define how attribute access is handled for objects. They provide the programmer with a way to customize attribute access on their classes by defining methods for getting, setting, or deleting attributes. In simpler terms, descriptors let you define how your class’s properties will behave.

Descriptors can be thought of as an intermediate object between the class definition and its instances. When an attribute gets accessed from an instance of a class containing descriptors, the descriptor intervenes before returning or setting the actual value.

Importance of understanding descriptors for Python programming

Understanding descriptors is crucial for any developer who wants to master object-oriented programming in Python. By manipulating attribute access with descriptors, developers can ensure data quality by enforcing constraints like data type validation or range checking while keeping the logic out of the application code.

Descriptors also offer flexibility when working with large-scale applications since they allow changes to occur without modifying existing code; this helps prevent bugs and errors stemming from changes made when developing new features. Additionally, knowing how to use descriptors opens up new possibilities for designing elegant APIs that provide expressive interfaces without exposing implementation details.

Overview of what the article will cover

In this comprehensive examination of Python Descriptors, we will delve into the definition and explanation of descriptors, followed by a detailed explanation of how they work in Python code. We will look at examples of how to use descriptors in code, as well as the different types of descriptors – data and non-data descriptors.

The article will also provide detailed steps on how to implement descriptors in code, with examples demonstrating how to define a descriptor class and use it with attributes. We will look at various use cases for data and non-data descriptors, including validation, type checking, caching, method binding and attribute access control.

We will wrap up by discussing common pitfalls when using descriptors and best practices for development with them. This article aims to provide a comprehensive understanding of Python Descriptors that is both practical and valuable to all developers.

Understanding Descriptors

Definition and explanation of descriptors in Python

Descriptors are objects that define how attributes are accessed and modified in a class. They allow an attribute to be managed by methods defined within the descriptor, rather than directly on the instance of the object. In other words, descriptors act as intermediaries between classes and their attributes.

They make it possible for developers to control how attribute access is handled within a class, which can be incredibly powerful. Descriptors are themselves objects with special methods: __get__(), __set__(), and __delete__().

These methods define how getter, setter, and deleter operations behave for a particular attribute. When an attribute is accessed or modified on an instance of a class that uses descriptor objects, these special methods are called instead of the default ones.

How descriptors work in Python code

Descriptors work by modifying how Python looks up attributes on instances of classes. Normally, when you try to access an attribute on an object (such as Python will look for that attribute first in the object’s dictionary (x.__dict__[“foo”]) and then in its class’s dictionary (x.__class__.__dict__[“foo”]).

If the attribute isn’t found there, it will continue looking up the inheritance chain until it either finds it or raises an AttributeError. When a descriptor is used for an attribute instead of a plain value, however, Python modifies this lookup process slightly.

Instead of looking up the value directly from either the instance’s or class’s dictionary as before, it first calls one or more special methods on the descriptor object itself (namely __get__() if accessing or __set__() if assigning). These methods then have full control over how that particular access operation is performed and can manipulate the value as needed before returning it to the caller.

Examples of how to use descriptors in code

Here’s an example of how you might create a simple descriptor object that modifies access behavior for a particular attribute:

python class MyDescriptor:

def __get__(self, instance, owner): print("Getting attribute!")

return instance._value def __set__(self, instance, value):

print("Setting attribute!") instance._value = value

class MyClass: def __init__(self):

self._value = None my_descriptor = MyDescriptor()

In this example, we create a descriptor object called MyDescriptor that simply prints out messages when the attribute it’s associated with is accessed or modified. We then define a class called MyClass that has an instance variable called _value.

We define an attribute of type MyDescriptor in our class called my_descriptor. Now let’s see how this works when we try to access or modify the _value attribute on an instance of our class:

python >>> my_instance = MyClass()

>>> my_instance.my_descriptor Getting attribute!

>>> my_instance.my_descriptor = 42 Setting attribute!

>>> my_instance.my_descriptor Getting attribute!


As you can see, when we try to access or modify the _value using our descriptor object Python calls our custom-defined __get__()/__set__()s methods instead of falling back on its default behavior. This allows us to control exactly what happens at each step in the process and make sure that everything works smoothly.

Types of Descriptors

Descriptors in Python come in two types: Data Descriptors and Non-Data Descriptors.

Data Descriptors

A data descriptor is a descriptor that defines both the __get__() and __set__() methods. This class of descriptors allows for data to be set and retrieved via some external means, such as an object or method.

When these methods are called, they can perform actions such as validation or type checking before allowing the data to be set or retrieved. An example of a data descriptor is the property() built-in function in Python.

