[1.7.x] Fixed #22570 -- Made Form.__getitem__ KeyError more descriptive.

Backport of df60db0e78 from master

django/forms/forms.py

@@ -142,7 +142,8 @@ class BaseForm(object):
         try:
             field = self.fields[name]
         except KeyError:
-            raise KeyError('Key %r not found in Form' % name)
+            raise KeyError(
+                "Key %r not found in '%s'" % (name, self.__class__.__name__))
         return BoundField(self, field, name)
 
     @property

tests/forms_tests/tests/test_forms.py

@@ -57,11 +57,11 @@ class FormsTestCase(TestCase):
         self.assertHTMLEqual(str(p['first_name']), '<input type="text" name="first_name" value="John" id="id_first_name" />')
         self.assertHTMLEqual(str(p['last_name']), '<input type="text" name="last_name" value="Lennon" id="id_last_name" />')
         self.assertHTMLEqual(str(p['birthday']), '<input type="text" name="birthday" value="1940-10-9" id="id_birthday" />')
-        try:
+
+        nonexistenterror = "Key u?'nonexistentfield' not found in 'Person'"
+        with self.assertRaisesRegexp(KeyError, nonexistenterror):
             p['nonexistentfield']
             self.fail('Attempts to access non-existent fields should fail.')
-        except KeyError:
-            pass
 
         form_output = []