How to keep ssh session not expired using paramiko?

Question

I intend to run several commands on remote host using paramiko, but the ssh session closed after running a command.
The code listed below:

from paramiko import SSHClient  
import paramiko  
ssh = SSHClient()
ssh.load_system_host_keys()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect(host, 22, user, passwd, timeout=3)
stdin, stdout, stderr = ssh.exec_command('uname -a')

So is there any way to stop the ssh session from closing? Or any alternatives for paramiko?

Update:
I was able to keeping calling exec_command on my Macbook when connected to a Linux server, but the ssh session closed automatically after exec_command once on a Linux server when connected to a switch and raised an
SSHException: paramiko.ssh_exception.SSHException: SSH session not active

>>> print ssh.get_transport()  
>>> <paramiko.Transport at 0xf00216d0L (unconnected)>  
>>> print ssh.get_transport().is_active()  
>>> False  
>>> print ssh.get_transport().is_authenticated()  
>>> False

Is there any way to keep paramiko ssh session active all the time?

The paramiko debug mode info returned as following:

starting thread (client mode): 0x2657e10L
Connected (version 1.99, client Comware-5.20)
kex algos:[u'diffie-hellman-group-exchange-sha1', u'diffie-hellman- group14-sha1', u'diffie-hellman-group1-sha1'] server key:[u'ssh-rsa'] client encrypt:[u'aes128-cbc', u'3des-cbc', u'des-cbc'] server encrypt:[u'aes128-cbc', u'3des-cbc', u'des-cbc'] client mac:[u'hmac-sha1', u'hmac-sha1-96', u'hmac-md5', u'hmac-md5-96'] server mac:[u'hmac-sha1', u'hmac-sha1-96', u'hmac-md5', u'hmac-md5-96'] client compress:[u'none'] server compress:[u'none'] client lang:[u''] server lang:[u''] kex follows?False
Ciphers agreed: local=aes128-cbc, remote=aes128-cbc
using kex diffie-hellman-group14-sha1; server key type ssh-rsa; cipher: local aes128-cbc, remote aes128-cbc; mac: local hmac-sha1, remote hmac-sha1; compression: local none, remote none
Switch to new keys ...
userauth is OK
Authentication (password) successful!
[chan 0] Max packet in: 32768 bytes
[chan 1] Max packet in: 32768 bytes
[chan 0] Max packet out: 32496 bytes
Secsh channel 0 opened.
Secsh channel 2 open FAILED:
Resource shortage: Resource shortage
[chan 0] Sesch channel 0 request ok
[chan 0] EOF sent (0)

Answer 1

You can implement an interactive shell using paramiko, that way the channel is not closed after a command is executed on the remote shell.

import paramiko
import re


class ShellHandler:

    def __init__(self, host, user, psw):
        self.ssh = paramiko.SSHClient()
        self.ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
        self.ssh.connect(host, username=user, password=psw, port=22)

        channel = self.ssh.invoke_shell()
        self.stdin = channel.makefile('wb')
        self.stdout = channel.makefile('r')

    def __del__(self):
        self.ssh.close()

    @staticmethod
    def _print_exec_out(cmd, out_buf, err_buf, exit_status):
        print('command executed: {}'.format(cmd))
        print('STDOUT:')
        for line in out_buf:
            print(line, end="")
        print('end of STDOUT')
        print('STDERR:')
        for line in err_buf:
            print(line, end="")
        print('end of STDERR')
        print('finished with exit status: {}'.format(exit_status))
        print('------------------------------------')
        pass

    def execute(self, cmd):
        """

        :param cmd: the command to be executed on the remote computer
        :examples:  execute('ls')
                    execute('finger')
                    execute('cd folder_name')
        """
        cmd = cmd.strip('\n')
        self.stdin.write(cmd + '\n')
        finish = 'end of stdOUT buffer. finished with exit status'
        echo_cmd = 'echo {} $?'.format(finish)
        self.stdin.write(echo_cmd + '\n')
        shin = self.stdin
        self.stdin.flush()

        shout = []
        sherr = []
        exit_status = 0
        for line in self.stdout:
            if str(line).startswith(cmd) or str(line).startswith(echo_cmd):
                # up for now filled with shell junk from stdin
                shout = []
            elif str(line).startswith(finish):
                # our finish command ends with the exit status
                exit_status = int(str(line).rsplit(maxsplit=1)[1])
                if exit_status:
                    # stderr is combined with stdout.
                    # thus, swap sherr with shout in a case of failure.
                    sherr = shout
                    shout = []
                break
            else:
                # get rid of 'coloring and formatting' special characters
                shout.append(re.compile(r'(\x9B|\x1B\[)[0-?]*[ -/]*[@-~]').sub('', line).
                             replace('\b', '').replace('\r', ''))

        # first and last lines of shout/sherr contain a prompt
        if shout and echo_cmd in shout[-1]:
            shout.pop()
        if shout and cmd in shout[0]:
            shout.pop(0)
        if sherr and echo_cmd in sherr[-1]:
            sherr.pop()
        if sherr and cmd in sherr[0]:
            sherr.pop(0)

        self._print_exec_out(cmd=cmd, out_buf=shout, err_buf=sherr, exit_status=exit_status)
        return shin, shout, sherr

Answer 2

I see you are using the timeout parameter in your connect call:

ssh.connect(host, 22, user, passwd, timeout=3)

From the documentation:

timeout (float) – an optional timeout (in seconds) for the TCP connect

In one of my scripts I simply do:

ssh = paramiko.SSHClient()
ssh.connect(host, username=settings.user)

which keeps the connection open until I call

ssh.close()