I'm working with Retrofit 2 and I'm trying to request some data from a Web Service which I made with Python.
Actually it throws a "java.lang.IllegalStateException" but when I call the GET method the API returns code 200 and also I can see a printed version for debugging of the response JSON. The problem is the Call method, as it is always executing OnFailure.
Please, help!
This is my (simple) Python Web Service, api-prueba:
    from flask import Flask, jsonify
    import json
    import mysql.connector
    import collections
    app = Flask(__name__)
    cnx = mysql.connector.connect(user="root", password="", host="localhost", database="gym")
    @app.route('/gym/api/v1.0/clases', methods=['GET'])
    def getClases():
        sql = "SELECT id, nombre, descripcion FROM clases"
        cursor = cnx.cursor()
        cursor.execute(sql)
        entradas = cursor.fetchall()
        # Convertir la query en un array de entradas
        objects_list = []
        for row in entradas:
            objects_list.append(row)
            j = json.dumps(objects_list)
        return j
    @app.route('/')
    def index():
        return 'Hello, World!'
    if __name__ == '__main__':
        app.run(debug=True)
This is my ApiClient:
    public class ApiClient {
    private static ApiService REST_CLIENT;
    private static final String API_URL = "http://10.0.2.2:5000/gym/api/v1.0/"; //Change according to your API path.
    static {
        setupRestClient();
    }
    private ApiClient() {
    }
    public static ApiService get() {
        return REST_CLIENT;
    }
    private static void setupRestClient() {
        OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
        //Uncomment these lines below to start logging each request.
            HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
            logging.setLevel(HttpLoggingInterceptor.Level.BODY);
            httpClient.addInterceptor(logging);
            Retrofit retrofit = new Retrofit.Builder()
                    .baseUrl(API_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .client(httpClient.build())
                    .build();
            REST_CLIENT = retrofit.create(ApiService.class);
        }
    }
My API interace, ApiService:
    public interface ApiService {
       @GET("clases")
       Call> getClases();
    }
This is the fragment which queries the API and fetch the data, ClaseFragment:
 
    public class ClaseFragment extends Fragment implements View.OnClickListener {
    private List clases;
    private ListView listView;
    private Button btnRefresh;
    public ClaseFragment() {
        // Required empty public constructor
    }
    public static ClaseFragment newInstance() {
        ClaseFragment fragment = new ClaseFragment();
        Bundle args = new Bundle();
        fragment.setArguments(args);
        return fragment;
    }
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
        }
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View root = inflater.inflate(R.layout.fragment_clase, container, false);
        listView = (ListView) root.findViewById(R.id.claseList);
        btnRefresh = (Button) root.findViewById(R.id.btnRefresh);
        btnRefresh.setOnClickListener(this);
        getListaClases();
        return root;
     }
      @Override
      public void onClick(View v) {
        //do what you want to do when button is clicked
        switch (v.getId()) {
            case R.id.btnRefresh:
                Log.d(getTag(), "hello");
                getListaClases();
                break;
            default:
                break;
        }
    }
     private void getListaClases() {
        final ProgressDialog loading = ProgressDialog.show(getActivity(),
                getContext().getString(R.string.loading_title),
                getContext().getString(R.string.loading_please_wait),
                false,
                false);
        Call> call = ApiClient.get().getClases();
        call.enqueue(new Callback>() {
            @Override
            public void onFailure(Call> call, Throwable t) {
                Log.d("ApiService", "Error Occured: " + t.getMessage());
                loading.dismiss();
            }
            @Override
            public void onResponse(Call> call, Response> response) {
                Log.d("ApiService", "Successfully response fetched");
                loading.dismiss();
                if (response.code() == 200 && response.isSuccessful()) {
                    clases = response.body();
                    showList();
                } else {
                    Log.d("APIPlug", "No item found");
                }
            }
        });
    }
    //Our method to show list
     private void showList() {
        Log.d("ApiService", "Show List");
        ClaseAdapter adapter = new ClaseAdapter(getActivity(), clases);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView parent, View view, int position, long id) {
                Clase clickedObj = (Clase) parent.getItemAtPosition(position);
                //If you have detail activity about your object define in here
            /*
            Intent detail = new Intent(getContext(), ActorDetail.class);
            detail.putExtra("actorObject", clickedObj);
            startActivity(detail);
            */
            }
        });
    }
    }   
This is the actual response from my web service which I want to display into a ListView:
 [
    [1, "halterofilia", "Profesor: XXXX YYYY. Clase de Halterofilia."],
    [2, "crossfit", "Profesor:ZZZZ TTTT. Clase de CrossFit"],
    [3, "halterofilia", "Profesor:XXXX YYYY. Clase de halterofilia"]
 ]
EDIT:
Traceback from Android Studio:
 D/OkHttp: --> GET http://10.0.2.2:5000/gym/api/v1.0/clases http/1.1
 D/OkHttp: --> END GET
 D/OkHttp: <-- 200 OK http://10.0.2.2:5000/gym/api/v1.0/clases (8ms)
 D/OkHttp: Content-Type: text/html; charset=utf-8
 D/OkHttp: Content-Length: 192
 D/OkHttp: Server: Werkzeug/0.11.11 Python/2.7.12
 D/OkHttp: Date: Sat, 08 Oct 2016 23:19:00 GMT
 D/OkHttp: OkHttp-Sent-Millis: 1475968739892
 D/OkHttp: OkHttp-Received-Millis: 1475968739897
 D/OkHttp: [[1, "halterofilia", "Profesor: XXXX YYYY. Clase de   Halterofilia."], [2, "crossfit", "Profesor:ZZZZ TTTT. Clase de CrossFit"], [3,   "halterofilia", "Profesor:XXXX YYYY. Clase de halterofilia"]]
 D/OkHttp: <-- END HTTP (192-byte body)
 D/ApiService: Error Occured: java.lang.IllegalStateException: Expected          BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 3 path $[0]
This is from PyCharm where I'm running the Web Service:
  * Detected change in 'D:\\Proyectos\\PyCharm\\api-prueba\\api-prueba.py', reloading
 * Restarting with stat
 * Debugger is active!
 * Debugger pin code: 195-275-846
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
127.0.0.1 - - [09/Oct/2016 00:21:38] "GET /gym/api/v1.0/clases HTTP/1.1" 200 -
 
    