gh-60402: Fix Tab in a string in IDLE

Open the file name completion list only if some file name starts
with the text before the cursor; otherwise insert a tab.

Co-authored-by: Roger Serwy <roger.serwy@gmail.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people

Lib/idlelib/autocomplete.py

@@ -153,6 +153,11 @@ def open_completions(self, args):
         comp_lists = self.fetch_completions(comp_what, mode)
         if not comp_lists[0]:
             return None
+        if (complete and mode == FILES
+                and not any(name.startswith(comp_start)
+                            for name in comp_lists[0])):
+            # Nothing to complete; insert a tab.
+            return None
         self.autocompletewindow = self._make_autocomplete_window()
         return not self.autocompletewindow.show_window(
                 comp_lists, "insert-%dc" % len(comp_start),

Lib/idlelib/idle_test/test_autocomplete.py

@@ -218,6 +218,12 @@ def make_acw(): return self.dummy_acw()
         self.assertTrue(acp.open_completions(ac.TAB))
         self.text.delete('1.0', 'end')
 
+        # No file name starts with the text (gh-60402).
+        self.text.insert('1.0', '"hello wor')
+        self.assertIsNone(acp.open_completions(ac.TAB))
+        self.assertTrue(acp.open_completions(ac.FORCE))
+        self.text.delete('1.0', 'end')
+
     def test_completion_kwds(self):
         self.assertIn('and', ac.completion_kwds)
         self.assertIn('case', ac.completion_kwds)

Misc/NEWS.d/next/IDLE/2026-09-15-21-44-35.gh-issue-60402.iFz6X6.rst

@@ -0,0 +1,2 @@
+In IDLE, Tab in a string that is not the start of a file name inserts a tab
+instead of opening the completion list.