Clear the copy of the globs dict after running examples. This helps to

break cycles, which are a special problem when running generator tests
that provoke exceptions by invoking the .next() method of a named
generator-iterator:  then the iterator is named in globs, and the
iterator's frame gets a tracekback object pointing back to globs, and
gc doesn't chase these types so the cycle leaks.

Also changed _run_examples() to make a copy of globs itself, so its
callers (direct and indirect) don't have to (and changed the callers
to stop making their own copies); *that* much is a change I've been
meaning to make for a long time (it's more robust the new way).

Here's a way to provoke the symptom without doctest; it leaks at a
prodigious rate; if the last two "source" lines are replaced with
    g().next()
the iterator isn't named and then there's no leak:

source = """\
def g():
    yield 1/0

k = g()
k.next()
"""

code = compile(source, "<source>", "exec")

def f(globs):
    try:
        exec code in globs
    except ZeroDivisionError:
        pass

while 1:
    f(globals().copy())

After this change, running test_generators in an infinite loop still leaks,
but reduced from a flood to a trickle.

Lib/doctest.py

@@ -529,23 +529,34 @@ def _run_examples_inner(out, fakeout, examples, globs, verbose, name):
 
     return failures, len(examples)
 
-# Run list of examples, in context globs.  Return (#failures, #tries).
+# Run list of examples, in a shallow copy of context (dict) globs.
+# Return (#failures, #tries).
+# CAUTION:  globs is cleared before returning.  This is to help break
+# cycles that may have been created by the examples.
 
 def _run_examples(examples, globs, verbose, name):
     import sys
     saveout = sys.stdout
+    globs = globs.copy()
     try:
         sys.stdout = fakeout = _SpoofOut()
         x = _run_examples_inner(saveout.write, fakeout, examples,
                                 globs, verbose, name)
     finally:
         sys.stdout = saveout
+        # While Python gc can clean up most cycles on its own, it doesn't
+        # chase frame objects.  This is especially irksome when running
+        # generator tests that raise exceptions, because a named generator-
+        # iterator gets an entry in globs, and the generator-iterator
+        # object's frame's traceback info points back to globs.  This is
+        # easy to break just by clearing the namespace.
+        globs.clear()
     return x
 
 def run_docstring_examples(f, globs, verbose=0, name="NoName"):
     """f, globs, verbose=0, name="NoName" -> run examples from f.__doc__.
 
-    Use dict globs as the globals for execution.
+    Use (a shallow copy of) dict globs as the globals for execution.
     Return (#failures, #tries).
 
     If optional arg verbose is true, print stuff even if there are no
@@ -735,7 +746,7 @@ see its docs for details.
         f = t = 0
         e = _extract_examples(s)
         if e:
-            f, t = _run_examples(e, self.globs.copy(), self.verbose, name)
+            f, t = _run_examples(e, self.globs, self.verbose, name)
         if self.verbose:
             print f, "of", t, "examples failed in string", name
         self.__record_outcome(name, f, t)
@@ -773,8 +784,7 @@ see its docs for details.
                     "when object.__name__ doesn't exist; " + `object`)
         if self.verbose:
             print "Running", name + ".__doc__"
-        f, t = run_docstring_examples(object, self.globs.copy(),
-                                      self.verbose, name)
+        f, t = run_docstring_examples(object, self.globs, self.verbose, name)
         if self.verbose:
             print f, "of", t, "examples failed in", name + ".__doc__"
         self.__record_outcome(name, f, t)