Why does passing variables to subprocess.Popen not work despite passing a list of arguments?

Question

I have a script which calls another Python script by subprocess.Popen. But since I have arguments stored in variable(s)

servers[server]['address']
servers[server]['port']
servers[server]['pass']

I am unable to perform the command

p = subprocess.Popen(
    ["python mytool.py -a ", servers[server]['address'], "-x", servers[server]['port'], "-p", servers[server]['pass'], "some additional command"],
    shell=True,
    stdout=subprocess.PIPE
)

Answer 1

Drop shell=True. The arguments to Popen() are treated differently on Unix if shell=True:

import sys
from subprocess import Popen, PIPE

# populate list of arguments
args = ["mytool.py"]
for opt, optname in zip("-a -x -p".split(), "address port pass".split()):
    args.extend([opt, str(servers[server][optname])])
args.extend("some additional command".split())

# run script
p = Popen([sys.executable or 'python'] + args, stdout=PIPE)
# use p.stdout here...
p.stdout.close()
p.wait()

Note that passing shell=True for commands with external input is a security hazard, as described by a warning in the docs.