Python subprocess.Popen.communicate() does not wait for the process to complete

Question

On Ubuntu 22, using Python 3.10, I need to set up a VPN connection and analyze the response from the VPN server. I do this as follows:

connect_command = ['ip', 'netns', 'exec', 'vpn_test_namespace', 'ipsec', 'up', 'vpn_test_config']
connect_process = subprocess.Popen(connect_command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
connect_output, connect_error = connect_process.communicate()

However, the process does not wait for completion, and communicate() executes instantly. As a result, connect_output and connect_error are empty. If I add a pause:

connect_command = ['ip', 'netns', 'exec', 'vpn_test_namespace', 'ipsec', 'up', 'vpn_test_config']
connect_process = subprocess.Popen(connect_command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
time.sleep(3)
connect_output, connect_error = connect_process.communicate()

Or wait using a breakpoint, then everything is okay, and I see the result. What am I doing wrong?

I tried various methods recommended on StackOverflow or in Google, such as call(), check_call(), wait(), run(), but I was unable to achieve the desired result of making the script wait for the process to complete.