[ python-Bugs-1573394 ] struct module doesn't use weakref for cache

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Bugs item #1573394, was opened at 2006-10-08 19:37
Message generated for change (Comment added) made by etrepum
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Category: Python Library
Group: Python 2.5
Status: Closed
Resolution: Wont Fix
Priority: 5
Submitted By: Mark Flacy (markaflacy)
Assigned to: Bob Ippolito (etrepum)
Summary: struct module doesn't use weakref for cache

Initial Comment:
At the moment, when you attempt to add your 101st
different Struct object to the cache, all the other 100
entries are tossed into the garbage.  That seems a
trifle odd.

The Struct cache would appear to be a perfect use for a
weakref dictionary.

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>Comment By: Bob Ippolito (etrepum)
Date: 2006-10-12 13:54

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Yes, that code is different. You haven't shown that it's
better though. Using popitem probably doesn't have very good
cache effects.

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Comment By: Žiga Seilnacht (zseil)
Date: 2006-10-12 13:36

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I was thinking abot something like this:

Index: Lib/struct.py
===================================================================
--- Lib/struct.py	(revision 52316)
+++ Lib/struct.py	(working copy)
@@ -35,7 +35,7 @@
 def _compile(fmt):
     # Internal: compile struct pattern
     if len(_cache) >= _MAXCACHE:
-        _cache.clear()
+        _cache.popitem()
     s = Struct(fmt)
     _cache[fmt] = s
     return s

(sorry, I don't have the rights to attach a file)


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Comment By: Bob Ippolito (etrepum)
Date: 2006-10-12 12:56

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Weakrefs would slow it down.. probably to the point where
the cache wouldn't be an improvement at all.

The re module does the same thing with the regular
expression cache. This isn't a bug until someone presents a
patch that proves another strategy is better for performance.

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Comment By: Žiga Seilnacht (zseil)
Date: 2006-10-12 11:52

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WeakValueDictionary would be useless here; the cache is
the only thing that holds references to Struct objects.
Replacing the _cache.clear() call with _cache.popitem()
seems like a better solution.

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