bpo-32826: Add "encoding=utf-8" to open() in idle_test/test_help_about. (GH-5639)

GUI test test_file_buttons() only looks at initial ascii-only lines,
but failed on systems where open() defaults to 'ascii' because
readline() internally reads and decodes far enough ahead to encounter
a non-ascii character in CREDITS.txt.

Lib/idlelib/idle_test/test_help_about.py

@@ -50,31 +50,33 @@ class LiveDialogTest(unittest.TestCase):
     def test_printer_buttons(self):
         """Test buttons whose commands use printer function."""
         dialog = self.dialog
-        button_sources = [(dialog.py_license, license),
-                          (dialog.py_copyright, copyright),
-                          (dialog.py_credits, credits)]
+        button_sources = [(dialog.py_license, license, 'license'),
+                          (dialog.py_copyright, copyright, 'copyright'),
+                          (dialog.py_credits, credits, 'credits')]
 
-        for button, printer in button_sources:
+        for button, printer, name in button_sources:
+            with self.subTest(name=name):
                 printer._Printer__setup()
                 button.invoke()
                 get = dialog._current_textview.viewframe.textframe.text.get
-            self.assertEqual(printer._Printer__lines[0], get('1.0', '1.end'))
-            self.assertEqual(
-                    printer._Printer__lines[1], get('2.0', '2.end'))
+                lines = printer._Printer__lines
+                self.assertEqual(lines[0], get('1.0', '1.end'))
+                self.assertEqual(lines[1], get('2.0', '2.end'))
                 dialog._current_textview.destroy()
 
     def test_file_buttons(self):
         """Test buttons that display files."""
         dialog = self.dialog
-        button_sources = [(self.dialog.readme, 'README.txt'),
-                          (self.dialog.idle_news, 'NEWS.txt'),
-                          (self.dialog.idle_credits, 'CREDITS.txt')]
+        button_sources = [(self.dialog.readme, 'README.txt', 'readme'),
+                          (self.dialog.idle_news, 'NEWS.txt', 'news'),
+                          (self.dialog.idle_credits, 'CREDITS.txt', 'credits')]
 
-        for button, filename in button_sources:
+        for button, filename, name in button_sources:
+            with  self.subTest(name=name):
                 button.invoke()
                 fn = findfile(filename, subdir='idlelib')
                 get = dialog._current_textview.viewframe.textframe.text.get
-            with open(fn) as f:
+                with open(fn, encoding='utf-8') as f:
                     self.assertEqual(f.readline().strip(), get('1.0', '1.end'))
                     f.readline()
                     self.assertEqual(f.readline().strip(), get('3.0', '3.end'))

Misc/NEWS.d/next/IDLE/2018-02-12-11-05-22.bpo-32826.IxNZrk.rst

+Add "encoding=utf-8" to open() in IDLE's test_help_about.
+GUI test test_file_buttons() only looks at initial ascii-only lines,
+but failed on systems where open() defaults to 'ascii' because
+readline() internally reads and decodes far enough ahead to encounter
+a non-ascii character in CREDITS.txt.