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This is a peculiar regression in 3.5 where a comma-separated cookie following an expires field is considered invalid:
Ex: which results in a silent error/empty cookie in 3.5.1
Python 3.5.1 (default, Dec 26 2015, 18:11:22) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from http.cookies import SimpleCookie
>>> SimpleCookie('A=B; expires=Thu, 01-Jan-1970 00:00:00 GMT, C=D')
<SimpleCookie: >
compared to 3.4.2 and 2.7.9
Python 3.4.2 (default, Jan 29 2015, 06:45:30) 
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from http.cookies import SimpleCookie
>>> SimpleCookie('A=B; expires=Thu, 01-Jan-1970 00:00:00 GMT, C=D')
<SimpleCookie: A='B' C='D'>
Python 2.7.9 (default, Jan 29 2015, 06:28:46) 
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from Cookie import SimpleCookie
>>> SimpleCookie('A=B; expires=Thu, 01-Jan-1970 00:00:00 GMT, C=D')
<SimpleCookie: A='B' C='D'>
This seems to specifically happen when the expires field is the last field in the cookie.
eg, this works as expected:
>>> SimpleCookie('A=B; expires=Thu, 01-Jan-1970 00:00:00 GMT; Path=/, C=D')
<SimpleCookie: A='B' C='D'>
I'm not sure if this is related to the other comma-related cookie bugs, such as issue26302 as this does not appear to use the same regex at first glance. I have not had a chance to track it down it further.

This is not a regression and you can see that if you do
print(SimpleCookie('A=B; expires=Thu, 01-Jan-1970 00:00:00 GMT, C=D'))
The values of expires and Path attribute were parsed incorrectly, even though you got two cookies. The problem as far as I can see is a non-standard separator between attributes that you're trying to use, i.e. a comma. If you were to use a semi-colon (or a space) there would not be a problem in any of the versions.