Understanding Python Decorators: Definitions, Syntax, and Practical Use Cases

1. Definition of Decorators

A decorator is a function that adds extra functionality to an existing function; essentially it is a closure.

Features of decorators:

Do not modify the source code of the original function.

Do not change the way the original function is called.

Add additional behavior to the original function.

Distinguishing closures and decorators:

If a closure function has exactly one parameter and that parameter is a function, the closure is considered a decorator.

Write code following the Open‑Closed Principle: existing code should not be modified but can be extended.

2. Example Code

<code># 定义装饰器
def decorator(func):
    def inner():
        # 在内部函数里面对已有函数进行装饰
        print('已添加登录认证')
        func()
    return inner


def comment():
    print('发表评论')

# 调用装饰器对已有函数进行装饰，左边的 comment = inner
comment = decorator(comment)

# 调用方式不变
comment()</code>

3. Syntactic Sugar for Decorators

When many functions need the same login‑validation logic, repeatedly writing func = decorator(func) becomes cumbersome. Python provides a syntactic sugar using the @ symbol.

<code># 定义装饰器
def decorator(func):
    def inner():
        # 在内部函数里面对已有函数进行装饰
        print('已添加登录认证')
        func()
    return inner

@decorator  # 等价于 comment = decorator(comment)
def comment():
    print('发表评论')

# 调用方式不变
comment()</code>

4. Execution Timing of Decorators

After a module is loaded, the decorator executes immediately, decorating the target function.

<code># 定义装饰器
def decorator(func):
    print('装饰器执行了')
    def inner():
        # 在内部函数里面对已有函数进行装饰
        print('已添加登录认证')
        func()
    return inner

@decorator
def comment():
    print('发表评论')

# 运行结果
# 装饰器执行了</code>

5. Uses of Decorators

5.1 Typical Scenarios

Measuring function execution time

Logging output information

5.2 Timing Example

<code>import time

def decorator(func):
    def inner():
        begin = time.time()
        func()
        end = time.time()
        result = end - begin
        print(f'函数执行完成耗时：{result}')
    return inner

@decorator
def work():
    for i in range(10000):
        print(i)

work()</code>

6. Generic Decorators

Generic decorators can wrap functions of any signature.

6.1 Decorating Functions with Parameters

<code>def decorator(func):
    def inner(num1, num2):
        print('正在努力执行加法计算')
        func(num1, num2)
    return inner

@decorator
def add_num(num1, num2):
    result = num1 + num2
    print(f'结果为：{result}')

add_num(1, 2)</code>

6.2 Decorating Functions with Return Values

<code>def decorator(func):
    def inner(num1, num2):
        print('正在努力执行加法计算')
        num = func(num1, num2)
        return num
    return inner

@decorator
def add_num(num1, num2):
    result = num1 + num2
    return result

result = add_num(1, 2)
print(f'结果为：{result}')</code>

6.3 Decorating Functions with Variable Arguments

<code>def decorator(func):
    def inner(*args, **kwargs):
        print('正在努力执行加法计算')
        # *args: positional arguments
        # **kwargs: keyword arguments
        num = func(*args, **kwargs)
        return num
    return inner

@decorator
def add_num(*args, **kwargs):
    result = 0
    for value in args:
        result += value
    for value in kwargs.values():
        result += value
    return result

result = add_num(1, 2, a=3)
print(f'结果为：{result}')</code>

7. Stacking Multiple Decorators

When multiple decorators are applied, they are executed from the innermost to the outermost.

<code>def make_div(func):
    print('make_div装饰器执行了')
    def inner():
        result = '<div>' + func() + '</div>'
        return result
    return inner

def make_p(func):
    print('make_p装饰器执行了')
    def inner():
        result = '<p>' + func() + '</p>'
        return result
    return inner

@make_div
@make_p
def content():
    return '人生苦短，我用python'

c = content()
print(c)</code>

Output:

<code>make_p装饰器执行了
make_div装饰器执行了
<div><p>人生苦短，我用python</p></div></code>

8. Decorators with Parameters

A decorator can accept its own arguments using an extra outer function.

<code>def return_decorator(flag):
    def decorator(func):
        def inner(a, b):
            if flag == '+':
                print('正在努力执行加法计算')
            elif flag == '-':
                print('正在努力执行减法计算')
            func(a, b)
        return inner
    return decorator

@return_decorator('+')
def add_num(a, b):
    result = a + b
    print(result)

@return_decorator('-')
def sub_num(a, b):
    result = a - b
    print(result)

add_num(1, 2)
sub_num(1, 2)</code>

Output:

<code>正在努力执行加法计算
3
正在努力执行减法计算
-1</code>

9. Class‑Based Decorators

A class can be used as a decorator by implementing the __call__ method.

<code>class MyDecorator(object):
    def __init__(self, func):
        self.__func = func
    def __call__(self, *args, **kwargs):
        print('马上就有下班啦')
        self.__func()

@MyDecorator
def show():
    print('快要下雪啦')

show()</code>

Output:

<code>马上就有下班啦
快要下雪啦</code>

10. Common Application Scenarios

Collecting operation or error logs for functions

Validating function usage permissions

Measuring function execution time

Dynamic data fetching in ORM/DB models via property methods

Caching function results

Customizing function input/output (serialization/deserialization)