Always evaluate {} as {}

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

README.md

@@ -360,6 +360,25 @@ 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
+>>> schema({'extra': 1})
+Traceback (most recent call last):
+    ...
+MultipleInvalid: not a valid value
+>>> 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)

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):