Specify a one or many field using marshmallow - python-3.x

Question

I would like to specify a schema field wichi accepts one or many resources. However I seem only able to specify one behavior or the other.
>>> class Resource(marshmallow.Schema):
... data = marshmallow.fields.Dict()
...
>>> class ContainerSchema(marshmallow.Schema):
... resource = marshmallow.fields.Nested(ResourceSchema, many=True)
...
>>> ContainerSchema().dump({'resource': [{'data': 'DATA'}]})
MarshalResult(data={'resource': [{'data': 'DATA'}]}, errors={})
In the above example a list must be defined. However I would prefer not to:
>>> ContainerSchema().dump({'resource': {'data': 'DATA'}})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/lib64/python3.6/site-packages/marshmallow/schema.py", line 513, in dump
**kwargs
File "/lib64/python3.6/site-packages/marshmallow/marshalling.py", line 147, in serialize
index=(index if index_errors else None)
File "/lib64/python3.6/site-packages/marshmallow/marshalling.py", line 68, in call_and_store
value = getter_func(data)
File "/lib64/python3.6/site-packages/marshmallow/marshalling.py", line 141, in <lambda>
getter = lambda d: field_obj.serialize(attr_name, d, accessor=accessor)
File "/lib64/python3.6/site-packages/marshmallow/fields.py", line 252, in serialize
return self._serialize(value, attr, obj)
File "/lib64/python3.6/site-packages/marshmallow/fields.py", line 448, in _serialize
schema._update_fields(obj=nested_obj, many=self.many)
File "/lib64/python3.6/site-packages/marshmallow/schema.py", line 760, in _update_fields
ret = self.__filter_fields(field_names, obj, many=many)
File "/lib64/python3.6/site-packages/marshmallow/schema.py", line 810, in __filter_fields
obj_prototype = obj[0]
KeyError: 0
Can I have a schema allowing both a single item or many of it?

Answer 1

The point with giving the arguments as a list - whether it's one or many - is so the schema knows how to handle it in either case. For the schema to process arguments of a different format, like not in a list, you need to add a preprocessor to the schema, like this:
class ContainerSchema(marshmallow.Schema):
resource = marshmallow.fields.Nested(ResourceSchema, many=True)
#pre_dump
def wrap_indata(self, indata):
if type(indata['resource']) is dict:
indata['resource'] = [indata['resource']]