gh-159190: IDLE: Show full syntax error diagnostics in Shell

Lib/idlelib/idle_test/test_runscript.py

@@ -62,6 +62,44 @@ def test_run_module_event_shell_busy_no_restart(self):
         interp.runcode.assert_not_called()
         ew._close()
 
+    def test_checksyntax_reports_error_in_shell(self):
+        ew = EditorWindow(root=self.root)
+        self.addCleanup(ew._close)
+
+        sb = runscript.ScriptBinding(ew)
+        shell = mock.Mock()
+        sb.flist = mock.Mock()
+        sb.flist.open_shell.return_value = shell
+
+        source = b"def hello()\n    print('hello')\n"
+
+        with (
+            mock.patch("builtins.open", mock.mock_open(read_data=source)),
+            mock.patch.object(sb, "errorbox") as errorbox,
+            mock.patch.object(ew, "colorize_syntax_error") as colorize,
+        ):
+            result = sb.checksyntax("example.py")
+        self.assertFalse(result)
+
+        # The Shell receives the complete syntax diagnostic.
+        output = ''.join(
+            call.args[0] for call in shell.stderr.write.call_args_list
+        )
+        self.assertIn("example.py", output)
+        self.assertIn("line 1", output)
+        self.assertIn("def hello()", output)
+        self.assertIn("^", output)
+        self.assertIn("SyntaxError: expected ':'", output)
+
+        # No modal error dialog.
+        errorbox.assert_not_called()
+
+        # Preserve error highlighting in the editor.
+        colorize.assert_called_once()
+        # Make the Shell visible and focus it.
+        shell.top.lift.assert_called_once()
+        shell.text.focus_set.assert_called_once()
+
 
 if __name__ == '__main__':
     unittest.main(verbosity=2)

Lib/idlelib/runscript.py

@@ -13,6 +13,7 @@
 import tabnanny
 import time
 import tokenize
+import traceback
 
 from tkinter import messagebox
 
@@ -94,14 +95,19 @@ def checksyntax(self, filename):
             # If successful, return the compiled code
             return compile(source, filename, "exec")
         except SyntaxError as value:
-            msg = value.msg or "<no detail available>"
             lineno = value.lineno or 1
             offset = value.offset or 0
             if offset == 0:
                 lineno += 1  #mark end of offending line
             pos = "0.0 + %d lines + %d chars" % (lineno-1, offset-1)
             editwin.colorize_syntax_error(text, pos)
-            self.errorbox(type(value).__name__, msg)
+
+            shell.stderr.write(
+                ''.join(traceback.format_exception_only(value))
+            )
+            shell.text.see("end")
+            shell.top.lift()
+            shell.text.focus_set()
             return False
         except Exception as value:
             msg = str(value) or "<no detail available>"

Misc/NEWS.d/next/IDLE/2026-10-11-18-13-38.gh-issue-159190.wduX9N.rst

@@ -0,0 +1 @@
+IDLE now displays detailed syntax error diagnostics in the Shell instead of a modal dialog, while preserving error highlighting in the editor.