Add a new validator Maybe(type) which can be used for values that are

either of a certain type or None

voluptuous/tests/tests.py

@@ -6,7 +6,7 @@
     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
@@ -371,6 +371,7 @@ def test_repr():
                    max_included=False, msg='number not in range')
     coerce_ = Coerce(int, msg="moo")
     all_ = All('10', Coerce(int), msg='all msg')
+    maybe_int = Maybe(int)
 
     assert_equal(repr(match), "Match('a pattern', msg='message')")
     assert_equal(repr(replace), "Replace('you', 'I', msg='you and I')")
@@ -380,6 +381,7 @@ def test_repr():
     )
     assert_equal(repr(coerce_), "Coerce(int, msg='moo')")
     assert_equal(repr(all_), "All('10', Coerce(int, msg=None), msg='all msg')")
+    assert_equal(repr(maybe_int), "Maybe(<type 'int'>)")
 
 
 def test_list_validation_messages():
@@ -499,6 +501,15 @@ def test_unordered():
     s([3, 2])
 
 
+def test_maybe():
+    assert_raises(TypeError, Maybe, lambda x: x)
+
+    s = Schema(Maybe(int))
+    assert s(1) == 1
+
+    assert_raises(Invalid, s, 'foo')
+
+
 def test_empty_list_as_exact():
     s = Schema([])
     assert_raises(Invalid, s, [1])

voluptuous/validators.py

@@ -459,6 +459,35 @@ def PathExists(v):
         raise PathInvalid("Not a Path")
 
 
+class Maybe(object):
+    """Validate that the object is of a given type or is None.
+
+    :raises Invalid: if the value is not of the type declared or None
+
+    >>> s = Schema(Maybe(int))
+    >>> s(10)
+    10
+    >>> with raises(Invalid):
+    ...  s("string")
+
+    """
+    def __init__(self, kind, msg=None):
+        if not isinstance(kind, type):
+            raise TypeError("kind has to be a type")
+
+        self.kind = kind
+        self.msg = msg
+
+    def __call__(self, v):
+        if v is not None and not isinstance(v, self.kind):
+            raise Invalid(self.msg or "%s must be None or of type %s" % (v, self.kind))
+
+        return v
+
+    def __repr__(self):
+        return 'Maybe(%s)' % str(self.kind)
+
+
 class Range(object):
     """Limit a value to a range.