maybe can be something of the given type or None

voluptuous/tests/tests.py

@@ -5,7 +5,7 @@
     Schema, Required, Optional, Extra, Invalid, In, Remove, Literal,
     Url, MultipleInvalid, LiteralInvalid, NotIn, Match, Email,
     Replace, Range, Coerce, All, Any, Length, FqdnUrl, ALLOW_EXTRA, PREVENT_EXTRA,
-    validate, ExactSequence, Equal, Unordered, Number
+    validate, ExactSequence, Equal, Unordered, Number, Maybe
 )
 from voluptuous.humanize import humanize_error
 from voluptuous.util import to_utf8_py2, u
@@ -544,6 +544,19 @@ def fn(arg):
     fn(1)
 
 
+def test_schema_decorator_no_args():
+    @validate(int, __return__=Maybe(int))
+    def fn(arg):
+        if arg == 0:
+            return None
+        else:
+            return arg
+
+    fn(1)
+    fn(0)
+
+
+
 def test_schema_decorator_unmatch_return_with_args():
     @validate(int, __return__=int)
     def fn(arg):

voluptuous/validators.py

@@ -459,6 +459,14 @@ def PathExists(v):
         raise PathInvalid("Not a Path")
 
 
+class Maybe(object):
+    def __init__(self, kind):
+        self.kind = kind
+
+    def __call__(self, v):
+        return v is None or isinstance(v, self.kind)
+
+
 class Range(object):
     """Limit a value to a range.