Can't json encode some validation errors, "unorderable types: str() < int()"

I’m using marshmallow with flask, which has a jsonify() function that sorts keys, and i’m using that to return the validation errors to the client. In some situations, the dicts with validation errors returned by marshmallow combine int and str keys, which results in a TypeError while attempting to serialize them.

Here’s a somewhat minimized test case:

import json
import marshmallow as ma

class RoleSchema(ma.Schema):
    name = ma.fields.Str()

class UserSchema(ma.Schema):
    roles = ma.fields.Nested(RoleSchema, many=True)

user, errors = UserSchema().load({'roles':['name']})
print(errors)
print(json.dumps(errors))
print(json.dumps(errors, sort_keys=True))

Output:

{'roles': {0: {}, '_schema': ['Invalid input type.']}}
{"roles": {"0": {}, "_schema": ["Invalid input type."]}}
Traceback (most recent call last):
  File "asd.py", line 13, in <module>
    print(json.dumps(errors, sort_keys=True))
  File "/usr/lib64/python3.5/json/__init__.py", line 237, in dumps
    **kw).encode(obj)
  File "/usr/lib64/python3.5/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib64/python3.5/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
TypeError: unorderable types: str() < int()

I think the relevant function is ErrorStore.get_errors(index=0)

https://github.com/marshmallow-code/marshmallow/blob/2.9.0/marshmallow/marshalling.py#L46

Stringifying the indexes in that function fixes it for me. Not sure if this is the best way to fix it though.

--- a/marshalling.py
+++ b/marshalling.py
@@ -45,6 +45,7 @@

     def get_errors(self, index=None):
         if index is not None:
+            index = str(index)
             errors = self.errors.get(index, {})
             self.errors[index] = errors
         else:

[deckar01]

This issue can probably be removed from the v3 milestone.

[deckar01]

The output of List(Nested(Schema)) for this use case is interesting:

{'roles': {0: {'_schema': ['Invalid input type.']}}}

That is probably how Nested(many=True) should be formatted. Roles shouldn’t have a schema error, because it is correctly supplied as a list. This isn’t the first time we have ran into inconsistencies with many=True where List(Nested) did the right thing. https://github.com/marshmallow-code/marshmallow/issues/315#issuecomment-228788849