Fix #216: Add documentation and tests for using TTLCache with datetime.

2 files changed, 24 insertions, 2 deletions

diff --git a/docs/index.rst b/docs/index.rst
index 147a19f..c9828b3 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -26,7 +26,7 @@ calls are provided, too.
 .. testsetup:: *
 
    import operator
-   from cachetools import cached, cachedmethod, LRUCache
+   from cachetools import cached, cachedmethod, LRUCache, TTLCache
 
    from unittest import mock
    urllib = mock.MagicMock()
@@ -122,7 +122,17 @@ computed when the item is inserted into the cache.
 
    By default, the time-to-live is specified in seconds and
    :func:`time.monotonic` is used to retrieve the current time.  A
-   custom `timer` function can be supplied if needed.
+   custom `timer` function can also be supplied:
+
+   .. testcode::
+
+      from datetime import datetime, timedelta
+
+      cache = TTLCache(maxsize=10, ttl=timedelta(hours=12), timer=datetime.now)
+
+   The expression `timer() + ttl` at the time of insertion defines the
+   expiration time of a cache item, and must be comparable against
+   later results of `timer()`.
 
    .. method:: expire(self, time=None)
 
diff --git a/tests/test_ttl.py b/tests/test_ttl.py
index f677c9b..6e51a59 100644
--- a/tests/test_ttl.py
+++ b/tests/test_ttl.py
@@ -182,3 +182,15 @@ class TTLCacheTest(unittest.TestCase, CacheTestMixin):
         with self.assertRaises(KeyError):
             cache[(1, 2, 3)]
         self.assertNotIn((1, 2, 3), cache)
+
+    def test_ttl_datetime(self):
+        from datetime import datetime, timedelta
+
+        cache = TTLCache(maxsize=1, ttl=timedelta(days=1), timer=datetime.now)
+
+        cache[1] = 1
+        self.assertEqual(1, len(cache))
+        cache.expire(datetime.now())
+        self.assertEqual(1, len(cache))
+        cache.expire(datetime.now() + timedelta(days=1))
+        self.assertEqual(0, len(cache))