Always evaluate {} as {}

If you want a dict with any content, use 'dict', not
Schema({}, extra=ALLOW_EXTRA).

README.md

@@ -360,6 +360,28 @@ token `extra` as a key:
 
 ```
 
+However, an empty dict (`{}`) is treated as is. If you want to specify a list that can
+contain anything, specify it as `dict`:
+
+```pycon
+>>> schema = Schema({}, extra=ALLOW_EXTRA)  # don't do this
+>>> try:
+...   schema({'extra': 1})
+...   raise AssertionError('MultipleInvalid not raised')
+... except MultipleInvalid as e:
+...   exc = e
+>>> str(exc) == "not a valid value"
+True
+>>> schema({})
+{}
+>>> schema = Schema(dict)  # do this instead
+>>> schema({})
+{}
+>>> schema({'extra': 1})
+{'extra': 1}
+
+```
+
 #### Required dictionary keys
 
 By default, keys in the schema are not required to be in the data:

voluptuous/schema_builder.py

@@ -201,7 +201,7 @@ def _compile(self, schema):
             return lambda _, v: v
         if isinstance(schema, Object):
             return self._compile_object(schema)
-        if isinstance(schema, collections.Mapping):
+        if isinstance(schema, collections.Mapping) and len(schema):
             return self._compile_dict(schema)
         elif isinstance(schema, list) and len(schema):
             return self._compile_list(schema)
@@ -366,7 +366,7 @@ def _compile_dict(self, schema):
 
         A dictionary schema will only validate a dictionary:
 
-            >>> validate = Schema({})
+            >>> validate = Schema({'prop': str})
             >>> with raises(er.MultipleInvalid, 'expected a dictionary'):
             ...   validate([])
 
@@ -381,7 +381,6 @@ def _compile_dict(self, schema):
             >>> with raises(er.MultipleInvalid, "extra keys not allowed @ data['two']"):
             ...   validate({'two': 'three'})
 
-
         Validation function, in this case the "int" type:
 
             >>> validate = Schema({'one': 'two', 'three': 'four', int: str})
@@ -391,10 +390,17 @@ def _compile_dict(self, schema):
             >>> validate({10: 'twenty'})
             {10: 'twenty'}
 
+        An empty dictionary is matched as value:
+
+            >>> validate = Schema({})
+            >>> with raises(er.MultipleInvalid, 'not a valid value'):
+            ...   validate([])
+
         By default, a "type" in the schema (in this case "int") will be used
         purely to validate that the corresponding value is of that type. It
         will not Coerce the value:
 
+            >>> validate = Schema({'one': 'two', 'three': 'four', int: str})
             >>> with raises(er.MultipleInvalid, "extra keys not allowed @ data['10']"):
             ...   validate({'10': 'twenty'})
 

voluptuous/tests/tests.py

@@ -479,6 +479,40 @@ def test_empty_list_as_exact():
     s([])
 
 
+def test_empty_dict_as_exact():
+    # {} always evaluates as {}
+    s = Schema({})
+    assert_raises(Invalid, s, {'extra': 1})
+    s = Schema({}, extra=ALLOW_EXTRA)  # this should not be used
+    assert_raises(Invalid, s, {'extra': 1})
+
+    # {...} evaluates as Schema({...})
+    s = Schema({'foo': int})
+    assert_raises(Invalid, s, {'foo': 1, 'extra': 1})
+    s = Schema({'foo': int}, extra=ALLOW_EXTRA)
+    s({'foo': 1, 'extra': 1})
+
+    # dict matches {} or {...}
+    s = Schema(dict)
+    s({'extra': 1})
+    s({})
+    s = Schema(dict, extra=PREVENT_EXTRA)
+    s({'extra': 1})
+    s({})
+
+    # nested {} evaluate as {}
+    s = Schema({
+        'inner': {}
+    }, extra=ALLOW_EXTRA)
+    assert_raises(Invalid, s, {'inner': {'extra': 1}})
+    s({})
+    s = Schema({
+        'inner': Schema({}, extra=ALLOW_EXTRA)
+    })
+    assert_raises(Invalid, s, {'inner': {'extra': 1}})
+    s({})
+
+
 def test_schema_decorator_match_with_args():
     @validate(int)
     def fn(arg):