SF #737473: Show up-to-date source code in tracebacks always.

And add an optional argument 'filename' to linecache.checkcache()
to enable checking caches per-file.

Lib/test/test_traceback.py

@@ -40,6 +40,47 @@ class TracebackCases(unittest.TestCase):
         self.assert_(len(err) == 3)
         self.assert_(err[1].strip() == "[x for x in x] = x")
 
+    def test_bug737473(self):
+        import sys, os, tempfile
+        savedpath = sys.path[:]
+        testdir = tempfile.mkdtemp()
+        try:
+            sys.path.insert(0, testdir)
+            testfile = os.path.join(testdir, 'test_bug737473.py')
+            print >> open(testfile, 'w'), """\
+def test():
+    raise ValueError"""
+
+            if hasattr(os, 'utime'):
+                os.utime(testfile, (0, 0))
+            else:
+                import time
+                time.sleep(3) # not to stay in same mtime.
+
+            if 'test_bug737473' in sys.modules:
+                del sys.modules['test_bug737473']
+            import test_bug737473
+
+            try:
+                test_bug737473.test()
+            except ValueError:
+                # this loads source code to linecache 
+                traceback.extract_tb(sys.exc_traceback)
+
+            print >> open(testfile, 'w'), """\
+def test():
+    raise NotImplementedError"""
+            reload(test_bug737473)
+            try:
+                test_bug737473.test()
+            except NotImplementedError:
+                src = traceback.extract_tb(sys.exc_traceback)[-1][-1]
+                self.failUnlessEqual(src, 'raise NotImplementedError')
+        finally:
+            sys.path[:] = savedpath
+            for f in os.listdir(testdir):
+                os.unlink(os.path.join(testdir, f))
+            os.rmdir(testdir)
 
 def test_main():
     run_unittest(TracebackCases)