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import calendar
import time
from email.utils import formatdate, parsedate, parsedate_tz
from datetime import datetime, timedelta
TIME_FMT = "%a, %d %b %Y %H:%M:%S GMT"
def expire_after(delta, date=None):
date = date or datetime.utcnow()
return date + delta
def datetime_to_header(dt):
return formatdate(calendar.timegm(dt.timetuple()))
class BaseHeuristic(object):
def warning(self, response):
"""
Return a valid 1xx warning header value describing the cache
adjustments.
The response is provided too allow warnings like 113
http://tools.ietf.org/html/rfc7234#section-5.5.4 where we need
to explicitly say response is over 24 hours old.
"""
return '110 - "Response is Stale"'
def update_headers(self, response):
"""Update the response headers with any new headers.
NOTE: This SHOULD always include some Warning header to
signify that the response was cached by the client, not
by way of the provided headers.
"""
return {}
def apply(self, response):
updated_headers = self.update_headers(response)
if updated_headers:
response.headers.update(updated_headers)
warning_header_value = self.warning(response)
if warning_header_value is not None:
response.headers.update({"Warning": warning_header_value})
return response
class OneDayCache(BaseHeuristic):
"""
Cache the response by providing an expires 1 day in the
future.
"""
def update_headers(self, response):
headers = {}
if "expires" not in response.headers:
date = parsedate(response.headers["date"])
expires = expire_after(timedelta(days=1), date=datetime(*date[:6]))
headers["expires"] = datetime_to_header(expires)
headers["cache-control"] = "public"
return headers
class ExpiresAfter(BaseHeuristic):
"""
Cache **all** requests for a defined time period.
"""
def __init__(self, **kw):
self.delta = timedelta(**kw)
def update_headers(self, response):
expires = expire_after(self.delta)
return {"expires": datetime_to_header(expires), "cache-control": "public"}
def warning(self, response):
tmpl = "110 - Automatically cached for %s. Response might be stale"
return tmpl % self.delta
class LastModified(BaseHeuristic):
"""
If there is no Expires header already, fall back on Last-Modified
using the heuristic from
http://tools.ietf.org/html/rfc7234#section-4.2.2
to calculate a reasonable value.
Firefox also does something like this per
https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching_FAQ
http://lxr.mozilla.org/mozilla-release/source/netwerk/protocol/http/nsHttpResponseHead.cpp#397
Unlike mozilla we limit this to 24-hr.
"""
cacheable_by_default_statuses = {
200, 203, 204, 206, 300, 301, 404, 405, 410, 414, 501
}
def update_headers(self, resp):
headers = resp.headers
if "expires" in headers:
return {}
if "cache-control" in headers and headers["cache-control"] != "public":
return {}
if resp.status not in self.cacheable_by_default_statuses:
return {}
if "date" not in headers or "last-modified" not in headers:
return {}
date = calendar.timegm(parsedate_tz(headers["date"]))
last_modified = parsedate(headers["last-modified"])
if date is None or last_modified is None:
return {}
now = time.time()
current_age = max(0, now - date)
delta = date - calendar.timegm(last_modified)
freshness_lifetime = max(0, min(delta / 10, 24 * 3600))
if freshness_lifetime <= current_age:
return {}
expires = date + freshness_lifetime
return {"expires": time.strftime(TIME_FMT, time.gmtime(expires))}
def warning(self, resp):
return None
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