Testing a while loop in python

Question

What is the best method for testing a method that looks like this

class Foo(object):

   is_running = False
   def run(self):
       self.is_running = True
       while self.is_running:
           do_some_work()

This is pretty standard code for a consumer, do work while the is_running flag is set.

But this is difficult to test because it's going to enter the loop and never come out unless I create a second thread to change is_running to false.

Are there any good strategies for testing this without spinning up a separate thread to run the code?

I haven't seen anything but I am thinking maybe the mock library would provide functionality that can return a [True, True, False] each time is_running is read but would that require me to change is_running from a member variable to a property or a method?

Why not replace it with a full descriptor instead of just a property? — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Oct 7, 2014 at 19:09
why not use coroutines: docs.python.org/3/library/asyncio-task.html — Stefano M
– Stefano M, Commented Oct 7, 2014 at 19:09
Try threading.Timer - it spins up the thread for you. This object looks like its supposed to run in a separate thread anyway, so unit testing it that way seems normal to me. — tdelaney
– tdelaney, Commented Oct 7, 2014 at 19:13
I'm with @tdelaney - if the class is designed to be used in conjunction with another thread, why not test it that way? Creating a thread to use for this sort of test would only take a few lines of code, so its not like it adds a ton of overhead. — dano
– dano, Commented Oct 7, 2014 at 19:15

dano · Accepted Answer · 2014-10-07 19:26:56Z

As I mentioned in the comments, I think testing this method using threads is a perfectly viable approach, and probably the best solution. However, if you really want to avoid threads, you could convert is_running to a property and then use mock.PropertyMock to mock the property:

import mock
import time

class Foo(object):

   def __init__(self):
       self._is_running = False

   @property
   def is_running(self):
       return self._is_running

   @is_running.setter
   def is_running(self, val):
       self._is_running = val 

   def run(self):
       self._is_running = True  # Don't go through the property here.
       while self.is_running:
           print("in here")
           time.sleep(.5)


with mock.patch('__main__.Foo.is_running', new_callable=mock.PropertyMock,
                side_effect=[True, True, False]) as m:
    f = Foo()
    f.run()

Output:

in here
in here
<done>

I would say that changing your production implementation this much just to enable a specific method of testing is not worth it, though. Just have your test function create a thread to set is_running after some period of time.

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