UnboundLocalError: local variable 'field' referenced before assignment

[yudongya]

import apistar
import yaml
import json
from apistar import Link
from typesystem import to_json_schema

def get_link(link: Link):
_data = {}
for field in link.fields:

    _data[field.name] = to_json_schema(field.schema)

return _data
#_data[link.fields[1].name] = to_json_schema(link.fields[1].schema)

str= open('API/api.json', encoding="utf-8")
openapi = json.load(str)
client = apistar.Client(openapi)
link = client.lookup_operation("uploadFile")
print(get_link(link))

run result