Issue *18081, #18242: Change Idle warnings capture in PyShell and run to stop

replacing warnings.formatwarnings and to reverse replacement of
warnings.showwarnings when import is complete and when main function exits.
Add test_warning.py. Vinay Sajip provided capture_warnings function.

Lib/idlelib/idle_test/test_warning.py

@@ -0,0 +1,73 @@
+'''Test warnings replacement in PyShell.py and run.py.
+
+This file could be expanded to include traceback overrides
+(in same two modules). If so, change name.
+Revise if output destination changes (http://bugs.python.org/issue18318).
+Make sure warnings module is left unaltered (http://bugs.python.org/issue18081).
+'''
+
+import unittest
+from test.support import captured_stderr
+
+import warnings
+# Try to capture default showwarning before Idle modules are imported.
+showwarning = warnings.showwarning
+# But if we run this file within idle, we are in the middle of the run.main loop
+# and default showwarnings has already been replaced.
+running_in_idle = 'idle' in showwarning.__name__
+
+from idlelib import run
+from idlelib import PyShell as shell
+
+# The following was generated from PyShell.idle_formatwarning
+# and checked as matching expectation.
+idlemsg = '''
+Warning (from warnings module):
+  File "test_warning.py", line 99
+    Line of code
+UserWarning: Test
+'''
+shellmsg = idlemsg + ">>> "
+
+class RunWarnTest(unittest.TestCase):
+
+    @unittest.skipIf(running_in_idle, "Does not work when run within Idle.")
+    def test_showwarnings(self):
+        self.assertIs(warnings.showwarning, showwarning)
+        run.capture_warnings(True)
+        self.assertIs(warnings.showwarning, run.idle_showwarning_subproc)
+        run.capture_warnings(False)
+        self.assertIs(warnings.showwarning, showwarning)
+
+    def test_run_show(self):
+        with captured_stderr() as f:
+            run.idle_showwarning_subproc(
+                    'Test', UserWarning, 'test_warning.py', 99, f, 'Line of code')
+            # The following uses .splitlines to erase line-ending differences
+            self.assertEqual(idlemsg.splitlines(), f.getvalue().splitlines())
+
+class ShellWarnTest(unittest.TestCase):
+
+    @unittest.skipIf(running_in_idle, "Does not work when run within Idle.")
+    def test_showwarnings(self):
+        self.assertIs(warnings.showwarning, showwarning)
+        shell.capture_warnings(True)
+        self.assertIs(warnings.showwarning, shell.idle_showwarning)
+        shell.capture_warnings(False)
+        self.assertIs(warnings.showwarning, showwarning)
+
+    def test_idle_formatter(self):
+        # Will fail if format changed without regenerating idlemsg
+        s = shell.idle_formatwarning(
+                'Test', UserWarning, 'test_warning.py', 99, 'Line of code')
+        self.assertEqual(idlemsg, s)
+
+    def test_shell_show(self):
+        with captured_stderr() as f:
+            shell.idle_showwarning(
+                    'Test', UserWarning, 'test_warning.py', 99, f, 'Line of code')
+            self.assertEqual(shellmsg.splitlines(), f.getvalue().splitlines())
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2, exit=False)