1. 起因

   临时有一个赛事需求，需要紧急上线几组接口，用于支持赛事的进行。这里主要涉及的两个接口分别是：

   1. 获取当前实验下的所有资料信息
   2. 获取当前实验下的公告信息

2. 问题表现

   1. 接口1返回数据正常
   2. 接口2返回了其他奇怪的公告信息，或者不显示

3. 问题排查

   这个问题其实很奇怪，接口单独测试都正常，但是一放到一起就出问题了。直到看到小伙伴写的接口后，我才发现，原来是默认参数的问题。

   在说之前，我先放一段代码和这段代码的运行结果。

   import typing as t
   
   from flask import Flask, request
   from flask.views import MethodView as _View
   
   app = Flask(__name__)
   
   
   class View(_View):
   
       def __init__(
           self,
           funs: t.Dict[str, t.Callable[[t.Dict[str, t.Any]], None]] = dict(),
       ):
           super().__init__()
           self.funs = funs
   
       def out(
           self, msg: t.Dict[str, t.Any], cook_fun: t.Callable[[t.Dict[str, t.Any]], None]
       ):
           if cook_fun:
               cook_fun(msg)
           return msg
   
       def get(self):
           msg = {
               "msg": "Hello from py-default-args-bug!",
               "status": "success",
               "data": {},
           }
   
           cook_fun = None
           if "get" in self.funs:
               cook_fun = self.funs["get"]
   
           return self.out(msg, cook_fun)
   
   
   class AView(View):
   
       def __init__(self):
           super().__init__(funs={"get": self.cook_get})
   
       def cook_get(self, msg: t.Dict[str, t.Any]):
           msg["_cook"] = "AView"
   
   
   class BView(View):
   
       def __init__(self):
           super().__init__()
   
       def cook_get_1(self, msg: t.Dict[str, t.Any]):
           msg["_cook"] = "BView_1"
   
       def cook_get_2(self, msg: t.Dict[str, t.Any]):
           msg["_cook"] = "BView_2"
   
       def get(self):
           if request.args.get("Q") == "1":
               self.funs["get"] = self.cook_get_1
           else:
               self.funs["get"] = self.cook_get_2
           return super().get()
   
   
   class CView(View):
   
       def __init__(self):
           super().__init__()
   
       def get(self):
           print(self.funs)
           return super().get()
   
   
   app.add_url_rule("/a", view_func=AView.as_view("a"))
   app.add_url_rule("/b", view_func=BView.as_view("b"))
   app.add_url_rule("/c", view_func=CView.as_view("c"))
   
   if __name__ == "__main__":
       app.run(debug=True)

   运行结果：

   (py-default-args-bug) C:\LearnNote\BugDocs\src\py_default_args_bug>curl http://127.0.0.1:5000/c
   {
   "data": {},
   "msg": "Hello from py-default-args-bug!",
   "status": "success"
   }
   
   (py-default-args-bug) C:\LearnNote\BugDocs\src\py_default_args_bug>curl http://127.0.0.1:5000/a
   {
   "_cook": "AView",
   "data": {},
   "msg": "Hello from py-default-args-bug!",
   "status": "success"
   }
   
   (py-default-args-bug) C:\LearnNote\BugDocs\src\py_default_args_bug>curl http://127.0.0.1:5000/b
   {
   "_cook": "BView_2",
   "data": {},
   "msg": "Hello from py-default-args-bug!",
   "status": "success"
   }
   
   (py-default-args-bug) C:\LearnNote\BugDocs\src\py_default_args_bug>curl http://127.0.0.1:5000/c
   {
   "_cook": "BView_2",
   "data": {},
   "msg": "Hello from py-default-args-bug!",
   "status": "success"
   }
   
   (py-default-args-bug) C:\LearnNote\BugDocs\src\py_default_args_bug>curl http://127.0.0.1:5000/b?Q=1
   {
   "_cook": "BView_1",
   "data": {},
   "msg": "Hello from py-default-args-bug!",
   "status": "success"
   }
   
   (py-default-args-bug) C:\LearnNote\BugDocs\src\py_default_args_bug>curl http://127.0.0.1:5000/c    
   {
   "_cook": "BView_1",
   "data": {},
   "msg": "Hello from py-default-args-bug!",
   "status": "success"
   }

   居然在方法中使用了可变的默认参数，导致了这个问题。到这里就有小伙伴问了，为什么之前没有这种情况。

   其实很简单，因为之前都是AView那种写法，没有使用可变的默认参数，所以没有出现这个问题。

   对这个不理解的小伙伴，可以参考一下流畅的Python一书中提及的幽灵公交案例。

4. 解决过程

   禁止在函数、方法中使用这种默认参数，直接修改源码View，如下

   class View(_View):
   
       def __init__(
           self,
           funs: t.Optional[t.Dict[str, t.Callable[[t.Dict[str, t.Any]], None]]] = None,
       ):
           super().__init__()
           if funs is None:
               funs = dict()
           self.funs = funs