How to do open_channel in python paramiko?

How to convert this ruby piece to python?

Net::SSH.start(machine, "ubuntu", {:keys => [ssh_key]}) do |ssh|
  ssh.open_channel do |ch|
    sent_cmd = false
    ch.request_pty do |ch,success|
      ch.exec( "sudo su -" ) do |ch, success|
        raise "fail" if( !success )
      end
    end
  end
end

I have done this so far:

ssh_key1 = paramiko.RSAKey.from_private_key_file( ssh_key, 'r' )


session = paramiko.SSHClient()
session.set_missing_host_key_policy( paramiko.AutoAddPolicy() )

session.connect( machine, port = 22, username = 'ABC', pkey = ssh_key1 )

How to do open.channel in python?