因为目前使用的是模拟账户连接api，因此受到限制，api的account_token只能用于获取价格，不能用于下单，所以我希望通过模拟登录网页版交易平台，获取可用于下单的session_token，但是发现有两个url，一个是表面上的url，另一个是提交用户名和密码的url，我不知道应该用那个url，另外无论我用哪一个url，都会报错ValueError: Expecting value: line 1 column 1 (char 0)，这好像是和utf-8有关，请大家帮忙，谢谢
程序：

import requests
import json

url = "https://trade.oanda.com/" 
          #另一个url:"https://fxgame-webapi.oanda.com/v1/user/login.json"
username = 'cawa11'
password = '1122334455'
params = {'username':username,'password':password}
headers = {'Connection': 'Keep-Alive',
           'Accept-Encoding': 'gzip,deflate',
           'charset':'utf-8',
           'User-Agent':'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}

r = requests.get(url,headers = headers, params=params)
print(r.json())

报错：

Traceback (most recent call last):
  File "C:\Users\lenovo\Desktop\oo.py", line 15, in <module>
    print(r.json())
  File "C:\Python34\lib\site-packages\requests-2.9.1-py3.4.egg\requests\models.py", line 808, in json
    return complexjson.loads(self.text, **kwargs)
  File "C:\Python34\lib\json\__init__.py", line 318, in loads
    return _default_decoder.decode(s)
  File "C:\Python34\lib\json\decoder.py", line 343, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "C:\Python34\lib\json\decoder.py", line 361, in raw_decode
    raise ValueError(errmsg("Expecting value", s, err.value)) from None
ValueError: Expecting value: line 1 column 1 (char 0)