# documentação Django 2.0.8

# Código fonte para django.middleware.common

    import re
    import warnings
    from urllib.parse import urlparse

    from django.conf import settings
    from django.core.exceptions import PermissionDenied
    from django.core.mail import mail_managers
    from django.http import HttpResponsePermanentRedirect
    from django.urls import is_valid_path
    from django.utils.cache import (
        cc_delim_re, get_conditional_response, set_response_etag,
    )
    from django.utils.deprecation import MiddlewareMixin, RemovedInDjango21Warning
    from django.utils.http import escape_leading_slashes

    class CommonMiddleware(MiddlewareMixin):
        """
        "Common" middleware for taking care of some basic operations:

            - Forbid access to User-Agents in settings.DISALLOWED_USER_AGENTS

            - URL rewriting: Based on the APPEND_SLASH and PREPEND_WWW settings,
              append missing slashes and/or prepends missing "www."s.

                - If APPEND_SLASH is set and the initial URL doesn't end with a
                  slash, and it is not found in urlpatterns, form a new URL by
                  appending a slash at the end. If this new URL is found in
                  urlpatterns, return an HTTP redirect to this new URL; otherwise
                  process the initial URL as usual.

              This behavior can be customized by subclassing CommonMiddleware and
              overriding the response_redirect_class attribute.

            - ETags: If the USE_ETAGS setting is set, ETags will be calculated from
              the entire page content and Not Modified responses will be returned
              appropriately. USE_ETAGS is deprecated in favor of
              ConditionalGetMiddleware.
        """

        response_redirect_class = HttpResponsePermanentRedirect

        def process_request(self, request):
            """
            Check for denied User-Agents and rewrite the URL based on
            settings.APPEND_SLASH and settings.PREPEND_WWW
            """

            # Check for denied User-Agents
            if 'HTTP_USER_AGENT' in request.META:
                for user_agent_regex in settings.DISALLOWED_USER_AGENTS:
                    if user_agent_regex.search(request.META['HTTP_USER_AGENT']):
                        raise PermissionDenied('Forbidden user agent')

            # Check for a redirect based on settings.PREPEND_WWW
            host = request.get_host()
            must_prepend = settings.PREPEND_WWW and host and not host.startswith('www.')
            redirect_url = ('%s://www.%s' % (request.scheme, host)) if must_prepend else ''

            # Check if a slash should be appended
            if self.should_redirect_with_slash(request):
                path = self.get_full_path_with_slash(request)
            else:
                path = request.get_full_path()

            # Return a redirect if necessary
            if redirect_url or path != request.get_full_path():
                redirect_url += path
                return self.response_redirect_class(redirect_url)

        def should_redirect_with_slash(self, request):
            """
            Return True if settings.APPEND_SLASH is True and appending a slash to
            the request path turns an invalid path into a valid one.
            """
            if settings.APPEND_SLASH and not request.path_info.endswith('/'):
                urlconf = getattr(request, 'urlconf', None)
                return (
                    not is_valid_path(request.path_info, urlconf) and
                    is_valid_path('%s/' % request.path_info, urlconf)
                )
            return False

        def get_full_path_with_slash(self, request):
            """
            Return the full path of the request with a trailing slash appended.

            Raise a RuntimeError if settings.DEBUG is True and request.method is
            POST, PUT, or PATCH.
            """
            new_path = request.get_full_path(force_append_slash=True)
            # Prevent construction of scheme relative urls.
            new_path = escape_leading_slashes(new_path)
            if settings.DEBUG and request.method in ('POST', 'PUT', 'PATCH'):
                raise RuntimeError(
                    "You called this URL via %(method)s, but the URL doesn't end "
                    "in a slash and you have APPEND_SLASH set. Django can't "
                    "redirect to the slash URL while maintaining %(method)s data. "
                    "Change your form to point to %(url)s (note the trailing "
                    "slash), or set APPEND_SLASH=False in your Django settings." % {
                        'method': request.method,
                        'url': request.get_host() + new_path,
                    }
                )
            return new_path

        def process_response(self, request, response):
            """
            Calculate the ETag, if needed.

            When the status code of the response is 404, it may redirect to a path
            with an appended slash if should_redirect_with_slash() returns True.
            """
            # If the given URL is "Not Found", then check if we should redirect to
            # a path with a slash appended.
            if response.status_code == 404:
                if self.should_redirect_with_slash(request):
                    return self.response_redirect_class(self.get_full_path_with_slash(request))

            if settings.USE_ETAGS and self.needs_etag(response):
                warnings.warn(
                    "The USE_ETAGS setting is deprecated in favor of "
                    "ConditionalGetMiddleware which sets the ETag regardless of "
                    "the setting. CommonMiddleware won't do ETag processing in "
                    "Django 2.1.",
                    RemovedInDjango21Warning
                )
                if not response.has_header('ETag'):
                    set_response_etag(response)

                if response.has_header('ETag'):
                    return get_conditional_response(
                        request,
                        etag=response['ETag'],
                        response=response,
                    )
            # Add the Content-Length header to non-streaming responses if not
            # already set.
            if not response.streaming and not response.has_header('Content-Length'):
                response['Content-Length'] = str(len(response.content))

            return response

        def needs_etag(self, response):
            """Return True if an ETag header should be added to response."""
            cache_control_headers = cc_delim_re.split(response.get('Cache-Control', ''))
            return all(header.lower() != 'no-store' for header in cache_control_headers)

    class BrokenLinkEmailsMiddleware(MiddlewareMixin):

        def process_response(self, request, response):
            """Send broken link emails for relevant 404 NOT FOUND responses."""
            if response.status_code == 404 and not settings.DEBUG:
                domain = request.get_host()
                path = request.get_full_path()
                referer = request.META.get('HTTP_REFERER', '')

                if not self.is_ignorable_request(request, path, domain, referer):
                    ua = request.META.get('HTTP_USER_AGENT', '<none>')
                    ip = request.META.get('REMOTE_ADDR', '<none>')
                    mail_managers(
                        "Broken %slink on %s" % (
                            ('INTERNAL ' if self.is_internal_request(domain, referer) else ''),
                            domain
                        ),
                        "Referrer: %s\nRequested URL: %s\nUser agent: %s\n"
                        "IP address: %s\n" % (referer, path, ua, ip),
                        fail_silently=True)
            return response

        def is_internal_request(self, domain, referer):
            """
            Return True if the referring URL is the same domain as the current
            request.
            """
            # Different subdomains are treated as different domains.
            return bool(re.match("^https?://%s/" % re.escape(domain), referer))

        def is_ignorable_request(self, request, uri, domain, referer):
            """
            Return True if the given request *shouldn't* notify the site managers
            according to project settings or in situations outlined by the inline
            comments.
            """
            # The referer is empty.
            if not referer:
                return True

            # APPEND_SLASH is enabled and the referer is equal to the current URL
            # without a trailing slash indicating an internal redirect.
            if settings.APPEND_SLASH and uri.endswith('/') and referer == uri[:-1]:
                return True

            # A '?' in referer is identified as a search engine source.
            if not self.is_internal_request(domain, referer) and '?' in referer:
                return True

            # The referer is equal to the current URL, ignoring the scheme (assumed
            # to be a poorly implemented bot).
            parsed_referer = urlparse(referer)
            if parsed_referer.netloc in ['', domain] and parsed_referer.path == uri:
                return True

            return any(pattern.search(uri) for pattern in settings.IGNORABLE_404_URLS)