Django 自定義異常處理頁面源碼解讀
這個解讀來源於一個讀者的反饋，於是花了幾分鐘看了下這部分源碼，打算用十分鐘的時間寫一下，預計閱讀需要 5 分鐘。
自定義異常頁面
Django 提供了常見的錯誤的頁面，比如
- 說用戶訪問了一個不存在的路徑，引發的 404
- 系統發生了一個異常，出現了 500
一個好的網站應該可以給用戶友好的信息提示，比如：“服務器提了一個問題”之類的，然後給用戶一個引導。對於商業網站需要注意的是錯誤頁面的流量也是流量，應該有明確的引導。
在 Django 中定義這類處理很簡單，只需要在 urls.py 中配置：
# 參考：https://github.com/the5fire/typeidea/blob/deploy-to-cloud/typeidea/typeidea/urls.py#L24
handler404 = Handler404.as_view()
handler500 = Handler50x.as_view()
當然你需要定義這裏面的 Handler50x：
class Handler404(CommonViewMixin, TemplateView):
template_name = '404.html'
def get(self, request, *args, **kwargs):
context = self.get_context_data(**kwargs)
return self.render_to_response(context, status=404)
class Handler50x(CommonViewMixin, TemplateView):
template_name = '50x.html'
def get(self, request, *args, **kwargs):
context = self.get_context_data(**kwargs)
return self.render_to_response(context, status=500)
這樣就可以簡單的控制出錯時展示給用戶的頁面了。需要注意的是，這個配置只會在非 Debug 模式下有效。
Django Error Handler 源碼解析
要看這部分源碼的第一步是判斷 Django 可能會在哪處理這個異常。有很多方法，這裏是說一種，從請求的入口開始擼。
注意我看到版本是 Django 2.0.1
1 WSGI Handler 的部分
# 代碼：https://github.com/the5fire/django-inside/blob/84f272e1206554b43c86c0f7a50f37d1f3efbc28/django/core/handlers/wsgi.py#L135
class WSGIHandler(base.BaseHandler):
request_class = WSGIRequest
def __init__(self, *args, **kwargs):
super(WSGIHandler, self).__init__(*args, **kwargs)
self.load_middleware()
def __call__(self, environ, start_response):
set_script_prefix(get_script_name(environ))
signals.request_started.send(sender=self.__class__, environ=environ)
request = self.request_class(environ)
response = self.get_response(request) # the5fire: 注意這兒
# ... the5fire:省略其他
2 BaseHandler 中的 get_response
# ref: https://github.com/the5fire/django-inside/blob/84f272e1206554b43c86c0f7a50f37d1f3efbc28/django/core/handlers/base.py#L94
def get_response(self, request):
"""Return an HttpResponse object for the given HttpRequest."""
# Setup default url resolver for this thread
set_urlconf(settings.ROOT_URLCONF)
response = self._middleware_chain(request) # the5fire: 這裏進去
response._closable_objects.append(request)
# If the exception handler returns a TemplateResponse that has not
# been rendered, force it to be rendered.
if not getattr(response, 'is_rendered', True) and callable(getattr(response, 'render', None)):
response = response.render()
if response.status_code == 404:
logger.warning(
'Not Found: %s', request.path,
extra={'status_code': 404, 'request': request},
)
return response
3 被包裝的 _middleware_chain
# https://github.com/the5fire/django-inside/blob/84f272e1206554b43c86c0f7a50f37d1f3efbc28/django/core/handlers/base.py#L76
def load_middleware(self):
"""
Populate middleware lists from settings.MIDDLEWARE.
Must be called after the environment is fixed (see __call__ in subclasses).
"""
self._request_middleware = []
self._view_middleware = []
self._template_response_middleware = []
self._response_middleware = []
self._exception_middleware = []
handler = convert_exception_to_response(self._get_response)
for middleware_path in reversed(settings.MIDDLEWARE):
middleware = import_string(middleware_path)
# ... the5fire：忽略中間這些代碼
handler = convert_exception_to_response(mw_instance)
# We only assign to this when initialization is complete as it is used
# as a flag for initialization being complete.
self._middleware_chain = handler
4 具體處理異常的部分
def convert_exception_to_response(get_response):
"""
Wrap the given get_response callable in exception-to-response conversion.
All exceptions will be converted. All known 4xx exceptions (Http404,
PermissionDenied, MultiPartParserError, SuspiciousOperation) will be
converted to the appropriate response, and all other exceptions will be
converted to 500 responses.
This decorator is automatically applied to all middleware to ensure that
no middleware leaks an exception and that the next middleware in the stack
can rely on getting a response instead of an exception.
"""
@wraps(get_response)
def inner(request):
try:
response = get_response(request)
except Exception as exc:
response = response_for_exception(request, exc) # the5fire: 這裏進去
return response
return inner
def response_for_exception(request, exc):
if isinstance(exc, Http404):
if settings.DEBUG:
response = debug.technical_404_response(request, exc)
else:
response = get_exception_response(request, get_resolver(get_urlconf()), 404, exc)
# ... the5fire: 省略掉一大坨類似的代碼
else:
signals.got_request_exception.send(sender=None, request=request)
# the5fire: 下面這一行，具體的處理邏輯。
response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
# Force a TemplateResponse to be rendered.
if not getattr(response, 'is_rendered', True) and callable(getattr(response, 'render', None)):
response = response.render()
return response
5 異常處理邏輯
# https://github.com/the5fire/django-inside/blob/84f272e1206554b43c86c0f7a50f37d1f3efbc28/django/core/handlers/exception.py#L107
def handle_uncaught_exception(request, resolver, exc_info):
"""
Processing for any otherwise uncaught exceptions (those that will
generate HTTP 500 responses).
"""
if settings.DEBUG_PROPAGATE_EXCEPTIONS:
raise
logger.error(
'Internal Server Error: %s', request.path,
exc_info=exc_info,
extra={'status_code': 500, 'request': request},
)
if settings.DEBUG:
return debug.technical_500_response(request, *exc_info)
# Return an HttpResponse that displays a friendly error message.
# the5fire: 這裏會解析到對應的handler ，比如我們定義的那個
callback, param_dict = resolver.resolve_error_handler(500)
return callback(request, **param_dict)
6 最終解析到 urls/resolvers.py 中
# 完整代碼: https://github.com/the5fire/django-inside/blob/84f272e1206554b43c86c0f7a50f37d1f3efbc28/django/urls/resolvers.py#L555
def resolve_error_handler(self, view_type):
callback = getattr(self.urlconf_module, 'handler%s' % view_type, None) # the5fire: 這裏就是去獲取 urls.py 中對應的配置
if not callback:
# No handler specified in file; use lazy import, since
# django.conf.urls imports this file.
from django.conf import urls
callback = getattr(urls, 'handler%s' % view_type)
return get_callable(callback), {}
最後
實際上花了比預計更多的時間來把完整的代碼貼出來，以及明確對應的版本。在 Django 1.11 中的處理邏輯有些不同。
實際閱讀時間也會比預計的久，但如果能理解這個過程，你對於Django也會有更深的進步。