from subprocess import Popen, PIPE

class foo(object):

def __init__(self):
    self.process = None
    self.command = 'some command'

def _start_process(self, duration):
    self.process = Popen([self.command, '-d'], stdout=PIPE)

def _terminate_process(self):
    self.process.terminate()

import pytest
from _pytest.monkeypatch import MonkeyPatch
from mock import MagicMock, patch
monkeypatch = MonkeyPatch()
import file.foo

def bar():
    test_object = file.foo()
    monkeypatch.setattr('subprocess.Popen', MagicMock())
    test_object._start_process()