Avoid allocating space for tagged values unless they're used. This saves another few percent.slots

2 files changed, 19 insertions, 3 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index c5de832..b976ba1 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -17,6 +17,16 @@
   an undocumented private C API function, and helps make some
   instances require less memory. See `PR 154 <https://github.com/zopefoundation/zope.interface/pull/154>`_.
 
+- Reduce memory usage in other ways based on observations of usage
+  patterns in Zope (3) and Plone code bases.
+
+  - Specifications with no dependents are common (more than 50%) so
+    avoid allocating a ``WeakKeyDictionary`` unless we need it.
+  - Likewise, tagged values are relatively rare, so don't allocate a
+    dictionary to hold them until they are used.
+
+  The changes in this release resulted in a 7% memory reduction after
+  loading about 6,000 modules that define about 2,200 interfaces.
 
 4.7.1 (2019-11-11)
 ==================
diff --git a/src/zope/interface/interface.py b/src/zope/interface/interface.py
index c374cef..f501077 100644
--- a/src/zope/interface/interface.py
+++ b/src/zope/interface/interface.py
@@ -67,7 +67,9 @@ class Element(object):
 
         self.__name__ = __name__
         self.__doc__ = __doc__
-        self.__tagged_values = {}
+        # Tagged values are rare, especially on methods or attributes.
+        # Deferring the allocation can save substantial memory.
+        self.__tagged_values = None
 
     def getName(self):
         """ Returns the name of the object. """
@@ -79,18 +81,22 @@ class Element(object):
 
     def getTaggedValue(self, tag):
         """ Returns the value associated with 'tag'. """
+        if not self.__tagged_values:
+            raise KeyError(tag)
         return self.__tagged_values[tag]
 
     def queryTaggedValue(self, tag, default=None):
         """ Returns the value associated with 'tag'. """
-        return self.__tagged_values.get(tag, default)
+        return self.__tagged_values.get(tag, default) if self.__tagged_values else default
 
     def getTaggedValueTags(self):
         """ Returns a list of all tags. """
-        return self.__tagged_values.keys()
+        return self.__tagged_values.keys() if self.__tagged_values else ()
 
     def setTaggedValue(self, tag, value):
         """ Associates 'value' with 'key'. """
+        if self.__tagged_values is None:
+            self.__tagged_values = {}
         self.__tagged_values[tag] = value