Python Requests：使用Session.get()时为何出现参数数量错误？

问题 #

在为Requests请求实现504错误重试机制时，运行代码报错：get() takes 2 positional arguments but 3 were given，但明明只传了两个参数。未使用Session和mount的旧代码可以正常运行，具体信息如下：

错误栈 #

Exception has occurred: TypeError
get() takes 2 positional arguments but 3 were given
  File "...", line 113, in my_function
    s.get("https://epqs.nationalmap.gov/v1/json", payload)
  File "...", line 158, in <module>
    d = my_function(l)
TypeError: get() takes 2 positional arguments but 3 were given

出错代码 #

payload = {
   "x": coordinate[0],
   "y": coordinate[1],
   "wkid": 4326,
   "units": "Meter",
   "includeDate": "false",
}
s = requests.Session()
retries = Retry(
   total=10,
   backoff_factor=1,
   status_forcelist=[502, 503, 504],
)
s.mount("https://", HTTPAdapter(max_retries=retries))
s.get("https://epqs.nationalmap.gov/v1/json", payload)

可正常运行的旧代码 #

payload = {
   "x": coordinate[0],
   "y": coordinate[1],
   "wkid": 4326,
   "units": "Meter",
   "includeDate": "false",
}
r = requests.get("https://epqs.nationalmap.gov/v1/json", payload)

问题原因与解决方法 #

问题出在参数传递的方式差异：

• 全局的requests.get()函数允许将请求参数作为第二个位置参数传入（对应params参数）
• 但requests.Session.get()方法的第二个位置参数是timeout，而非params。直接传payload时，Session方法会自动绑定自身为第一个参数self，导致payload被当成第三个位置参数，触发参数数量错误。

修正只需将payload显式指定为params关键字参数：

修正后的代码 #

payload = {
   "x": coordinate[0],
   "y": coordinate[1],
   "wkid": 4326,
   "units": "Meter",
   "includeDate": "false",
}
s = requests.Session()
retries = Retry(
   total=10,
   backoff_factor=1,
   status_forcelist=[502, 503, 504],
)
s.mount("https://", HTTPAdapter(max_retries=retries))
# 显式指定params参数
s.get("https://epqs.nationalmap.gov/v1/json", params=payload)

建议日常编写代码时都显式指定params关键字参数，避免因API方法差异引发类似问题。

内容的提问来源于stack exchange，提问作者the_meter413