Change Tor exit node only by script

I have a python script which uses Tor as SOCKS proxy.

I would like to change Tor's exit node only by script, I mean Tor shouldn't change exit node by itself.

How I change exit node when needed by the script:

    from stem import Signal
    from stem.control import Controller

    with Controller.from_port(port = 9051) as controller:
        controller.authenticate()
        controller.signal(Signal.NEWNYM)

How I periodically check which ip has my exit node:

    import re
    import requests

    response = requests.get('https://check.torproject.org/')
    if response.status_code == 200:
        match = re.search(r'([0-9]{1,3}.){3}[0-9]{1,3}', response.text)
        if match:
            return match.group(0)

My torrc file:

    SOCKSPort 9050
    SocksBindAddress 127.0.0.1
    MaxCircuitDirtiness 86400 # == a day
    DataDirectory $a_dir_in_my_home
    ControlPort 9051
    HashedControlPassword $my_hased_password

How I run Tor:

    tor -f $my_torrc_file

But unfortunately by the logs of the script I can see that sometimes (more than 10 in an hour) the exit node changes unexpectedly.

What am I doing wrong?