Add default values handling. Fixes #12.

README.rst

@@ -225,6 +225,28 @@ You can mark a key as optional as follows:
     ...         Optional('occupation'): str}).validate({'name': 'Sam'})
     {'name': 'Sam'}
 
+And if you want to give a default value, simply use the keyword argument
+``default``:
+
+.. code:: python
+
+    >>> from schema import Optional, Use, Or
+    >>> Schema({'event': str,
+    ...        # use the default argument for schema classes
+    ...        Optional('date'): Use(int, default=2012),
+    ...        # wrap other objects in a Schema class to add a default value
+    ...        Optional('comment'): Schema(str, default='Initial import'),
+    ...        }).validate({'event': 'First commit'})
+    {'comment': 'Initial import', 'date': 2012, 'event': 'First commit'}
+    >>> # advanced use:
+    ... Schema({
+    ...        # Optional key is a type, it gets instantiated as a default
+    ...        Optional(int): Or(True, False, default=False),
+    ...        # but it is possible to use the default attribute as well
+    ...        Optional(int, default=42): Use(str, default='The answer'),
+    ...        }).validate({})
+    {0: False, 42: 'The answer'}
+
 **schema** has classes ``And`` and ``Or`` that help validating several schemas
 for the same data:
 

schema.py

@@ -23,10 +23,44 @@ def uniq(seq):
         return '\n'.join(a)
 
 
+def handle_default(init):
+
+    """Add default handling to the __init__ method
+    Meant to be used as a decorator"""
+
+    def init2(self, *args, **kw):
+        # get default from the ``default`` keyword argument
+        if 'default' in kw:
+            self.default = kw['default']
+            del(kw['default'])
+        # if auto_default is set, get default from first argument
+        elif hasattr(self, 'auto_default') and self.auto_default:
+            self.default = args[0]
+            if hasattr(self.default, 'default'):
+                self.default = self.default.default
+            elif issubclass(type(self.default), type):
+                self.default = self.default()
+            elif hasattr(self.default, 'validate'):
+                # e.g. {Optional(Use(int)): ...}
+                delattr(self, 'default')
+        # normal init
+        init(self, *args, **kw)
+        # validate default
+        if hasattr(self, 'default'):
+            try:
+                self.default = self.validate(self.default)
+            except SchemaError:
+                raise ValueError('%s does not validate its default: %s' % (
+                    self, self.default))
+    return init2
+
+
 class And(object):
 
+    @handle_default
     def __init__(self, *args, **kw):
         self._args = args
+        assert len(args)
         assert list(kw) in (['error'], [])
         self._error = kw.get('error')
 
@@ -55,6 +89,7 @@ def validate(self, data):
 
 class Use(object):
 
+    @handle_default
     def __init__(self, callable_, error=None):
         assert callable(callable_)
         self._callable = callable_
@@ -91,6 +126,7 @@ def priority(s):
 
 class Schema(object):
 
+    @handle_default
     def __init__(self, schema, error=None):
         self._schema = schema
         self._error = error
@@ -136,15 +172,22 @@ def validate(self, data):
                     if x is not None:
                         raise SchemaError(['invalid value for key %r' % key] +
                                           x.autos, [e] + x.errors)
-            coverage = set(k for k in coverage if type(k) is not Optional)
             required = set(k for k in s if type(k) is not Optional)
-            if coverage != required:
+            # missed keys
+            if not required.issubset(coverage):
                 raise SchemaError('missed keys %r' % (required - coverage), e)
+            # wrong keys
             if len(new) != len(data):
                 wrong_keys = set(data.keys()) - set(new.keys())
                 s_wrong_keys = ', '.join('%r' % k for k in sorted(wrong_keys))
                 raise SchemaError('wrong keys %s in %r' % (s_wrong_keys, data),
                                   e)
+            # default for optional keys
+            for k in set(s) - required - coverage:
+                try:
+                    new[k.default] = s[k].default
+                except AttributeError:
+                    pass
             return new
         if hasattr(s, 'validate'):
             try:
@@ -179,3 +222,5 @@ def validate(self, data):
 class Optional(Schema):
 
     """Marker for an optional part of Schema."""
+
+    auto_default = True

test_schema.py

@@ -12,6 +12,8 @@
     basestring = str  # Python 3 does not have basestring
 
 
+AE = raises(AssertionError)
+VE = raises(ValueError)
 SE = raises(SchemaError)
 
 
@@ -56,18 +58,19 @@ def test_validate_file():
 
 
 def test_and():
+    with AE: And()
     assert And(int, lambda n: 0 < n < 5).validate(3) == 3
     with SE: And(int, lambda n: 0 < n < 5).validate(3.33)
     assert And(Use(int), lambda n: 0 < n < 5).validate(3.33) == 3
     with SE: And(Use(int), lambda n: 0 < n < 5).validate('3.33')
 
 
 def test_or():
+    with AE: Or()
     assert Or(int, dict).validate(5) == 5
     assert Or(int, dict).validate({}) == {}
     with SE: Or(int, dict).validate('hai')
     assert Or(int).validate(4)
-    with SE: Or().validate(2)
 
 
 def test_validate_list():
@@ -151,6 +154,35 @@ def test_dict_optional_keys():
     assert Schema({'a': 1, Optional('b'): 2}).validate({'a': 1}) == {'a': 1}
     assert Schema({'a': 1, Optional('b'): 2}).validate(
             {'a': 1, 'b': 2}) == {'a': 1, 'b': 2}
+    assert Schema({'a': 1, Optional(Use(int)): 2}).validate(
+            {'a': 1, '4': 2}) == {'a': 1, 4: 2}
+
+
+def test_dict_optional_keys_defaults():
+    assert Schema({'a': 1, Optional('b'): Schema(2, default=2)}).validate(
+            {'a': 1}) == {'a': 1, 'b': 2}
+    assert Schema({'a': 1, Optional('b'): Use(int, default='4')}).validate(
+            {'a': 1}) == {'a': 1, 'b': 4}
+    assert Schema({'a': 1, Optional('b'): Use(int, default=4)}).validate(
+            {'a': 1}) == {'a': 1, 'b': 4}
+    assert Schema({'a': 1, Optional('b'): Or(2, 4, default=4)}).validate(
+            {'a': 1}) == {'a': 1, 'b': 4}
+    assert Schema({'a': 1, Optional('b'): And(
+            lambda x: x > 0,
+            lambda x: x < 16,
+            default=4)}).validate({'a': 1}) == {'a': 1, 'b': 4}
+    assert Schema({'a': 1, Optional(int): Schema(2, default=2)}).validate(
+            {'a': 1}) == {'a': 1, 0: 2}
+    assert Schema({
+        'a': 1,
+        Optional(int, default=4): Schema(2, default=2)
+    }).validate({'a': 1}) == {'a': 1, 4: 2}
+    with VE: Use(int, default='two')
+    with VE: Schema(lambda x: x < 42, default=1337)
+    assert Schema({
+        'a': 1,
+        Optional(Use(int, default=4)): Schema(int, default=2)
+    }).validate({'a': 1}) == {'a': 1, 4: 2}
 
 
 def test_complex():