Issue #18988: The "Tab" key now works when a word is already autocompleted.

Lib/idlelib/AutoComplete.py

@@ -160,12 +160,9 @@ class AutoComplete:
         if not comp_lists[0]:
             return
         self.autocompletewindow = self._make_autocomplete_window()
-        self.autocompletewindow.show_window(comp_lists,
-                                            "insert-%dc" % len(comp_start),
-                                            complete,
-                                            mode,
-                                            userWantsWin)
-        return True
+        return not self.autocompletewindow.show_window(
+                comp_lists, "insert-%dc" % len(comp_start),
+                complete, mode, userWantsWin)
 
     def fetch_completions(self, what, mode):
         """Return a pair of lists of completions for something. The first list

Lib/idlelib/AutoCompleteWindow.py

@@ -157,13 +157,14 @@ class AutoCompleteWindow:
         self.start = self.widget.get(self.startindex, "insert")
         if complete:
             completed = self._complete_string(self.start)
+            start = self.start
             self._change_start(completed)
             i = self._binary_search(completed)
             if self.completions[i] == completed and \
                (i == len(self.completions)-1 or
                 self.completions[i+1][:len(completed)] != completed):
                 # There is exactly one matching completion
-                return
+                return completed == start
         self.userwantswindow = userWantsWin
         self.lasttypedstart = self.start
 

Misc/NEWS

@@ -293,6 +293,8 @@ C API
 IDLE
 ----
 
+- Issue #18988: The "Tab" key now works when a word is already autocompleted.
+
 - Issue #18489: Add tests for SearchEngine. Original patch by Phil Webster.
 
 - Issue #18429: Format / Format Paragraph, now works when comment blocks