In the documentation of subprocess.Popen the docs say:

"args should be a string, or a sequence of program arguments. The
program to execute is normally the first item in the args sequence or
string, but can be explicitly set by using the executable argument. "

The statement that the program to exectute is the "first item in the
args sequence or string" indicates that the first item in a string like
"ls -l", i.e. the "ls" will be the program. The next paragraph however
makes it clear that this is not the case, but by then the damage is
done, and you think that passing in "ls -l" will work.

Rewording this, perhaps by saying "The program to execute is normally
the first item in the args sequence, or the string if a string a given",
would make this clear.