Issue #5124: Paste with selection should always replace.

This is how paste work on Windows, Mac, modern Linux apps, and ttk widgets.
The exception was X11 tk widgets.  Original patch by Serhiy Storchake.

Lib/idlelib/PyShell.py

@@ -1396,6 +1396,17 @@ class PseudoInputFile(PseudoFile):
         self.shell.close()
 
 
+def fix_x11_paste(root):
+    "Make paste replace selection on x11.  See issue #5124."
+    if root._windowingsystem == 'x11':
+        for cls in 'Text', 'Entry', 'Spinbox':
+            root.bind_class(
+                cls,
+                '<<Paste>>',
+                'catch {%W delete sel.first sel.last}\n' +
+                        root.bind_class(cls, '<<Paste>>'))
+
+
 usage_msg = """\
 
 USAGE: idle  [-deins] [-t title] [file]*
@@ -1528,8 +1539,10 @@ def main():
                                     'editor-on-startup', type='bool')
     enable_edit = enable_edit or edit_start
     enable_shell = enable_shell or not enable_edit
+
     # start editor and/or shell windows:
     root = Tk(className="Idle")
+    root.withdraw()
 
     # set application icon
     icondir = os.path.join(os.path.dirname(__file__), 'Icons')
@@ -1544,7 +1557,7 @@ def main():
         root.wm_iconphoto(True, *icons)
 
     fixwordbreaks(root)
-    root.withdraw()
+    fix_x11_paste(root)
     flist = PyShellFileList(root)
     macosxSupport.setupApp(root, flist)
 

Lib/idlelib/idle_test/test_editmenu.py

+'''Test (selected) IDLE Edit menu items.
+
+Edit modules have their own test files files
+'''
+from test.support import requires
+requires('gui')
+import tkinter as tk
+import unittest
+from idlelib import PyShell
+
+class PasteTest(unittest.TestCase):
+    '''Test pasting into widgets that allow pasting.
+
+    On X11, replacing selections requires tk fix.
+    '''
+    @classmethod
+    def setUpClass(cls):
+        cls.root = root = tk.Tk()
+        PyShell.fix_x11_paste(root)
+        cls.text = tk.Text(root)
+        cls.entry = tk.Entry(root)
+        cls.spin = tk.Spinbox(root)
+        root.clipboard_clear()
+        root.clipboard_append('two')
+
+    @classmethod
+    def tearDownClass(cls):
+        del cls.text, cls.entry, cls.spin
+        cls.root.clipboard_clear()
+        cls.root.update_idletasks()
+        cls.root.destroy()
+        del cls.root
+
+    def test_paste_text(self):
+        "Test pasting into text with and without a selection."
+        text = self.text
+        for tag, ans in ('', 'onetwo\n'), ('sel', 'two\n'):
+            with self.subTest(tag=tag, ans=ans):
+                text.delete('1.0', 'end')
+                text.insert('1.0', 'one', tag)
+                text.event_generate('<<Paste>>')
+                self.assertEqual(text.get('1.0', 'end'), ans)
+
+    def test_paste_entry(self):
+        "Test pasting into an entry with and without a selection."
+        # On 3.6, generated <<Paste>> fails without empty select range
+        # for 'no selection'.  Live widget works fine.
+        entry = self.entry
+        for end, ans in (0, 'onetwo'), ('end', 'two'):
+            with self.subTest(entry=entry, end=end, ans=ans):
+                entry.delete(0, 'end')
+                entry.insert(0, 'one')
+                entry.select_range(0, end)  # see note
+                entry.event_generate('<<Paste>>')
+                self.assertEqual(entry.get(), ans)
+
+    def test_paste_spin(self):
+        "Test pasting into a spinbox with and without a selection."
+        # See note above for entry.
+        spin = self.spin
+        for end, ans in (0, 'onetwo'), ('end', 'two'):
+            with self.subTest(end=end, ans=ans):
+                spin.delete(0, 'end')
+                spin.insert(0, 'one')
+                spin.selection('range', 0, end)  # see note
+                spin.event_generate('<<Paste>>')
+                self.assertEqual(spin.get(), ans)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)