Add default values handling. Fixes keleshev#12.

README.rst

@@ -219,6 +219,21 @@ 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 for your optional key, pass a keyword
+argument ``default``, or use the ``Default`` class:
+
+.. code:: python
+
+    >>> from schema import Optional, Default
+    >>> Schema({'event': str,
+    ...        Optional('date'): Use(int, default=2012),
+    ...        }).validate({'event': 'First commit'})
+    {'date': 2012, 'event': 'First commit'}
+    >>> Schema({'account': str,
+    ...        Optional('amount'): Default(int),
+    ...        }).validate({'account': 'Piggy bank'})
+    {'account': 'Piggy bank', 'amount': 0}
+
 **schema** has classes ``And`` and ``Or`` that help validating several schemas
 for the same data:
 

schema.py

@@ -27,10 +27,40 @@ def uniq(seq):
         return '\n'.join(a)
 
 
+def handle_default(init):
+
+    """Add default handling to __init__ method; meant for decorators"""
+
+    def init2(self, *args, **kw):
+        # find default for ``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 type(self.default) == type:
+                self.default = 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')
 
@@ -59,6 +89,7 @@ def validate(self, data):
 
 class Use(object):
 
+    @handle_default
     def __init__(self, callable_, error=None):
         assert callable(callable_)
         self._callable = callable_
@@ -94,6 +125,7 @@ def priority(s):
 
 class Schema(object):
 
+    @handle_default
     def __init__(self, schema, error=None):
         self._schema = schema
         self._error = error
@@ -138,12 +170,19 @@ def validate(self, data):
                                           x.autos, [e] + x.errors)
                     else:
                         raise SchemaError('key %r is required' % skey, e)
-            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):
                 raise SchemaError('wrong keys %r in %r' % (new, 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:
@@ -178,3 +217,12 @@ def validate(self, data):
 class Optional(Schema):
 
     """Marker for an optional part of Schema."""
+
+    auto_default = True
+
+
+class Default(Schema):
+
+    """Wrapper automatically adding a default value if possible"""
+
+    auto_default = True

test_schema.py

@@ -3,7 +3,7 @@
 
 from pytest import raises
 
-from schema import Schema, Use, And, Or, Optional, SchemaError
+from schema import Schema, Use, And, Or, Optional, Default, SchemaError
 
 
 try:
@@ -12,6 +12,7 @@
     basestring = str  # Python 3 does not have basestring
 
 
+VE = raises(ValueError)
 SE = raises(SchemaError)
 
 
@@ -56,18 +57,19 @@ def test_validate_file():
 
 
 def test_and():
+    with raises(AssertionError): 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 raises(AssertionError): 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():
@@ -138,6 +140,33 @@ def test_dict_optional_keys():
             {'a': 1, 'b': 2}) == {'a': 1, 'b': 2}
 
 
+def test_dict_optional_keys_defaults():
+    assert Schema({'a': 1, Optional('b'): Default(2)}).validate(
+            {'a': 1}) == {'a': 1, 'b': 2}
+    assert Schema({'a': 1, Optional('b'): Default(2)}).validate(
+            {'a': 1}) == {'a': 1, 'b': 2}
+    assert Schema({'a': 1, Optional('b'): Default(int)}).validate(
+            {'a': 1}) == {'a': 1, 'b': 0}
+    assert Schema({'a': 1, Optional('b'): Default(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'): 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): Default(2)}).validate(
+            {'a': 1}) == {'a': 1, 0: 2}
+    assert Schema({'a': 1, Optional(int, default=4): Default(2)}).validate(
+            {'a': 1}) == {'a': 1, 4: 2}
+    with VE: Use(int, default='two')
+    with VE: Default(lambda x: x < 42, default=1337)
+
+
 def test_complex():
     s = Schema({'<file>': And([Use(open)], lambda l: len(l)),
                 '<path>': os.path.exists,