Add ExpiresAfter heuristic. Fixes #48

This also fixed some tests that were broken.

Here is an example ExpiresAfter:

  sess = CacheControl(
      requests.Session(),
      heuristic=ExpiresAfter(days=1)
  )

The arguments to the constructor are the same as a python
datetime.timedelta.

cachecontrol/heuristics.py

@@ -5,6 +5,15 @@
 from datetime import datetime, timedelta
 
 
+def expire_after(delta, date=None):
+    date = date or datetime.now()
+    return date + delta
+
+
+def datetime_to_header(dt):
+    return formatdate(calendar.timegm(dt.timetuple()))
+
+
 class BaseHeuristic(object):
 
     def warning(self):
@@ -39,7 +48,28 @@ def update_headers(self, response):
 
         if 'expires' not in response.headers:
             date = parsedate(response.headers['date'])
-            expires = datetime(*date[:6]) + timedelta(days=1)
-            headers['expires'] = formatdate(calendar.timegm(expires.timetuple()))
-
+            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):
+        tmpl = '110 - Automatically cached for %s. Response might be stale'
+        return tmpl % self.delta

conftest.py

@@ -23,15 +23,11 @@ def optional_cacheable_request(self, env, start_response):
         cached. Yet, we might still choose to cache it via a
         heuristic."""
 
-        headers = {
-            'content-encoding': 'gzip',
-            'transfer-encoding': 'chunked',
-            'server': 'nginx/1.2.6 (Ubuntu)',
-            'last-modified': 'Mon, 21 Jul 2014 17:45:39 GMT',
-            'connection': 'keep-alive',
-            'date': 'Wed, 23 Jul 2014 04:57:40 GMT',
-            'content-type': 'text/html'
-        }
+        headers = [
+            ('server', 'nginx/1.2.6 (Ubuntu)'),
+            ('last-modified', 'Mon, 21 Jul 2014 17:45:39 GMT'),
+            ('content-type', 'text/html'),
+        ]
 
         start_response('200 OK', headers)
         return [pformat(env).encode("utf8")]
@@ -68,6 +64,13 @@ def etag(self, env, start_response):
             start_response('200 OK', headers)
         return [pformat(env).encode("utf8")]
 
+    def no_cache(self, env, start_response):
+        headers = [
+            ('Cache-Control', 'no-cache'),
+        ]
+        start_response('200 OK', headers)
+        return [pformat(env).encode("utf8")]
+
     def __call__(self, env, start_response):
         func = self.dispatch(env)
 

tests/test_expires_heuristics.py

@@ -1,8 +1,10 @@
 from mock import Mock
 
-from requests import Session
+from requests import Session, get
 from cachecontrol import CacheControl
-from cachecontrol.heuristics import OneDayCache
+from cachecontrol.heuristics import OneDayCache, ExpiresAfter
+
+from pprint import pprint
 
 
 class TestUseExpiresHeuristic(object):
@@ -22,11 +24,37 @@ def setup(self):
         )
 
     def test_cache_for_one_day(self, url):
-        hurl = url + '/optional_cacheable_request'
-        r = self.sess.get(hurl)
+        the_url = url + 'optional_cacheable_request'
+        r = self.sess.get(the_url)
+
+        assert 'expires' in r.headers
+        assert 'warning' in r.headers
+
+        pprint(dict(r.headers))
+
+        r = self.sess.get(the_url)
+        pprint(dict(r.headers))
+        assert r.from_cache
+
+
+class TestExpiresAfter(object):
+
+    def setup(self):
+        self.sess = Session()
+        self.cache_sess = CacheControl(
+            self.sess, heuristic=ExpiresAfter(days=1)
+        )
+
+    def test_expires_after_one_day(self, url):
+        the_url = url + 'no_cache'
+        resp = get(the_url)
+        assert resp.headers['cache-control'] == 'no-cache'
+
+        r = self.sess.get(the_url)
 
         assert 'expires' in r.headers
         assert 'warning' in r.headers
+        assert r.headers['cache-control'] == 'public'
 
-        r = self.sess.get(hurl)
+        r = self.sess.get(the_url)
         assert r.from_cache