I would say that both string and list should be accepted in args, and depending on the shell parameter, the module should create a list or a string from the specified list/string.

We already have a list2cmdline function to convert a list to string, we would only need to create the inverse of this function to convert a string to a list.

Executing a program whose parameters are coming from external source (eg. user input) can result security problems if those are not specified as a list and in this case I would be really happy to specify my parameters as a list to Popen and it would do the appropriate conversions as above.