def make_conditional(self, request_or_environ):
"""Make the response conditional to the request. This method works
best if an etag was defined for the response already. The `add_etag`
method can be used to do that. If called without etag just the date
header is set.
This does nothing if the request method in the request or environ is
anything but GET or HEAD.
It does not remove the body of the response because that's something
the :meth:`__call__` function does for us automatically.
Returns self so that you can do ``return resp.make_conditional(req)``
but modifies the object in-place.
:param request_or_environ: a request object or WSGI environment to be
used to make the response conditional
against.
"""
environ = _get_environ(request_or_environ)
if environ['REQUEST_METHOD'] in ('GET', 'HEAD'):
# if the date is not in the headers, add it now. We however
# will not override an already existing header. Unfortunately
# this header will be overriden by many WSGI servers including
# wsgiref.
if 'date' not in self.headers:
self.headers['Date'] = http_date()
if self.automatically_set_content_length and 'content-length' not in self.headers:
length = self.calculate_content_length()
if length is not None:
self.headers['Content-Length'] = length
if not is_resource_modified(environ, self.headers.get('etag'), None,
self.headers.get('last-modified')):
self.status_code = 304
return self
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