Implement Predicate decorator

README.rst

@@ -109,6 +109,19 @@ If ``Schema(...)`` encounters a callable (function, class, or object with
     ...
     SchemaError: <lambda>(-12) should evaluate to True
 
+.. code:: python
+
+    >>> from schema import Predicate
+    >>> @Predicate(error="Zero or negative number")
+    ... def positive(x):
+    ...     return x >= 0
+
+    >>> positive.validate(-12)
+    Traceback (most recent call last):
+    ...
+    SchemaError: Zero or negative number
+
+
 "Validatables"
 ~~~~~~~~~~~~~~
 
@@ -344,6 +357,7 @@ instead of a built-in one.
     ...
     SchemaError: Invalid year
 
+
 You can see all errors that occurred by accessing exception's ``exc.autos``
 for auto-generated error messages, and ``exc.errors`` for errors
 which had ``error`` text passed to them.

schema.py

@@ -4,9 +4,10 @@
 
 import re
 
+
 __version__ = '0.6.7'
 __all__ = ['Schema',
-           'And', 'Or', 'Regex', 'Optional', 'Use', 'Forbidden', 'Const',
+           'And', 'Or', 'Regex', 'Optional', 'Use', 'Forbidden', 'Const', 'Predicate',
            'SchemaError',
            'SchemaWrongKeyError',
            'SchemaMissingKeyError',
@@ -378,6 +379,15 @@ def validate(self, data):
         return data
 
 
+def Predicate(error=None, ignore_extra_keys=False):
+    """
+    Decorator to wrap function as Schema.
+    """
+    def _Predicate(f):
+        return Schema(f, error=error, ignore_extra_keys=ignore_extra_keys)
+    return _Predicate
+
+
 def _callable_str(callable_):
     if hasattr(callable_, '__name__'):
         return callable_.__name__

test_schema.py

@@ -12,7 +12,7 @@
 from schema import (Schema, Use, And, Or, Regex, Optional, Const,
                     SchemaError, SchemaWrongKeyError,
                     SchemaMissingKeyError, SchemaUnexpectedTypeError,
-                    SchemaForbiddenKeyError, Forbidden)
+                    SchemaForbiddenKeyError, Forbidden, Predicate)
 
 if sys.version_info[0] == 3:
     basestring = str  # Python 3 does not have basestring
@@ -592,3 +592,25 @@ def validate(self, data):
     v = {'k': 1, 'd': {'k': 2, 'l': [{'l': [3, 4, 5]}]}}
     d = MySchema(s).validate(v)
     assert d['k'] == 2 and d['d']['k'] == 3 and d['d']['l'][0]['l'] == [4, 5, 6]
+
+
+def test_predicate():
+    @Predicate()
+    def p1(x):
+        return x == 1
+    assert p1._error is None and not p1._ignore_extra_keys
+
+    @Predicate("ERROR2", ignore_extra_keys=True)
+    def p2(x):
+        return x == 2
+    assert p2._error is "ERROR2" and p2._ignore_extra_keys
+
+    p2.validate(2)  # success
+
+    with SE:
+        try:
+            p2.validate(0)  # failure
+        except SchemaError as e:
+            assert e.autos == ['p2(0) should evaluate to True']
+            assert e.errors == ["ERROR2"]
+            raise