通过Python装饰器实现DRY(不重复代码)原则_python教程_开发教程

Python 装饰器是一个消除冗余的强大工具。随着将功能模块化为大小合适的方法，即使是最复杂的工作流，装饰器也能使它变成简洁的功能。

例如让我们看看 Django web框架，该框架处理请求的方法接收一个方法对象，返回一个响应对象：

def handle_request(request):
    return HttpResponse("Hello, World")

defhandle_request(request):
    return HttpResponse("Hello, World")

我最近遇到一个案例，需要编写几个满足下述条件的api方法：

返回json响应
如果是GET请求，那么返回错误码

做为一个注册api端点例子，我将会像这样编写：

def register(request):
    result = None
    # check for post only
    if request.method != 'POST':
        result = {"error": "this method only accepts posts!"}
    else:
        try:
            user = User.objects.create_user(request.POST['username'],
                                            request.POST['email'],
                                            request.POST['password'])
            # optional fields
            for field in ['first_name', 'last_name']:
                if field in request.POST:
                    setattr(user, field, request.POST[field])
            user.save()
            result = {"success": True}
        except KeyError as e:
            result = {"error": str(e) }
    response = HttpResponse(json.dumps(result))
    if "error" in result:
        response.status_code = 500
    return response

defregister(request):
    result = None
    # check for post only
    if request.method != 'POST':
        result = {"error": "this method only accepts posts!"}
    else:
        try:
            user = User.objects.create_user(request.POST['username'],
                                            request.POST['email'],
                                            request.POST['password'])
            # optional fields
            for fieldin ['first_name', 'last_name']:
                if fieldin request.POST:
                    setattr(user, field, request.POST[field])
            user.save()
            result = {"success": True}
        exceptKeyErroras e:
            result = {"error": str(e) }
    response = HttpResponse(json.dumps(result))
    if "error" in result:
        response.status_code = 500
    return response

然而这样我将会在每个api方法中编写json响应和错误返回的代码。这将会导致大量的逻辑重复。所以让我们尝试用装饰器实现DRY原则吧。

装饰器简介

如果你不熟悉装饰器，我可以简单解释一下，实际上装饰器就是有效的函数包装器，python解释器加载函数的时候就会执行包装器，包装器可以修改函数的接收参数和返回值。举例来说，如果我想要总是返回比实际返回值大一的整数结果，我可以这样写装饰器：

# a decorator receives the method it's wrapping as a variable 'f'
def increment(f):
    # we use arbitrary args and keywords to
    # ensure we grab all the input arguments.
    def wrapped_f(*args, **kw):
        # note we call f against the variables passed into the wrapper,
        # and cast the result to an int and increment .
        return int(f(*args, **kw)) + 1
    return wrapped_f  # the wrapped function gets returned.

# a decorator receives the method it's wrapping as a variable 'f'
defincrement(f):
    # we use arbitrary args and keywords to
    # ensure we grab all the input arguments.
    defwrapped_f(*args, **kw):
        # note we call f against the variables passed into the wrapper,
        # and cast the result to an int and increment .
        return int(f(*args, **kw)) + 1
    return wrapped_f  # the wrapped function gets returned.

现在我们就可以用@符号和这个装饰器去装饰另外一个函数了：

@increment
def plus(a, b):
    return a + b

result = plus(4, 6)
assert(result == 11, "We wrote our decorator wrong!")

@increment
defplus(a, b):
    return a + b
 
result = plus(4, 6)
assert(result == 11, "We wrote our decorator wrong!")

装饰器修改了存在的函数，将装饰器返回的结果赋值给了变量。在这个例子中，’plus’的结果实际指向increment(plus)的结果。

对于非post请求返回错误

现在让我们在一些更有用的场景下应用装饰器。如果在django中接收的不是POST请求，我们用装饰器返回一个错误响应。

def post_only(f):
    """ Ensures a method is post only """
    def wrapped_f(request):
        if request.method != "POST":
            response = HttpResponse(json.dumps(
                {"error": "this method only accepts posts!"}))
            response.status_code = 500
            return response
        return f(request)
    return wrapped_f

defpost_only(f):
    """ Ensures a method is post only """
    defwrapped_f(request):
        if request.method != "POST":
            response = HttpResponse(json.dumps(
                {"error": "this method only accepts posts!"}))
            response.status_code = 500
            return response
        return f(request)
    return wrapped_f