This function allows for setting and getting values of attributes on an object, while also enabling validation and type checking before setting the value. Here’s an example:

class Person: def __init__(self, name):

self._name = name @property

def name(self): return self._name

@name.setter def name(self, value):

if not isinstance(value, str): raise TypeError("Name must be a string")

self._name = value person = Person("John") = "Mary" # sets the name attribute to "Mary" = 123 # raises TypeError: Name must be a string

In this example, the `@property` decorator creates a getter method for retrieving the value of `_name`, while `@name.setter` creates a setter method for setting `_name`. The setter method performs type checking before allowing `_name` to be set.

Non-Data Descriptors

Non-data descriptors define only the __get__() method but not the __set__() method. They allow for controlling access to attributes on an object by manipulating how those attributes are retrieved via dot notation or accessing them indirectly through calls. An example of non-data descriptors is the `staticmethod()` built-in function in Python.

This function allows for creating static methods on a class, which are methods that belong to the class itself rather than any particular instance of the class.

class Math:

@staticmethod def add(a, b):

return a + b result = Math.add(2, 3) # returns 5

In this example, `@staticmethod` decorator creates a method called `add()` that can be accessed directly on the class without creating an instance of the class. Non-data descriptors can also be used for attribute access control and method binding.

Descriptors in Python are powerful tools that allow for defining how attributes are accessed and manipulated on an object. Understanding their types and purposes is important to writing clean and efficient code.

Implementing Descriptors In Code

Descriptors are implemented as classes. To define a descriptor class, you must define at least one of the following three methods: `__get__()`, `__set__()`, or `__delete__()`.

These methods define how the descriptor behaves when accessed like an attribute of an object. The `__get__()` method is called when the attribute the descriptor is attached to is accessed.

The method takes two arguments: self and instance. Self refers to the descriptor object itself, while instance refers to the instance of the class that owns the attribute.

The `__get__()` method returns a value that will be used as the value of the attribute. The `__set__()` method is called when a new value is assigned to an attribute with a descriptor attached.

The method takes three arguments: self, instance, and value. Self and instance have their same meanings as in `__get__()`.

Value refers to the value being assigned to the attribute. If you do not want your descriptor to be settable, you can raise an AttributeError in this method.

How to Define a Descriptor Class

Here’s an example snippet code for defining a simple Descriptor:

class Descriptor: def __init__(self):

self.value = None def __get__(self, obj, objtype):

return self.value def __set__(self, obj, value):

self.value = value

In this example code snippet above we have defined a very basic descriptor that will simply store any values it receives through its set method and always return those values through its get method.

How To Use A Descriptor Class With Attributes

Descriptors can be used with attributes by adding them as class variables in your class definition. Typically they would appear first among whatever other attributes are defined in your class.

Here’s an example of how that could look:

class MyClass:

my_attribute = Descriptor() def __init__(self, initial_value):

self.my_attribute = initial_value

In this example code snippet above, we have defined a class called `MyClass` and added a descriptor called `my_attribute` as a class variable.

We also include an initialization method that sets an initial value for the attribute. When the initialization method is run, it uses the descriptor to set the value of `my_attribute`.

Examples Demonstrating How To Implement A Descriptor

Here’s an example of how to use a descriptor for type checking:

class TypeChecked: def __init__(self, name, expected_type): = name self.expected_type = expected_type

def __get__(self, obj, objtype): return obj.__dict__[]

def __set__(self, obj, value): if not isinstance(value,self.expected_type):

raise TypeError("Expected "+str(self.expected_type)) obj.__dict__[] = value

class MyClass: number = TypeChecked('number', int)

def __init__(self,number): self.number=number

In this example code snippet above, we define a custom descriptor called `TypeChecked`. This descriptor is used to check whether or not the values being assigned to attributes are of the correct type.

In our example case it checks whether number is assigned with integer. We then define our class called `MyClass`.

Notice that now we’ve added another argument in our initialization method and then set its attribute using TypeChecked Descriptor with ‘number’ and int as parameters. This ensures that if any other data type is assigned to MyClass’ attribute number during runtime will raise TypeError.

Descriptor Use Cases

Descriptors are a powerful tool in Python that can be used to achieve a variety of goals. In this section, we will explore some common use cases for descriptors, with a focus on data and non-data descriptors.

Use cases for data descriptors

Data descriptors are descriptors that implement both __get__() and __set__() methods. This means they can be used to control access to an attribute’s value. The following are some common use cases for data descriptors:


Data descriptors can be used to validate the values of attributes before they are set by the user. For example, if we have an attribute that should always be positive, we can define a descriptor that raises an exception if the value is negative.

