import threading
import os
from subprocess import Popen, PIPE, STDOUT

cmd_to_call=['/usr/bin/grep', '-q', 'localhost', '/etc/hosts']
# variant for nonempty stdout:
# cmd_to_call=['/usr/bin/grep', 'localhost', '/etc/hosts']

class AsyncReader(threading.Thread):

    def __init__(self, channel):
        super(AsyncReader, self).__init__()
        self.chan = channel
        self.output = ''
        self.start()
        self.join()

    def run(self):
        try:
            while True:
                line = self.chan.read(2048)
                if not line:
                    # Pipe can remain open after output has completed
                    print("EOF received, to break")
                    break
                self.output = self.output + line
        except (ValueError, IOError):
            # pipe has closed, meaning command output is done
            print("pipe closed")
            pass

    def get_contents(self):
        return self.output

p = Popen(cmd_to_call, shell=False, stdout=PIPE,
          stderr=PIPE, bufsize=-1,# env=os.environ.copy(),
          close_fds=True)

reader = AsyncReader(p.stdout)
stdout = reader.get_contents()
pollstatus = p.poll()
print("output: '%s', return code: %s, pollstatus=%s" % (stdout, p.returncode, pollstatus))