现在我们可以在上述注册api中应用这个装饰器：

@post_only
def register(request):
    result = None
    try:
        user = User.objects.create_user(request.POST['username'],
                                        request.POST['email'],
                                        request.POST['password'])
        # optional fields
        for field in ['first_name', 'last_name']:
            if field in request.POST:
                setattr(user, field, request.POST[field])
        user.save()
        result = {"success": True}
    except KeyError as e:
        result = {"error": str(e) }
    response = HttpResponse(json.dumps(result))
    if "error" in result:
        response.status_code = 500
    return response

@post_only
defregister(request):
    result = None
    try:
        user = User.objects.create_user(request.POST['username'],
                                        request.POST['email'],
                                        request.POST['password'])
        # optional fields
        for fieldin ['first_name', 'last_name']:
            if fieldin request.POST:
                setattr(user, field, request.POST[field])
        user.save()
        result = {"success": True}
    exceptKeyErroras e:
        result = {"error": str(e) }
    response = HttpResponse(json.dumps(result))
    if "error" in result:
        response.status_code = 500
    return response

现在我们就有了一个可以在每个api方法中重用的装饰器。

发送json响应

为了发送json响应（同时处理500状态码），我们可以新建另外一个装饰器：

def json_response(f):
    """ Return the response as json, and return a 500 error code if an error exists """
    def wrapped(*args, **kwargs):
        result = f(*args, **kwargs)
        response = HttpResponse(json.dumps(result))
        if type(result) == dict and 'error' in result:
            response.status_code = 500
        return response

defjson_response(f):
    """ Return the response as json, and return a 500 error code if an error exists """
    defwrapped(*args, **kwargs):
        result = f(*args, **kwargs)
        response = HttpResponse(json.dumps(result))
        if type(result) == dictand 'error' in result:
            response.status_code = 500
        return response

现在我们就可以在原方法中去除json相关的代码，添加一个装饰器做为代替：

@post_only
@json_response
def register(request):
    try:
        user = User.objects.create_user(request.POST['username'],
                                        request.POST['email'],
                                        request.POST['password'])
        # optional fields
        for field in ['first_name', 'last_name']:
            if field in request.POST:
                setattr(user, field, request.POST[field])
        user.save()
        return {"success": True}
    except KeyError as e:
        return {"error": str(e) }

@post_only
@json_response
defregister(request):
    try:
        user = User.objects.create_user(request.POST['username'],
                                        request.POST['email'],
                                        request.POST['password'])
        # optional fields
        for fieldin ['first_name', 'last_name']:
            if fieldin request.POST:
                setattr(user, field, request.POST[field])
        user.save()
        return {"success": True}
    exceptKeyErroras e:
        return {"error": str(e) }

现在，如果我需要编写新的方法，那么我就可以使用装饰器做冗余的工作。如果我要写登录方法，我只需要写真正相关的代码：

@post_only
@json_response
def login(request):
    if request.user is not None:
        return {"error": "User is already authenticated!"}
    user = auth.authenticate(request.POST['username'], request.POST['password'])
    if user is not None:
        if not user.is_active:
            return {"error": "User is inactive"}
        auth.login(request, user)
        return {"success": True, "id": user.pk}
    else:
        return {"error": "User does not exist with those credentials"}

@post_only
@json_response
deflogin(request):
    if request.useris not None:
        return {"error": "User is already authenticated!"}
    user = auth.authenticate(request.POST['username'], request.POST['password'])
    if useris not None:
        if not user.is_active:
            return {"error": "User is inactive"}
        auth.login(request, user)
        return {"success": True, "id": user.pk}
    else:
        return {"error": "User does not exist with those credentials"}

BONUS: 参数化你的请求方法

我曾经使用过 Tubogears 框架，其中请求参数直接解释转递给方法这一点我很喜欢。所以要怎样在Django中模仿这一特性呢？嗯，装饰器就是一种解决方案！

例如:

def parameterize_request(types=("POST",)):
    """
    Parameterize the request instead of parsing the request directly.
    Only the types specified will be added to the query parameters.

