diff --git a/mongoengine/base/fields.py b/mongoengine/base/fields.py
index ae86bc8d..347940f5 100644
--- a/mongoengine/base/fields.py
+++ b/mongoengine/base/fields.py
@@ -1,5 +1,4 @@
 import operator
-import warnings
 import weakref
 
 from bson import DBRef, ObjectId, SON
@@ -18,7 +17,7 @@ class BaseField:
     may be added to subclasses of `Document` to define a document's schema.
     """
 
-    name = None
+    name = None  # set in TopLevelDocumentMetaclass
     _geo_index = False
     _auto_gen = False  # Call `generate` to generate a value
     _auto_dereference = True
@@ -265,7 +264,9 @@ class ComplexBaseField(BaseField):
     items in a list / dict rather than one at a time.
     """
 
-    field = None
+    def __init__(self, field=None, **kwargs):
+        self.field = field
+        super().__init__(**kwargs)
 
     def __get__(self, instance, owner):
         """Descriptor to automatically dereference references."""
diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index ce93579b..4ef5dafc 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -556,11 +556,11 @@ class DateTimeField(BaseField):
         if callable(value):
             return value()
 
-        if not isinstance(value, str):
+        if isinstance(value, str):
+            return self._parse_datetime(value)
+        else:
             return None
 
-        return self._parse_datetime(value)
-
     @staticmethod
     def _parse_datetime(value):
         # Attempt to parse a datetime from a string
@@ -923,10 +923,9 @@ class ListField(ComplexBaseField):
     """
 
     def __init__(self, field=None, max_length=None, **kwargs):
-        self.field = field
         self.max_length = max_length
         kwargs.setdefault("default", lambda: [])
-        super().__init__(**kwargs)
+        super().__init__(field=field, **kwargs)
 
     def __get__(self, instance, owner):
         if instance is None:
@@ -1008,14 +1007,9 @@ class SortedListField(ListField):
         to perform a push operation.
     """
 
-    _ordering = None
-    _order_reverse = False
-
     def __init__(self, field, **kwargs):
-        if "ordering" in kwargs.keys():
-            self._ordering = kwargs.pop("ordering")
-        if "reverse" in kwargs.keys():
-            self._order_reverse = kwargs.pop("reverse")
+        self._ordering = kwargs.pop("ordering", None)
+        self._order_reverse = kwargs.pop("reverse", False)
         super().__init__(field, **kwargs)
 
     def to_mongo(self, value, use_db_field=True, fields=None):
@@ -1065,11 +1059,10 @@ class DictField(ComplexBaseField):
     """
 
     def __init__(self, field=None, *args, **kwargs):
-        self.field = field
         self._auto_dereference = False
 
         kwargs.setdefault("default", lambda: {})
-        super().__init__(*args, **kwargs)
+        super().__init__(*args, field=field, **kwargs)
 
     def validate(self, value):
         """Make sure that a list of valid fields is being used."""