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An interesting read, but I am lost in what the goal is.
e.g., on AIX, which I know well, the system default is /bin/ksh (aka /usr/bin/ksh). However, /bin/sh (/usr/bin/sh) is available as well.
My expectation is that on Linux the default shell is /bin/bash, and like AIX /bin/sh is also available.
/bin/sh, /bin/ksh, /bin/bash are all (potentially) default shells. However, something every posix system is "sh" aka borne shell.
Aside: windows is different, and I expect has a different syntax. 
So, if the goal is to GET the pathname of the so-called "default" shell -again, interesting - BUT - what does that buy me?
For Popen or now subprocess I would want a consistent shell syntax, i.e., borne shell on posix, cmd.exe on windows, and whatever is correct on platforms that do not fit in these two.
Hence, on posix platforms where /bin/sh does not exist (hard)code the correct path for that platform.
FYI: CS_PATH does not exist on AIX
IMHO: (as others have stated) having . in PATH is bad for security, being dependent on PATH opens other security concerns as well.
IMHO2: KISS principle. After several years there is still no consenus, so make it simple - the popen and subprocess shell is one of /bin/sh, cmd.exe, or some other "hard-coded" shell.
(Although I thought that subprocess called "program" directly. Python is still new, so if not applicable for subprocess - please ignore
rereading, direct call would be when shell=False I expect.
Anyway, my two cents.