详解Flask开发技巧之异常处理
目录
- 一、Flask内置异常处理
- 二、HTTPException类分析
- 三、自定义异常处理类
- 四、方便的定义自己的错误类
- 五、注意事项
一、Flask内置异常处理
要想在Flask中处理好异常,有一套自己的异常处理机制,首先,我们必须先知道Flask自己是如何处理异常的。去flask的源码里找一找会发现,在flask源码的app.py文件下,有很多会抛出异常的方法,其中拿一个举例:
def handle_exception(self, e): """Default exception handling that kicks in when an exception occurs that is not caught. In debug mode the exception will be re-raised immediately, otherwise it is logged and the handler for a 500 internal server error is used. If no such handler exists, a default 500 internal server error message is displayed. .. versionadded:: 0.3 """ exc_type, exc_value, tb = sys.exc_info() got_request_exception.send(self, exception=e) handler = self._find_error_handler(InternalServerError()) if self.propagate_exceptions: # if we want to repropagate the exception, we can attempt to # raise it with the whole traceback in case we can do that # (the function was actually called from the except part) # otherwise, we just raise the error again if exc_value is e: reraise(exc_type, exc_value, tb) else: raise e self.log_exception((exc_type, exc_value, tb)) if handler is None: return InternalServerError() return self.finalize_request(handler(e), from_error_handler=True)
我们发现在flask内部对于500异常,会抛出这样一个错误类InternalServerError()
class InternalServerError(HTTPException):
......
至此我们发现flask内部异常通过继承这个HTTPException类来处理,那么这个HTTPException类就是我们研究的重点。
二、HTTPException类分析
@implements_to_string class HTTPException(Exception): """Baseclass for all HTTP exceptions. This exception can be called as WSGI application to render a default error page or you can catch the subclasses of it independently and render nicer error messages. """ code = None description = None def __init__(self, description=None, response=None): super(HTTPException, self).__init__() if description is not None: self.description = description self.response = response @classmethod def wrap(cls, exception, name=None): """Create an exception that is a subclass of the calling HTTP exception and the ``exception`` argument. The first argument to the class will be passed to the wrapped ``exception``, the rest to the HTTP exception. If ``e.args`` is not empty and ``e.show_exception`` is ``True``, the wrapped exception message is added to the HTTP error description. .. versionchanged:: 0.15.5 The ``show_exception`` attribute controls whether the description includes the wrapped exception message. .. versionchanged:: 0.15.0 The description includes the wrapped exception message. """ class newcls(cls, exception): _description = cls.description show_exception = False def __init__(self, arg=None, *args, **kwargs): super(cls, self).__init__(*args, **kwargs) if arg is None: exception.__init__(self) else: exception.__init__(self, arg) @property def description(self): if self.show_exception: return "{}\n{}: {}".format( self._description, exception.__name__, exception.__str__(self) ) return self._description @description.setter def description(self, value): self._description = value newcls.__module__ = sys._getframe(1).f_globals.get("__name__") name = name or cls.__name__ + exception.__name__ newcls.__name__ = newcls.__qualname__ = name return newcls @property def name(self): """The status name.""" from .http import HTTP_STATUS_CODES return HTTP_STATUS_CODES.get(self.code, "Unknown Error") def get_description(self, environ=None): """Get the description.""" return u"<p>%s</p>" % escape(self.description).replace("\n", "<br>") def get_body(self, environ=None): """Get the HTML body.""" return text_type( ( u'<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">\n' u"<title>%(code)s %(name)s</title>\n" u"<h1>%(name)s</h1>\n" u"%(description)s\n" ) % { "code": self.code, "name": escape(self.name), "description": self.get_description(environ), } ) def