Use Matcher({'a': absent'}) to prohibit a field.

mockupdb/__init__.py

@@ -106,7 +106,7 @@ def reraise(exctype, value, trace=None):
     'Request', 'Command', 'OpQuery', 'OpGetMore', 'OpKillCursors', 'OpInsert',
     'OpUpdate', 'OpDelete', 'OpReply',
 
-    'Matcher',
+    'Matcher', 'absent',
 ]
 
 
@@ -753,6 +753,9 @@ def __repr__(self):
         return rep + ')'
 
 
+absent = object()
+
+
 class Matcher(object):
     """Matches a subset of `.Request` objects.
 
@@ -790,6 +793,13 @@ def matches(self, *args, **kwargs):
         >>> Matcher({'a': 1}).matches({'a': 1, 'b': 1})
         True
 
+        Prohibit a field:
+
+        >>> Matcher({'a': absent}).matches({'a': 1})
+        False
+        >>> Matcher({'a': absent}).matches({'b': 1})
+        True
+
         Order matters if you use an OrderedDict:
 
         >>> doc0 = OrderedDict([('a', 1), ('b', 1)])
@@ -896,7 +906,7 @@ def matches(self, *args, **kwargs):
         for i, doc in enumerate(self._prototype.docs):
             other_doc = request.docs[i]
             for key, value in doc.items():
-                if other_doc.get(key) != value:
+                if other_doc.get(key, absent) != value:
                     return False
             if isinstance(doc, (OrderedDict, bson.SON)):
                 if not isinstance(other_doc, (OrderedDict, bson.SON)):