I'm not familiar with Python, but you need to pass the command text after the -Command switch.
import subprocess
powershell_command = 'choco update haskell-stack'
powershell_process = subprocess.Popen(["powerShell", "-NoExit", "-Command", powershell_command], creationflags=subprocess.CREATE_NEW_CONSOLE)
You might also need to put brackets {} around the command text.
powershell_command = '{choco update haskell-stack}'