Fix #157: Add TLRU cache implementation.

docs/index.rst

@@ -140,6 +140,34 @@ computed when the item is inserted into the cache.
       items that have expired by the current value returned by
       :attr:`timer`.
 
+.. autoclass:: TLRUCache(maxsize, ttu, timer=time.monotonic, getsizeof=None)
+   :members: popitem, timer, ttu
+
+   Similar to :class:`TTLCache`, this class also associates an
+   expiration time with each item.  However, for :class:`TLRUCache`
+   items, expiration time is calculated by a user-provided time-to-use
+   (`ttu`) function, which is passed three arguments at the time of
+   insertion: the new item's key and value, as well as the current
+   value of `timer()`.  The expression `ttu(key, value, timer())`
+   defines the expiration time of a cache item, and must be comparable
+   against later results of `timer()`.
+
+   Items that expire because they have exceeded their time-to-use will
+   be no longer accessible, and will be removed eventually.  If no
+   expired items are there to remove, the least recently used items
+   will be discarded first to make space when necessary.
+
+   .. method:: expire(self, time=None)
+
+      Expired items will be removed from a cache only at the next
+      mutating operation, e.g. :meth:`__setitem__` or
+      :meth:`__delitem__`, and therefore may still claim memory.
+      Calling this method removes all items whose time-to-live would
+      have expired by `time`, so garbage collection is free to reuse
+      their memory.  If `time` is :const:`None`, this removes all
+      items that have expired by the current value returned by
+      :attr:`timer`.
+
 
 Extending cache classes
 -----------------------

src/cachetools/__init__.py

@@ -7,6 +7,7 @@
     "LRUCache",
     "MRUCache",
     "RRCache",
+    "TLRUCache",
     "TTLCache",
     "cached",
     "cachedmethod",
@@ -17,6 +18,7 @@
 import collections
 import collections.abc
 import functools
+import heapq
 import random
 import time
 
@@ -497,6 +499,165 @@ def __getlink(self, key):
         return value
 
 
+@functools.total_ordering
+class _TLRUItem:
+
+    __slots__ = ("key", "expire", "removed")
+
+    def __init__(self, key=None, expire=None):
+        self.key = key
+        self.expire = expire
+        self.removed = False
+
+    def __lt__(self, other):
+        return self.expire < other.expire
+
+
+class TLRUCache(Cache):
+    """Time aware Least Recently Used (TLRU) cache implementation."""
+
+    def __init__(self, maxsize, ttu, timer=time.monotonic, getsizeof=None):
+        Cache.__init__(self, maxsize, getsizeof)
+        self.__items = collections.OrderedDict()
+        self.__order = []
+        self.__timer = _Timer(timer)
+        self.__ttu = ttu
+
+    def __contains__(self, key):
+        try:
+            item = self.__items[key]  # no reordering
+        except KeyError:
+            return False
+        else:
+            return self.__timer() < item.expire
+
+    def __getitem__(self, key, cache_getitem=Cache.__getitem__):
+        try:
+            item = self.__getitem(key)
+        except KeyError:
+            expired = False
+        else:
+            expired = not (self.__timer() < item.expire)
+        if expired:
+            return self.__missing__(key)
+        else:
+            return cache_getitem(self, key)
+
+    def __setitem__(self, key, value, cache_setitem=Cache.__setitem__):
+        with self.__timer as time:
+            expire = self.__ttu(key, value, time)
+            if not (time < expire):
+                return  # skip expired items
+            self.expire(time)
+            cache_setitem(self, key, value)
+        # removing an existing item would break the heap structure, so
+        # only mark it as removed for now
+        try:
+            self.__getitem(key).removed = True
+        except KeyError:
+            pass
+        self.__items[key] = item = _TLRUItem(key, expire)
+        heapq.heappush(self.__order, item)
+
+    def __delitem__(self, key, cache_delitem=Cache.__delitem__):
+        with self.__timer as time:
+            # no self.expire() for performance reasons, e.g. self.clear() [#67]
+            cache_delitem(self, key)
+        item = self.__items.pop(key)
+        item.removed = True
+        if not (time < item.expire):
+            raise KeyError(key)
+
+    def __iter__(self):
+        for curr in self.__order:
+            # "freeze" time for iterator access
+            with self.__timer as time:
+                if time < curr.expire and not curr.removed:
+                    yield curr.key
+
+    def __len__(self):
+        time = self.__timer()
+        count = 0
+        for curr in self.__order:
+            if time < curr.expire and not curr.removed:
+                count += 1
+        return count
+
+    def __repr__(self, cache_repr=Cache.__repr__):
+        with self.__timer as time:
+            self.expire(time)
+            return cache_repr(self)
+
+    @property
+    def currsize(self):
+        with self.__timer as time:
+            self.expire(time)
+            return super().currsize
+
+    @property
+    def timer(self):
+        """The timer function used by the cache."""
+        return self.__timer
+
+    @property
+    def ttu(self):
+        """The local time-to-use function used by the cache."""
+        return self.__ttu
+
+    def expire(self, time=None):
+        """Remove expired items from the cache."""
+        if time is None:
+            time = self.__timer()
+        items = self.__items
+        order = self.__order
+        # clean up the heap if too many items are marked as removed
+        if len(order) > len(items) * 2:
+            self.__order = order = [item for item in order if not item.removed]
+            heapq.heapify(order)
+        cache_delitem = Cache.__delitem__
+        while order and (order[0].removed or not (time < order[0].expire)):
+            item = heapq.heappop(order)
+            if not item.removed:
+                cache_delitem(self, item.key)
+                del items[item.key]
+
+    def clear(self):
+        with self.__timer as time:
+            self.expire(time)
+            Cache.clear(self)
+
+    def get(self, *args, **kwargs):
+        with self.__timer:
+            return Cache.get(self, *args, **kwargs)
+
+    def pop(self, *args, **kwargs):
+        with self.__timer:
+            return Cache.pop(self, *args, **kwargs)
+
+    def setdefault(self, *args, **kwargs):
+        with self.__timer:
+            return Cache.setdefault(self, *args, **kwargs)
+
+    def popitem(self):
+        """Remove and return the `(key, value)` pair least recently used that
+        has not already expired.
+
+        """
+        with self.__timer as time:
+            self.expire(time)
+            try:
+                key = next(iter(self.__items))
+            except StopIteration:
+                raise KeyError("%s is empty" % self.__class__.__name__) from None
+            else:
+                return (key, self.pop(key))
+
+    def __getitem(self, key):
+        value = self.__items[key]
+        self.__items.move_to_end(key)
+        return value
+
+
 def cached(cache, key=hashkey, lock=None):
     """Decorator to wrap a function with a memoizing callable that saves
     results in a cache.