    e.g. convert a=test

defparameterize_request(types=("POST",)):
    """
    Parameterizetherequestinsteadofparsingtherequestdirectly.
    Onlythetypesspecifiedwillbeaddedto thequeryparameters.
 
    e.g. convert a=test

注意这是一个参数化装饰器的例子。在这个例子中，函数的结果是实际的装饰器。

现在我就可以用参数化装饰器编写方法了！我甚至可以选择是否允许GET和POST，或者仅仅一种请求参数类型。

@post_only
@json_response
@parameterize_request(["POST"])
def register(request, username, email, password,
             first_name=None, last_name=None):
    user = User.objects.create_user(username, email, password)
    user.first_name=first_name
    user.last_name=last_name
    user.save()
    return {"success": True}

@post_only
@json_response
@parameterize_request(["POST"])
defregister(request, username, email, password,
            first_name=None, last_name=None):
    user = User.objects.create_user(username, email, password)
    user.first_name=first_name
    user.last_name=last_name
    user.save()
    return {"success": True}

现在我们有了一个简洁的、易于理解的api。

BONUS #2: 使用functools.wraps保存docstrings和函数名

很不幸，使用装饰器的一个副作用是没有保存方法名(__name__)和docstring(__doc__)值：

def increment(f):
    """ Increment a function result """
    wrapped_f(a, b):
        return f(a, b) + 1
    return wrapped_f

@increment
def plus(a, b)
    """ Add two things together """
    return a + b

plus.__name__  # this is now 'wrapped_f' instead of 'plus'
plus.__doc__   # this now returns 'Increment a function result' instead of 'Add two things together'

defincrement(f):
    """ Increment a function result """
    wrapped_f(a, b):
        return f(a, b) + 1
    return wrapped_f
 
@increment
defplus(a, b)
    """ Add two things together """
    return a + b
 
plus.__name__  # this is now 'wrapped_f' instead of 'plus'
plus.__doc__  # this now returns 'Increment a function result' instead of 'Add two things together'

这将对使用反射的应用造成麻烦，比如Sphinx，一个自动生成文档的应用。

为了解决这个问题，我们可以使用’wraps’装饰器附加上名字和docstring：

from functools import wraps

def increment(f):
    """ Increment a function result """
    @wraps(f)
    wrapped_f(a, b):
        return f(a, b) + 1
    return wrapped_f

@increment
def plus(a, b)
    """ Add two things together """
    return a + b

plus.__name__  # this returns 'plus'
plus.__doc__   # this returns 'Add two things together'

fromfunctoolsimportwraps
 
defincrement(f):
    """ Increment a function result """
    @wraps(f)
    wrapped_f(a, b):
        return f(a, b) + 1
    return wrapped_f
 
@increment
defplus(a, b)
    """ Add two things together """
    return a + b
 
plus.__name__  # this returns 'plus'
plus.__doc__  # this returns 'Add two things together'

BONUS #3: 使用’decorator’装饰器

如果仔细看看上述使用装饰器的方式，在包装器声明和返回的地方也有不少重复。

你可以安装python egg ‘decorator’，其中包含一个提供装饰器模板的’decorator’装饰器！

使用easy_install:

$ sudo easy_install decorator

$ sudoeasy_installdecorator

或者Pip:

$ pip install decorator

$ pipinstalldecorator

然后你可以简单的编写：

from decorator import decorator

@decorator
def post_only(f, request):
    """ Ensures a method is post only """
    if request.method != "POST":
        response = HttpResponse(json.dumps(
            {"error": "this method only accepts posts!"}))
        response.status_code = 500
        return response
    return f(request)

fromdecoratorimportdecorator
 
@decorator
defpost_only(f, request):
    """ Ensures a method is post only """
    if request.method != "POST":
        response = HttpResponse(json.dumps(
            {"error": "this method only accepts posts!"}))
        response.status_code = 500
        return response
    return f(request)

这个装饰器更牛逼的一点是保存了__name__和__doc__的返回值，也就是它封装了

functools.wraps的

功能！

(责任编辑：好模板)