Closes #46 by instantiating a new default instance for every field by request.

mongoengine/base.py

@@ -52,7 +52,10 @@ class BaseField(object):
         # Get value from document instance if available, if not use default
         value = instance._data.get(self.name)
         if value is None:
-            value = self.default
+            if callable(self.default): # fixes #46
+                value = self.default()
+            else:
+                value = self.default
             # Allow callable default values
             if callable(value):
                 value = value()

mongoengine/fields.py

@@ -263,7 +263,7 @@ class ListField(BaseField):
             raise ValidationError('Argument to ListField constructor must be '
                                   'a valid field')
         self.field = field
-        kwargs.setdefault('default', [])
+        kwargs.setdefault('default', lambda: [])
         super(ListField, self).__init__(**kwargs)
 
     def __get__(self, instance, owner):
@@ -356,7 +356,7 @@ class DictField(BaseField):
     def __init__(self, basecls=None, *args, **kwargs):
         self.basecls = basecls or BaseField
         assert issubclass(self.basecls, BaseField)
-        kwargs.setdefault('default', {})
+        kwargs.setdefault('default', lambda: {})
         super(DictField, self).__init__(*args, **kwargs)
 
     def validate(self, value):