validators: Add check_int_in validator.

This is a useful helper for our enum format fields where we want to
only allow a fixed list of integer values.

zerver/lib/validator.py

@@ -30,7 +30,7 @@ import ujson
 from django.utils.translation import ugettext as _
 from django.core.exceptions import ValidationError
 from django.core.validators import validate_email, URLValidator
-from typing import Iterable, Optional, Tuple, cast
+from typing import Iterable, Optional, Tuple, cast, List
 
 from datetime import datetime
 from zerver.lib.request import JsonableError
@@ -92,6 +92,17 @@ def check_int(var_name: str, val: object) -> Optional[str]:
         return _('%s is not an integer') % (var_name,)
     return None
 
+def check_int_in(possible_values: List[int]) -> Validator:
+    def validator(var_name: str, val: object) -> Optional[str]:
+        not_int = check_int(var_name, val)
+        if not_int is not None:
+            return not_int
+        if val not in possible_values:
+            return _("Invalid %s") % (var_name,)
+        return None
+
+    return validator
+
 def check_float(var_name: str, val: object) -> Optional[str]:
     if not isinstance(val, float):
         return _('%s is not a float') % (var_name,)