get_headers(self, environ=None): """Get a list of headers.""" return [("Content-Type", "text/html; charset=utf-8")] def get_response(self, environ=None): """Get a response object. If one was passed to the exception it's returned directly. :param environ: the optional environ for the request. This can be used to modify the response depending on how the request looked like. :return: a :class:`Response` object or a subclass thereof. """ from .wrappers.response import Response if self.response is not None: return self.response if environ is not None: environ = _get_environ(environ) headers = self.get_headers(environ) return Response(self.get_body(environ), self.code, headers)
- 截取这个类比较重要的几个方法分析,
get_headers
方法定义了这个返回的响应头,返回的是html文档。 get_body
方法定义了返回的响应体,对应也是一段html的内容。- 最后在Response中将响应体,状态码,响应头定义好返回。
分析至此,其实这个HTTPException中做的事也不难理解,就是定义好响应体,状态码,还有响应头,做了一个返回。当然这个类返回是html类型的,现在前后端分离交互都是json形式的返回,所以我们可以继承自这个类,定义我们自己的异常处理类。
三、自定义异常处理类
首先我们理解我们自己的这个异常处理类,应该继承自HTTPException来改写。而我们自定义的内容应该包含以下几点:
- 需要定义我们自己想要返回的错误信息的json格式,比如内部错误码、错误信息等我们想记录的信息。
- 需要更改返回的响应头,返回json格式的信息响应头就应该设为
'Content-Type': 'application/json'
- 同样需要和HTTPException一样定义好状态码
如下定义我们自己的异常类APIException,返回的信息包括内部错误码,错误信息,请求的url
class APIException(HTTPException): code = 500 msg = 'sorry, we made a mistake!' error_code = 999 def __init__(self, msg=None, code=None, error_code=None, headers=None): if code: self.code = code if error_code: self.error_code = error_code if msg: self.msg = msg super(APIException, self).__init__(msg, None) def get_body(self, environ=None): body = dict( msg=self.msg, error_code=self.error_code, request=request.method + ' ' + self.get_url_no_param() ) text = json.dumps(body) return text def get_headers(self, environ=None): """Get a list of headers.""" return [('Content-Type', 'application/json')] @staticmethod def get_url_no_param(): full_path = str(request.full_path) main_path = full_path.split('?') return main_path[0]
四、方便的定义自己的错误类
有了上面我们改写好的APIException类,我们就可以自由的定义各种状态码的错误以及对应的错误信息,然后在合适的位置抛出。比如:
class Success(APIException): code = 201 msg = 'ok' error_code = 0 class DeleteSuccess(APIException): code = 202 msg = 'delete ok' error_code = 1 class UpdateSuccess(APIException): code = 200 msg = 'update ok' error_code = 2 class ServerError(APIException): code = 500 msg = 'sorry, we made a mistake!' error_code = 999 class ParameterException(APIException): code = 400 msg = 'invalid parameter' error_code = 1000 class NotFound(APIException): code = 404 msg = 'the resource are not found' error_code = 1001 class AuthFailed(APIException): code = 401 msg = 'authorization failed' error_code = 1005 class Forbidden(APIException): code = 403 error_code = 1004 msg = 'forbidden, not in scope'
有了这些自定义的错误类,我们不仅可以直接在需要的地方抛出,而且有了自定义的错误码,发生错误时,只要对照错误码去查找对应的错误类,非常方便。而且特别说明的是,虽然说是错误类,但是也是可以定义响应成功的返回的,比如上面定义的200,201的类,同样可以作为一个成功的返回。
使用演示:
user = User.query.first() if not user: raise NotFound()
五、注意事项
尽管我们可以在我们认为可能出错的所有地方,继承自己的异常类,定义自己的错误类,然后抛出,但是也不是所有的异常都是我们可以提前预知的。比如我们接受前端传来的参数,参数类型或取值范围不正确,这些我们可以预知并处理好,但是如果是逻辑处理中出现了问题,这些不是我们程序员可以控制并处理。所以光有自定义错误类还不够,我们还需要在全局捕获异常来判断,利用AOP思想。
# 全局错误AOP处理 @app.errorhandler(Exception) def framework_error(e): api_logger.error("error info: %s" % e) # 对错误进行日志记录 if isinstance(e, APIException): return e if isinstance(e, HTTPException): code = e.code msg = e.description error_code = 1007 return APIException(msg, code, error_code) else: if not app.config['DEBUG']: return ServerError() else: return e
这里对于flask中抛出的所有的错误进行捕获,然后先进行日志的记录。然后判断如果是我们自定义的APIException,就直接返回。如果不是我们自定义的,但是是flask处理的HTTPException,包装成我们自定义的APIException再返回。如果都不是的话,说明是服务器出现的其他错误,问题一般出在我们的代码上,在生产环境下,一般统一返回一个500错误,在调试模式下,可以原样返回,便于我们定位修改自己的代码。
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