set description for trap as well as debug

test for key error description

flask/app.py

@@ -1583,9 +1583,9 @@ class Flask(_PackageBoundObject):
 
         # MultiDict passes the key to the exception, but that's ignored
         # when generating the response message. Set an informative
-        # description for key errors in debug mode.
+        # description for key errors in debug mode or when trapping errors.
         if (
-            self.debug
+            self.debug or self.config['TRAP_BAD_REQUEST_ERRORS']
             and isinstance(e, BadRequestKeyError)
             # only set it if it's still the default description
             and e.description is BadRequestKeyError.description

tests/test_basic.py

@@ -975,12 +975,17 @@ def test_trapping_of_bad_request_key_errors(app, client):
     def fail():
         flask.request.form['missing_key']
 
-    assert client.get('/fail').status_code == 400
+    rv = client.get('/fail')
+    assert rv.status_code == 400
+    assert b'missing_key' not in rv.data
 
     app.config['TRAP_BAD_REQUEST_ERRORS'] = True
+
     with pytest.raises(KeyError) as e:
         client.get("/fail")
+
     assert e.errisinstance(BadRequest)
+    assert 'missing_key' in e.value.description
 
 
 def test_trapping_of_all_http_exceptions(app, client):