Python進階: Decorator 裝飾器你太美

函數 -> 裝飾器

　　函數的4個核心概念

　　1.函數可以賦與變量

def func(message):
    print('Got a message: {}'.format(message))

send_message = func
send_message('hello world')
#輸出
#Got a message: hello world

　　2.函數可以當作函數的參數

def get_message(message):
    return 'Got a message: ' + message

def root_call(func, message):
    print(func(message))

root_call(get_message, 'hello world')
輸出
#Got a message: hello world

　　3.函數里嵌套函數

def func(message):
    def get_message(message):
        print('Got a message: {}'.format(message))
    return get_message(message)

func('hello world')
輸出
#Got a message: hello world

　　4.函數作為函數返回值(閉包)

def func_closure():
    def get_message(message):
        print('Got a message: {}'.format(message))
    return get_message

send_message = func_closure()
send_message('hello world')
#輸出
#Got a message: hello world

 

簡單裝飾器

　　例

def my_decorator(func):
    def wrapper():
        print('wrapper of decorator')
        func()
    return wrapper

def greet():
    print('hello world')

greet = my_decorator(greet)
greet()

　　使用語法糖 @

def my_decorator(func):
    def wrapper():
        print('wrapper of decorator')
        func()
    return wrapper

@my_decorator
def greet():
    print('hello world')

greet()
# 輸出
# wrapper of decorator
# hello world

帶有參數的裝飾器

　　直接在 wrapper函數中加上參數

def my_decorator(func):
    def wrapper(message):
        print('wrapper of decorator')
        func(message)
    return wrapper


@my_decorator #相當於 greet == wrapper(message)
def greet(message):
    print(message)

greet('hello world')
# 輸出
#wrapper of decorator
#hello world

　　這個裝飾器只能用在有一個參數的函數，如果想對任意參數的函數通用，可以這么寫

def my_decorator(func):
    def wrapper(*args, **kwargs):
        print('wrapper of decorator')
        func(*args, **kwargs)
    return wrapper

帶自定義參數的裝飾器

　　利用裝飾器自定義參數這特性，實現重復執行裝飾器內部函數

def repeat(num):
    def my_decorator(func):
        def wrapper(*args, **kwargs):
            for i in range(num):
                print('wrapper of decorator')
                func(*args, **kwargs)
        return wrapper
    return my_decorator


@repeat(4)
def greet(message):
    print(message)

greet('hello world')

# 輸出：
# wrapper of decorator
# hello world
# wrapper of decorator
# hello world
# wrapper of decorator
# hello world
# wrapper of decorator
# hello world

原函數還是原函數？

greet.__name__
#輸出
'wrapper'

help(greet)
# 輸出
Help on function wrapper in module __main__:

wrapper(*args, **kwargs)

　　可以看出，原函數的原信息會被wrapper取代

　　如果不想其改變，那么可用內置裝飾器@functools.wraps將原函數的元信息拷貝過去。

import functools

def my_decorator(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        print('wrapper of decorator')
        func(*args, **kwargs)
    return wrapper

@my_decorator
def greet(message):
    print(message)

greet.__name__

# 輸出
#'greet'

 類裝飾器

　　類裝飾器主要依賴於 __call__()函數，每當調用類實例時，__call__()函數會被執行一次

class Count:
    def __init__(self, func):
        self.func = func
        self.num_calls = 0

    def __call__(self, *args, **kwargs):
        self.num_calls += 1
        print('num of calls is: {}'.format(self.num_calls))
        return self.func(*args, **kwargs)

@Count
def example():
    print("hello world")

example()

# 輸出
# num of calls is: 1
# hello world

example()

# 輸出
# num of calls is: 2
# hello world

裝飾器的嵌套

@decorator1
@decorator2
@decorator3
def func():
    ...
#相當於 decorator1(decorator2(decorator3(func)))

import functools

def my_decorator1(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        print('execute decorator1')
        func(*args, **kwargs)
    return wrapper


def my_decorator2(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        print('execute decorator2')
        func(*args, **kwargs)
    return wrapper


@my_decorator1
@my_decorator2
def greet(message):
    print(message)


greet('hello world')

# 輸出
# execute decorator1
# execute decorator2
# hello world

裝飾器的實例用法

　　1)身份驗證，不登錄不允許操作

import functools

def authenticate(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        request = args[0]
        if check_user_logged_in(request): # 如果用戶處於登錄狀態
            return func(*args, **kwargs) # 執行函數 post_comment()
        else:
            raise Exception('Authentication failed')
    return wrapper

@authenticate
def post_comment(request, ...)
    ...

　　2)日志記錄 可測試函數的執行時間

import time
import functools

def log_execution_time(func):
    def wrapper(*args, **kwargs):
        start = time.perf_counter()
        res = func(*args, **kwargs)
        end = time.perf_counter()
        print('{} took {} ms'.format(func.__name__, (end - start) * 1000))
        return res
    return wrapper

@log_execution_time
def calculate_similarity(items):
    ...

　　3) 合法性檢測

import functools

def validation_check(input):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        ... # 檢查輸入是否合法

@validation_check
def neural_network_training(param1, param2, ...):
    ...

LRU cache. @lru_cache緩存裝飾器
@lru_cache
def check(param1, param2, ...) # 檢查用戶設備類型，版本號等等
    ...

　　4) try...excaption

class ServerDebugHelper:
    @classmethod
    def debug(cls):
        def decorator(func):
            @wraps(func)
            def wrapper(*args, **kwargs):
                try:
                    return func(*args, **kwargs)
                except:
                    import traceback
                    print(traceback.format_exc())
            return wrapper
        return decorator

 

　　

參考

　　極客時間《Python核心技術與實戰》專欄