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I'm with Chris. The information should be stored in the os module. I suggest os.shell var or os.get_shell() function like this:
def get_shell():
 for path in confstr("CS_PATH").split(pathsep):
 sh = sep.join((path, "sh"))
 try:
 mode = stat(sh).st_mode
 except OSError:
 pass
 else:
 if st.S_ISREG(mode):
 return sh
 raise FileNotFound("sh")
According to all examples S_ISREG() is sufficient here.
On Windows the function should use the env var COMSPEC instead of hard coding "cmd.exe".