Detailed explanation of decorators such as Python decorators

1. Introduction

In Python, a decorator is a tool that allows us to dynamically add or modify functions and class functions without changing the original code.

Compared with function decorators, class decorators are more powerful and flexible, because they can not only modify class methods, but also class itself.

2. Basic concepts of decorators

A decorator is a higher-order function that accepts a function or class and returns a new function or class. They usually pass@Symbols are used.

When using a decorator, we place the decorator above the decorator so that the decorator will be automatically applied when the object is created.

2.1. Function Decorator Review

Before introducing class decorators, let’s briefly review the basic usage of function decorators.

def my_decorator(func):
    def wrapper(*args, **kwargs):
        print("Something is happening before the function is called.")
        result = func(*args, **kwargs)
        print("Something is happening after the function is called.")
        return result
    return wrapper

@my_decorator
def say_hello(name):
    print(f"Hello, {name}!")

say_hello("Alice")

Run the above code and the output is:

Something is happening before the function is called.
Hello, Alice!
Something is happening after the function is called.

2.2 Definition and use of class decorators

Class decorators are similar to function decorators, except that they take a class as an argument and return a new class.

The simplest class decorator can be defined like this:

def class_decorator(cls):
    class WrappedClass(cls):
        def new_method(self):
            print("This is a new method added by the decorator.")
        def existing_method(self):
            print("This method is overridden by the decorator.")
    return WrappedClass

@class_decorator
class MyClass:
    def existing_method(self):
        print("This method exists in the original class.")

obj = MyClass()
obj.new_method()          # Call the newly added methodobj.existing_method()     # Calling the rewritten method

Run the above code and the output is:

This is a new method added by the decorator.
This method is overridden by the decorator.

In this example,class_decoratoris a class decorator that accepts a classMyClassAs an argument, and return a new classWrappedClass。WrappedClassInherited fromMyClass, and some new and rewrite methods.

3. Application scenarios of class decorators

There are many application scenarios in actual development of class decorators. Here are a few common examples:

3.1. Automatically add properties

Class decorators can be used to automatically add some properties to classes.

For example, we can create a decorator that automatically adds one to the classcreated_atProperties, record the creation time of the object.

import datetime

def add_timestamp(cls):
    class WrappedClass(cls):
        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            self.created_at = ()
    return WrappedClass

@add_timestamp
class MyClass:
    def __init__(self, name):
         = name

obj = MyClass("Alice")
print()        # Output: Aliceprint(obj.created_at)  # Output: The creation time of the object

3.2 Access Control

Class decorators can be used to control the access permissions of classes.

For example, we can create a decorator that requires administrator privileges when calling certain methods.

def require_admin(cls):
    class WrappedClass(cls):
        def admin_method(self, *args, **kwargs):
            if not self.is_admin:
                raise PermissionError("Admin privileges required")
            return super().admin_method(*args, **kwargs)
    return WrappedClass

@require_admin
class MyClass:
    def __init__(self, is_admin):
        self.is_admin = is_admin

    def admin_method(self):
        print("This is an admin method.")

admin_obj = MyClass(is_admin=True)
admin_obj.admin_method()  # Normal call
user_obj = MyClass(is_admin=False)
user_obj.admin_method()   # Throw out PermissionError

3.3 Data Verification

Class decorators can be used to perform data verification before class method calls.

For example, we can create a decorator that verifies the type of method parameters.

def validate_params(cls):
    class WrappedClass(cls):
        def method_with_validation(self, x):
            if not isinstance(x, int):
                raise ValueError("Parameter x must be an integer")
            return super().method_with_validation(x)
    return WrappedClass

@validate_params
class MyClass:
    def method_with_validation(self, x):
        print(f"Received {x}")

obj = MyClass()
obj.method_with_validation(10)  # Normal callobj.method_with_validation("a") # Throw out ValueError

4. Comprehensive example

Next, we will show how to use class decorators with a comprehensive example.

This example will include a logging decorator, a permission check decorator, and a data verification decorator.

import datetime

def log_activity(cls):
    class WrappedClass(cls):
        def __getattribute__(self, name):
            attr = super().__getattribute__(name)
            if callable(attr):
                def logged(*args, **kwargs):
                    print(f"Calling {name} with arguments {args} and {kwargs} at {()}")
                    result = attr(*args, **kwargs)
                    print(f"{name} returned {result}")
                    return result
                return logged
            return attr
    return WrappedClass

def require_permission(permission):
    def decorator(cls):
        class WrappedClass(cls):
            def __getattribute__(self, name):
                attr = super().__getattribute__(name)
                if callable(attr):
                    def secured(*args, **kwargs):
                        if not self.has_permission(permission):
                            raise PermissionError(f"Permission {permission} required")
                        return attr(*args, **kwargs)
                    return secured
                return attr
        return WrappedClass
    return decorator

def validate_params(cls):
    class WrappedClass(cls):
        def method_with_validation(self, x):
            if not isinstance(x, int):
                raise ValueError("Parameter x must be an integer")
            return super().method_with_validation(x)
    return WrappedClass

@log_activity
@require_permission("admin")
@validate_params
class MyClass:
    def __init__(self, is_admin):
        self.is_admin = is_admin

    def has_permission(self, permission):
        return self.is_admin

    def method_with_validation(self, x):
        return f"Received {x}"

# Test comprehensive exampleadmin_obj = MyClass(is_admin=True)
print(admin_obj.method_with_validation(10))  # Normal call
try:
    admin_obj.method_with_validation("a")  # Throw ValueErrorexcept ValueError as e:
    print(e)

user_obj = MyClass(is_admin=False)
try:
    user_obj.method_with_validation(10)  # Throw PermissionErrorexcept PermissionError as e:
    print(e)

4.1 Output result

Calling __init__ with arguments (True,) and {} at 2024-07-11 14:23:45.123456
__init__ returned None
Calling method_with_validation with arguments (10,) and {} at 2024-07-11 14:23:45.123456
method_with_validation returned Received 10
Received 10
Calling method_with_validation with arguments ('a',) and {} at 2024-07-11 14:23:45.123456
Parameter x must be an integer
Calling __init__ with arguments (False,) and {} at 2024-07-11 14:23:45.123456
__init__ returned None
Calling method_with_validation with arguments (10,) and {} at 2024-07-11 14:23:45.123456
Permission admin required

4.2 Explanation

Logging Decorator: Record the method call of the class, including input parameters and return values.
Permission Check Decorator: Check whether the user has permission to call the method.
Data Verification Decorator: Verify the type of method parameters.

In this way, we can add additional functionality to the class without modifying the class itself.

Summarize

Class decorator is a very powerful tool in Python that can be used to extend and modify the behavior of classes. With the class decorator, we can add extra functionality to the class without modifying the original code.

In actual development, class decorators have many application scenarios, including logging, access control, data verification, etc.

The above is personal experience. I hope you can give you a reference and I hope you can support me more.