rewrote the initpkg mechanism and moved py lib implementation files to

_py/...  with py/__init__.py containing pointers into them

The new apipkg is only around 70 lines of code and allows
us to get rid of the infamous "py.__." by regular non-magical
"_py." imports. It is also available as a separately installable
package, see http://bitbucket.org/hpk42/apipkg

--HG--
branch : trunk

_py/path/cacheutil.py

@@ -0,0 +1,111 @@
+"""
+This module contains multithread-safe cache implementations.
+
+All Caches have 
+
+    getorbuild(key, builder) 
+    delentry(key) 
+
+methods and allow configuration when instantiating the cache class. 
+"""
+from time import time as gettime
+
+class BasicCache(object):
+    def __init__(self, maxentries=128):
+        self.maxentries = maxentries
+        self.prunenum = int(maxentries - maxentries/8)
+        self._dict = {}
+
+    def _getentry(self, key):
+        return self._dict[key]
+
+    def _putentry(self, key, entry):
+        self._prunelowestweight()
+        self._dict[key] = entry 
+
+    def delentry(self, key, raising=False):
+        try:
+            del self._dict[key]
+        except KeyError:
+            if raising:
+                raise
+
+    def getorbuild(self, key, builder):
+        try:
+            entry = self._getentry(key)
+        except KeyError:
+            entry = self._build(key, builder)
+            self._putentry(key, entry)
+        return entry.value
+
+    def _prunelowestweight(self):
+        """ prune out entries with lowest weight. """
+        numentries = len(self._dict)
+        if numentries >= self.maxentries:
+            # evict according to entry's weight
+            items = [(entry.weight, key) 
+                        for key, entry in self._dict.items()]
+            items.sort()
+            index = numentries - self.prunenum
+            if index > 0:
+                for weight, key in items[:index]:
+                    # in MT situations the element might be gone
+                    self.delentry(key, raising=False) 
+
+class BuildcostAccessCache(BasicCache):
+    """ A BuildTime/Access-counting cache implementation.
+        the weight of a value is computed as the product of
+
+            num-accesses-of-a-value * time-to-build-the-value
+
+        The values with the least such weights are evicted
+        if the cache maxentries threshold is superceded.
+        For implementation flexibility more than one object
+        might be evicted at a time.
+    """
+    # time function to use for measuring build-times
+
+    def _build(self, key, builder):
+        start = gettime()
+        val = builder()
+        end = gettime()
+        return WeightedCountingEntry(val, end-start)
+
+
+class WeightedCountingEntry(object):
+    def __init__(self, value, oneweight):
+        self._value = value
+        self.weight = self._oneweight = oneweight 
+
+    def value(self):
+        self.weight += self._oneweight
+        return self._value
+    value = property(value)
+
+class AgingCache(BasicCache):
+    """ This cache prunes out cache entries that are too old.
+    """
+    def __init__(self, maxentries=128, maxseconds=10.0):
+        super(AgingCache, self).__init__(maxentries)
+        self.maxseconds = maxseconds
+
+    def _getentry(self, key):
+        entry = self._dict[key]
+        if entry.isexpired():
+            self.delentry(key) 
+            raise KeyError(key) 
+        return entry
+
+    def _build(self, key, builder):
+        val = builder()
+        entry = AgingEntry(val, gettime() + self.maxseconds)
+        return entry
+
+class AgingEntry(object):
+    def __init__(self, value, expirationtime):
+        self.value = value
+        self.weight = expirationtime
+
+    def isexpired(self):
+        t = gettime()
+        return t >= self.weight