After I migrated my site to Python 3.5 from 3.4 many users of my site have not been able to login. After small research I found out that users cookies there was a key without value and new Python get empty SimpleCookie result. 
Example for Python 3.5.2:

>>> import http.cookies as Cookie
>>> _cookies = Cookie.SimpleCookie()
>>> _cookies.load("rgoods_1=2; __test;")
>>> _cookies
<SimpleCookie: >

Example for Python 3.4.5:

>>> import http.cookies as Cookie
>>> _cookies = Cookie.SimpleCookie()
>>> _cookies.load("rgoods_1=2; __test;")
>>> _cookies
<SimpleCookie: rgoods_1='2'>

I don't know many about right cookie format but drop the wrong key for me is the best solution. I have seen many discussions about Cookie parsers but I was not see words about single keys. If it will be dublicate I am sorry.