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。
@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)
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。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。
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:
'Content-Type': 'application/json'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]
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()
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。
jsjbwy