Python subprocess failed when called from service – CentOS

Below simple scripts, used to list keys in ssh-agent:

list_keys.sh:

#!/bin/bash
ssh-add -l

list_keys.py:

if __name__ == '__main__':
    """
    Creates log
    """
    ...

    print("Start")
    print subprocess.check_output(["/root/list_keys.sh"])

It works well when called directly from the terminal.

$python list_keys.py

The log shows as expected:

Start
2048 SHA256:+gkk***************************nQ .ssh/my_key (RSA)

But when I tried to make it as service, it fails.

Start
Traceback (most recent call last):
  File "/root/list_keys.py", line 43, in <module>
print subprocess.check_output(["/root/list_keys.sh"])
  File "/usr/lib64/python2.7/subprocess.py", line 575, in check_output
raise CalledProcessError(retcode, cmd, output=output)
subprocess
.
CalledProcessError
:
Command '['/root/list_keys.sh']' returned non-zero exit status 2

The service config:

list_keys.service:

[Unit]
Description=List Keys Service
After=multi-user.target
[email protected]

[Service]
Type=simple
ExecStart=/usr/bin/python /root/list_keys.py
StandardInput=tty-force

[Install]
WantedBy=multi-user.target

I’m working on Centos 7. Any clue how to make the list_keys.py as service?