Fix empty string casted to datetime today in DateTimeField (#1533)

mongoengine/fields.py

@@ -483,6 +483,10 @@ class DateTimeField(BaseField):
         if not isinstance(value, six.string_types):
             return None
 
+        value = value.strip()
+        if not value:
+            return None
+
         # Attempt to parse a datetime:
         if dateutil:
             try:

tests/fields/fields.py

@@ -35,6 +35,28 @@ __all__ = ("FieldTest", "EmbeddedDocumentListFieldTestCase")
 
 class FieldTest(MongoDBTestCase):
 
+    def test_datetime_from_empty_string(self):
+        """
+        Ensure an exception is raised when trying to
+        cast an empty string to datetime.
+        """
+        class MyDoc(Document):
+            dt = DateTimeField()
+
+        md = MyDoc(dt='')
+        self.assertRaises(ValidationError, md.save)
+
+    def test_datetime_from_whitespace_string(self):
+        """
+        Ensure an exception is raised when trying to
+        cast a whitespace-only string to datetime.
+        """
+        class MyDoc(Document):
+            dt = DateTimeField()
+
+        md = MyDoc(dt='   ')
+        self.assertRaises(ValidationError, md.save)
+
     def test_default_values_nothing_set(self):
         """Ensure that default field values are used when creating
         a document.