自定义 URL Converter

  转换器定义与注册:
      1 | class ListConverter(BaseConverter):
      2 |     regex = r"[\w-]+(?:,[\w-]+)*"
      3 |     def to_python(self, value): return value.split(",")
      4 |     def to_url(self, values): return ",".join(BaseConverter.to_url(self, v) for v in values)
      5 | class UserConverter(BaseConverter):
      6 |     regex = r"\d+"
      7 |     def to_python(self, value):
      8 |         uid = int(value)
      9 |         if uid not in USERS: raise ValidationError()
     10 |         return {"id": uid, "name": USERS[uid]}
     11 |     def to_url(self, user): return str(user["id"])
     12 | app.url_map.converters["list"] = ListConverter
     13 | app.url_map.converters["user"] = UserConverter
  GET /tags/a,b,c    → 200 list:['a', 'b', 'c']
  GET /tags/solo     → 200 list:['solo']
  GET /tags/a,,b     → 404 
  GET /u/1           → 200 dict:{'id': 1, 'name': 'ann'}
  GET /u/2           → 200 dict:{'id': 2, 'name': 'bob'}
  GET /u/3           → 404 
  GET /u/abc         → 404 

  url_for("tags", tags=["x","y","z"]) → /tags/x,y,z
  url_for("user", u={"id": 2, ...}) → /u/2
  url_for("tags", tags=["a b","c"]) → /tags/a%20b,c (to_url 里调用 BaseConverter.to_url 做了 URL 转义)

  使用未注册的 list 转换器 → LookupError: the converter 'list' does not exist

  GET /u/3   → 404;触发的回调:['before_request']
  GET /v/1   → 404;触发的回调:['before_request', 'view']

  自定义 user 转换器 73.5 µs/请求;内置 int 73.8 µs/请求;差 -0.3 µs