check_short_string: Verify string before checking length.

While the previous implementation was correct, this should give better
error messages.

zerver/lib/validator.py

@@ -40,11 +40,13 @@ def check_string(var_name: str, val: object) -> Optional[str]:
     return None
 
 def check_short_string(var_name: str, val: object) -> Optional[str]:
+    if not isinstance(val, str):
+        return _('%s is not a string') % (var_name,)
     max_length = 200
     if len(val) >= max_length:
         return _("{var_name} is longer than {max_length}.".format(
             var_name=var_name, max_length=max_length))
-    return check_string(var_name, val)
+    return None
 
 def check_int(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, int):

zerver/tests/test_decorators.py

@@ -36,7 +36,7 @@ from zerver.decorator import (
 )
 from zerver.lib.validator import (
     check_string, check_dict, check_dict_only, check_bool, check_float, check_int, check_list, Validator,
-    check_variable_type, equals, check_none_or, check_url,
+    check_variable_type, equals, check_none_or, check_url, check_short_string
 )
 from zerver.models import \
     get_realm, get_user, UserProfile, Client, Realm, Recipient
@@ -452,6 +452,17 @@ class ValidatorTestCase(TestCase):
         x = 4
         self.assertEqual(check_string('x', x), 'x is not a string')
 
+    def test_check_short_string(self):
+        # type: () -> None
+        x = "hello"  # type: Any
+        self.assertEqual(check_short_string('x', x), None)
+
+        x = 'x' * 201
+        self.assertEqual(check_short_string('x', x), 'x is longer than 200.')
+
+        x = 4
+        self.assertEqual(check_short_string('x', x), 'x is not a string')
+
     def test_check_bool(self):
         # type: () -> None
         x = True  # type: Any