Add expiry seconds option to cacheclass.

voussoirkit/cacheclass.py

@@ -1,42 +1,96 @@
 import collections
+import time
+
+from voussoirkit import sentinel
+
+NO_ITEM = sentinel.Sentinel('no item')
 
 class Cache:
-    def __init__(self, maxlen):
+    def __init__(self, maxlen, expiry=float('inf')):
         self.maxlen = maxlen
+        self.expiry = expiry
         self.cache = collections.OrderedDict()
 
+        # To prevent excessive purge loops during repeated setitem, only allow
+        # a purge once every this many seconds.
+        self.max_purge_frequency = 0.5
+
+        self._last_purge = 0
+
     def __contains__(self, key):
-        return key in self.cache
+        return self.get(key, fallback=NO_ITEM) is not NO_ITEM
 
     def __getitem__(self, key):
-        value = self.cache.pop(key)
-        self.cache[key] = value
+        '''
+        Return the key's value, or raise KeyError.
+        '''
+        # Let KeyError raise to caller.
+        (value, timestamp) = self.cache.pop(key)
+
+        now = time.time()
+        if (now - timestamp) > self.expiry:
+            raise KeyError(key)
+
+        self.cache[key] = (value, timestamp)
         return value
 
     def __len__(self):
+        '''
+        Purge expired items, then count the length.
+        Due to the purge, this method is not O(1) as most len methods are.
+        '''
+        self._purge_expired()
         return len(self.cache)
 
     def __setitem__(self, key, value):
+        # If the key was already present, we don't need to worry about maxlen
+        # because the net change is zero. If it was not present (KeyError) we
+        # check the maxlen and pop the oldest item if needed.
+        # Either way we update the timestamp.
         try:
             self.cache.pop(key)
         except KeyError:
-            if len(self.cache) >= self.maxlen:
+            if len(self) >= self.maxlen:
                 self.cache.popitem(last=False)
-        self.cache[key] = value
+        self.cache[key] = (value, time.time())
+
+    def _purge_expired(self):
+        now = time.time()
+        if now - self._last_purge < self.max_purge_frequency:
+            return
+
+        for (key, (value, timestamp)) in list(self.cache.items()):
+            if (now - timestamp) > self.expiry:
+                self.cache.pop(key)
+
+        self._last_purge = now
 
     def clear(self):
+        '''
+        Remove everything from the cache.
+        '''
         self.cache.clear()
 
     def get(self, key, fallback=None):
+        '''
+        Return the key's value, or fallback in case of KeyError.
+        '''
         try:
             return self[key]
         except KeyError:
             return fallback
 
     def pop(self, key):
-        return self.cache.pop(key)
+        '''
+        Remove the key and return its value, or raise KeyError.
+        '''
+        (value, timestamp) = self.cache.pop(key)
+        return value
 
     def remove(self, key):
+        '''
+        Remove the item and ignore KeyError.
+        '''
         try:
             self.pop(key)
         except KeyError: