Python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")
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
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.
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!")
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
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