bpo-33905: Add test for idlelib.stackview.StackBrowser. (GH-7852)

Increases coverage by 44%.

Lib/idlelib/idle_test/test_stackviewer.py

@@ -1,21 +1,34 @@
-"Test stackviewer, coverage 19%."
+"Test stackviewer, coverage 63%."
 
 from idlelib import stackviewer
 import unittest
 from test.support import requires
 from tkinter import Tk
 
+from idlelib.tree import TreeNode, ScrolledCanvas
+import sys
 
-class Test(unittest.TestCase):
+
+class StackBrowserTest(unittest.TestCase):
 
     @classmethod
     def setUpClass(cls):
+        svs = stackviewer.sys
+        try:
+            abc
+        except NameError:
+            svs.last_type, svs.last_value, svs.last_traceback = (
+                sys.exc_info())
+
         requires('gui')
         cls.root = Tk()
         cls.root.withdraw()
 
     @classmethod
     def tearDownClass(cls):
+        svs = stackviewer.sys
+        del svs.last_traceback, svs.last_type, svs.last_value
+
         cls.root.update_idletasks()
 ##        for id in cls.root.tk.call('after', 'info'):
 ##            cls.root.after_cancel(id)  # Need for EditorWindow.
@@ -23,13 +36,11 @@ class Test(unittest.TestCase):
         del cls.root
 
     def test_init(self):
-        try:
-            zzz
-        except NameError as e:
-            ex = e
-# Test runners suppress setting of sys.last_xyx, which stackviewer needs.
-# Revise stackviewer so following works.
-#        stackviewer.StackBrowser(self.root, ex=exc)
+        sb = stackviewer.StackBrowser(self.root)
+        isi = self.assertIsInstance
+        isi(stackviewer.sc, ScrolledCanvas)
+        isi(stackviewer.item, stackviewer.StackTreeItem)
+        isi(stackviewer.node, TreeNode)
 
 
 if __name__ == '__main__':