Mocking python class in unit test and verifying an instance

I'm trying to unit test an SFTP helper class that makes some calls to the pysftp module. I want to mock the actual network calls from pysftp so there are no side effects, and just make sure that the class is correctly calling the underlying SFTP methods with the correct paramaters.

Here is a simple example of my code so far:

# sftp_helper.py
import pysftp
class SFTPHelper(object):
    def __init__(self, host, username, password, files_dir):
        self.host = host
        self.username = username
        self.password = password
        self.files_dir = files_dir

    def list_files(self):
        with pysftp.Connection(
                self.host,
                username=self.username,
                password=self.password) as sftp:
            return sftp.listdir(self.files_dir)

# test_sftp_helper.py
import unittest
import mock
import sftp_helper
class TestSFTPHelper(unittest.TestCase)
    @mock.patch(sftp_helper.pysftp.Connection)
    def test_list_files(self, mock_connection)
        sftp_helper = SFTPHelper('somehost', 'someuser', 'somepassword', '/some/files/dir')
        sftp_helper.list_files()

        # this assertion passes
        mock_connection.assert_called_with(
            'somehost', password='somepassword', username='someuser')

        # this assertion does not pass
        mock_connection.listdir.assert_called_with('/some/files/dir')

The assertion error:

AssertionError: Expected call: listdir('/some/files/dir')
Not called

I assume it doesn't work because I need to assert that the function was called on the instance, but how do I get the instances of pysftp.Connection that was used in my method?