Document parsing lists in query strings; remove duck typing

CHANGELOG.rst

@@ -1,6 +1,18 @@
 Changelog
 ---------
 
+5.4.0 (unreleased)
+******************
+
+Changes:
+
+* Use explicit type check for `fields.DelimitedList` when deciding to
+  parse value with `getlist()` (`#406 (comment) <https://github.com/marshmallow-code/webargs/issues/406#issuecomment-514446228>`_ ).
+
+Support:
+
+* Add "Parsing Lists in Query Strings" section to docs (:issue:`406`).
+
 5.3.2 (2019-06-19)
 ******************
 

docs/quickstart.rst

@@ -195,6 +195,27 @@ Then decorate that function with :func:`Parser.error_handler <webargs.core.Parse
     def handle_error(error, req, schema, status_code, headers):
         raise CustomError(error.messages)
 
+Parsing Lists in Query Strings
+------------------------------
+
+Use `fields.DelimitedList <webargs.fields.DelimitedList>` to parse comma-separated
+lists in query parameters, e.g. ``/?permissions=read,write``
+
+.. code-block:: python
+
+    from webargs import fields
+
+    args = {"permissions": fields.DelimitedList(fields.Str())}
+
+If you expect repeated query parameters, e.g. ``/?repo=webargs&repo=marshmallow``, use
+`fields.List <marshmallow.fields.List>` instead.
+
+.. code-block:: python
+
+    from webargs import fields
+
+    args = {"repo": fields.List(fields.Str())}
+
 Nesting Fields
 --------------
 

src/webargs/core.py

@@ -65,7 +65,11 @@ class Meta(object):
 
 def is_multiple(field):
     """Return whether or not `field` handles repeated/multi-value arguments."""
-    return isinstance(field, ma.fields.List) and not hasattr(field, "delimiter")
+    from webargs import fields  # prevent circular import
+
+    return isinstance(field, ma.fields.List) and not isinstance(
+        field, fields.DelimitedList
+    )
 
 
 def get_mimetype(content_type):