>>> import shlex; shlex.quote('key={}'.format(shlex.quote('space separated list'))) '\'key=\'"\'"\'space separated list\'"\'"\'\''
is the issue.
>>> import shlex; shlex.quote( 'key={} '.format( shlex.quote( 'space separated list'))) \'"\'"\ 'space separated list\'"\'"\'\''
'\'key=
is the issue.