gh-140481: avoid misleading exception in tk.Tk.__getitem__

Lib/test/test_tkinter/widget_tests.py

@@ -60,6 +60,8 @@ def checkParam(self, widget, name, value, *, expected=_sentinel,
         t = widget.configure(name)
         self.assertEqual(len(t), 5)
         self.assertEqual2(t[4], expected, eq=eq)
+        self.assertEqual(name in widget, True)
+        self.assertEqual("invalid-option-tkinter" in widget, False)
 
     def checkInvalidParam(self, widget, name, value, errmsg=None):
         orig = widget[name]

Lib/tkinter/__init__.py

@@ -1845,10 +1845,18 @@ def configure(self, cnf=None, **kw):
 
     def cget(self, key):
         """Return the current value of the configuration option."""
-        return self.tk.call(self._w, 'cget', '-' + key)
+        return self.tk.call(self._w, 'cget', f'-{key}')
 
     __getitem__ = cget
 
+    def __contains__(self, option):
+        """Check if the given option exists in this widget"""
+        try:
+            self.cget(option)
+            return True
+        except TclError:
+            return False
+
     def __setitem__(self, key, value):
         self.configure({key: value})
 
@@ -4322,11 +4330,18 @@ def blank(self):
 
     def cget(self, option):
         """Return the value of OPTION."""
-        return self.tk.call(self.name, 'cget', '-' + option)
+        return self.tk.call(self.name, 'cget', f'-{option}')
     # XXX config
 
-    def __getitem__(self, key):
-        return self.tk.call(self.name, 'cget', '-' + key)
+    __getitem__ = cget
+
+    def __contains__(self, option):
+        """Check if the given option exists in this widget"""
+        try:
+            self.cget(option)
+            return True
+        except TclError:
+            return False
 
     def copy(self, *, from_coords=None, zoom=None, subsample=None):
         """Return a new PhotoImage with the same image as this widget.

Misc/NEWS.d/next/Library/2025-10-22-22-18-12.gh-issue-140481.llKxYZ.rst

@@ -0,0 +1 @@
+fixed misleading error and added ``__contain__`` method in ``tkinter.Misc``