Type checking

Another use case for data descriptors is type checking. We can define a descriptor that ensures the value assigned to an attribute is of a specific type. For example, if we have an attribute that should always be a string, we can define a descriptor that checks the type of the assigned value.


Data descriptors can also be used for caching. If we have an expensive computation or calculation associated with an attribute, we can define a descriptor that stores the result of the computation so it doesn’t need to be recalculated each time it’s accessed.

Use cases for non-data descriptors

Non-data descriptors are descriptors that only implement __get__(). They do not allow setting values on attributes but instead provide other functionality when accessing them. The following are some common use cases for non-data descriptors:

Method binding

Non-data descriptors can be used to bind methods to classes or instances instead of just functions as in regular methods. When accessing or calling such methods Python automatically passes either instance or class to the first argument of the method based on how it was accessed.

Attribute access control

Another use case for non-data descriptors is attribute access control. We can define a descriptor that controls what attributes are accessible on an object. For example, if we want to limit access to certain attributes on an object, we can define a descriptor that raises an exception when those attributes are accessed.

Lazily computed values

Non-data descriptors can also be used for lazily computed values. If we have an expensive computation or calculation that should only be done when a specific attribute is accessed, we can define a descriptor that performs the computation and replaces itself with the calculated value. This way, the computation is only done when needed and not before.

Descriptors are incredibly powerful in Python and provide great flexibility in controlling and managing attribute access at runtime. By understanding their functionality and potential use cases we can become better programmers able to write efficient code while maintaining simplicity and elegance in our programs.

Descriptor Pitfalls And Best Practices

Common pitfalls when using descriptors

While descriptors are a powerful tool in Python programming, they can also lead to some common pitfalls. One of the most common issues is the misuse of descriptor classes. It is important to understand that descriptor classes should be used for specific purposes, such as validation or type checking, and should not be overused or misapplied.

Another issue is the failure to handle exceptions properly when using descriptors. When implementing a descriptor, it is crucial to handle exceptions gracefully and provide clear feedback to users.

Another common pitfall when using descriptors is the creation of circular references between objects. This can happen when an attribute references another object that in turn refers back to the same attribute object.

This issue can cause memory leaks and performance problems if not addressed quickly. Another pitfall when using descriptors is overreliance on them for simple tasks that could be easily accomplished through other means, such as regular functions or properties.

Best practices when using descriptors

To avoid these pitfalls and ensure effective use of descriptors in Python programming, it is important to follow best practices. One best practice is careful consideration of how and where a descriptor class will be used before implementing it. Developers should consider factors such as the purpose of their implementation (validation, type checking) and whether there are simpler alternatives available.

Another best practice is proper exception handling for descriptor classes. Developers should ensure that exceptions are handled gracefully and provide clear feedback on any issues encountered during execution.

Additionally, developers should strive to avoid circular references between objects by carefully reviewing their code for possible issues before implementation. It’s essential to document the use of any descriptor classes thoroughly so that others working on your code can understand how they work and why they were implemented in certain ways.

While Python descriptors are an indispensable tool for many types of programming tasks, they can also lead to common pitfalls if not used properly. By following best practices such as carefully considering their implementation and handling exceptions properly, developers can avoid these issues and ensure effective use of descriptor classes in their code.


Summary of Key Points

In this comprehensive examination of Python descriptors, we covered a lot of ground. We defined and explained descriptors in Python, explored the types of descriptors, and implemented them in code.

Throughout this article, we learned how to use data and non-data descriptors for different use cases such as validation, type checking, method binding and attribute access control. We also addressed common pitfalls when using descriptors and best practices to avoid them.

Importance of Understanding and Implementing Python Descriptors

Understanding Python descriptors is essential to creating robust code that can handle complex tasks with ease. Descriptors allow us to customize attribute access in our object-oriented programs, leading to more efficient and effective coding practices.

By using data descriptors for validation or caching data on-the-fly, we can ensure that our programs stay accurate even as users interact with them differently over time. Non-data descriptors provide additional flexibility by allowing customization of method calls or attribute access control.

Python’s capabilities are vast, but it is only through understanding its unique features that programmers can unleash its full potential. The ability to implement custom behavior with property-like syntax is just one example of what makes Python so powerful.

Resources for Further Learning

For those interested in delving deeper into the world of Python descriptors after reading this article, there are several resources available: – The official documentation on descriptor protocol – “Python Cookbook” by David Beazley & Brian K. Jones

– “Fluent Python” by Luciano Ramalho – “Learning Python Design Patterns” by Chetan Giridhar

In addition to these resources, there are numerous online communities where programmers share their knowledge about working with language-specific concepts like Python’s descriptor protocol. By exploring these resources further, readers will be able to continue building their knowledge base in order to create powerful and efficient code.

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