Mock a Remote Host in Python

Question

I am writing some functions, using paramiko, to execute commands and create files on a remote host. I would like to write some unit tests for them, but I don't know what would be the simplest way to achieve this? This is what I envisage as being an example outline of my code:

import os
import paramiko
import pytest

def my_function(hostname, relpath='.', **kwargs):
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    ssh.connect(hostname, **kwargs)
    sftp = ssh.open_sftp()
    sftp.chdir(relpath)
    stdin, stdout, stderr = ssh.exec_command("echo hallo > test.txt")

@pytest.fixture("module")
def mock_remote_host():
    # start a remote host here with a local test path
    try:
        yield hostname, testpath, {"username":"bob", "password":"1234"}
    finally:
        # delete the test path
        # close the remote host

def test_my_function(mock_remote_host):
    hostname, dirpath, kwargs = mock_remote_host
    my_function(hostname, **kwargs)
    filepath = os.path.join(dirpath, 'test.txt')
    assert os.path.exists(filepath)

I have had a look at the paramiko test modules, but they seem quite complex for my use case and I'm not sure how to go about simplifying them.

Answer 1

I think what you really need to mock is paramiko.SSHClientobject. You are unittesting your function my_function, you can assume paramiko module works correctly and the only thing you need to unit test is if my_function calls methods of this paramiko.SSHClient in correct way.

To mock paramiko.SSH module you can use unittest.mock and decorate your test_my_function function with @mock.patch.object(paramiko.SSHClient, sshclientmock). You have to define sshclientmock as some kind of Mock or MagicMock first.

Also in python 2.7 there is some equivalent of unittest.mock but I dont remember where to find it exactly.

EDIT: As @chepner mentioned in comment. For python 2.7 you can find mock module in pypi and install it using pip install mock