[pypy-commit] pypy default: (antocuni, cfbolz around): reverse-engineer what's going on, and document it

[antocuni]

Author: Antonio Cuni <anto.cuni at gmail.com>
Branch: 
Changeset: r54499:c20aa5de87cf
Date: 2012-04-18 16:00 +0200
http://bitbucket.org/pypy/pypy/changeset/c20aa5de87cf/

Log:	(antocuni, cfbolz around): reverse-engineer what's going on, and
	document it

diff --git a/pypy/objspace/std/test/test_methodcache.py b/pypy/objspace/std/test/test_methodcache.py
--- a/pypy/objspace/std/test/test_methodcache.py
+++ b/pypy/objspace/std/test/test_methodcache.py
@@ -63,8 +63,31 @@
             assert a.f() == 42 + i
             A.f = eval("lambda self: %s" % (42 + i + 1, ))
         cache_counter = __pypy__.method_cache_counter("f")
-        # the cache hits come from A.f = ..., which first does a lookup on A as
-        # well
+        #
+        # a bit of explanation about what's going on.  (1) is the line "a.f()"
+        # and (2) is "A.f = ...".
+        #
+        # at line (1) we do the lookup on type(a).f
+        #
+        # at line (2) we do a setattr on A. However, descr_setattr does also a
+        # lookup of type(A).f i.e. type.f, to check if by chance 'f' is a data
+        # descriptor.
+        #
+        # At the first iteration:
+        # (1) is a miss because it's the first lookup of A.f. The result is cached
+        #
+        # (2) is a miss because it is the first lookup of type.f. The
+        # (non-existant) result is cached. The version of A changes, and 'f'
+        # is changed to be a cell object, so that subsequest assignments won't
+        # change the version of A
+        #
+        # At the second iteration:
+        # (1) is a miss because the version of A changed just before
+        # (2) is a hit, because type.f is cached. The version of A no longer changes
+        #
+        # At the third and subsequent iterations:
+        # (1) is a hit, because the version of A did not change
+        # (2) is a hit, see above
         assert cache_counter == (17, 3)
 
     def test_subclasses(self):