Partly broken example trying to test Process as a single thread.

test_process.py

# mock_process.py
"""Make a process single threaded for testing."""

import multiprocessing

def f(x):
    return x

def target():
    """Run a process by specifying a target function."""
    p = multiprocessing.Process(target=f, args=(1,))
    p.start()
    p.join()

class Worker(multiprocessing.Process):
    def __init__(self, x):
        super(Worker, self).__init__()
        self.x = x
    def run(self):
        return self.x

def subclass():
    """Run a process by subclassing Process."""
    p = Worker(1)
    p.start()
    p.join()

import pytest
import mock

@mock.patch('multiprocessing.Process')
def test_target(MockProcess):
    target()
    kwargs = MockProcess.call_args.kwargs
    assert kwargs['target'](*kwargs['args']) == 1
    mp = MockProcess.return_value
    assert mp.start.called
    assert mp.join.called

class DummyWorker(Worker):
    def __init__(self, x):
        super(DummyWorker, self).__init__()
    def start():
        self.ret = self.run()
    def join():
        pass

def mock_start(self):
    return self.run()

@mock.patch.object(Worker, 'join')
@mock.patch.object(Worker, 'start')
def test_subclass(mock_start, mock_join):
    subclass()
    import pdb; pdb.set_trace()
    assert 1 == subclass()

pytest.main()