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On 1/16/14 5:40 AM, James Chapman wrote:<br>
<blockquote
cite="mid:CAHvkzy=ghCOm-EV158ct0DhGZtprKy4h-93uMBhdt59ZF5yPAQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div class="gmail_quote">Originally sent to python tutor mailing
list but I suspect this might be a better list for my
question...<br>
<br>
<div dir="ltr">
<div>Hi all<br>
</div>
<div dir="ltr">
<div><br>
</div>
<div>
I have a question regarding mocking in unit testing.</div>
<div><br>
</div>
<div>Let's assume I have the following class:</div>
<div><br>
</div>
<div><font face="courier new, monospace">-------------------------------------------</font></div>
<div><font face="courier new, monospace">import subprocess<br>
</font></div>
<div><font face="courier new, monospace"><br>
</font></div>
<div>
<div><font face="courier new, monospace">class
Pinger(object):</font></div>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace"> def
ping_host(self, host_to_ping):</font></div>
<div><font face="courier new, monospace">
cmd_string = 'ping %s' % (host_to_ping)</font></div>
<div><font face="courier new, monospace">
cmd_args = cmd_string.split()</font></div>
<div><font face="courier new, monospace"> proc =
subprocess.Popen(cmd_args, shell=True)</font></div>
<div><font face="courier new, monospace">
proc.wait()</font></div>
<div><font face="courier new, monospace"> if
proc.returncode != 1:</font></div>
<div><font face="courier new, monospace">
raise Exception('Error code was: %d' %
(proc.returncode))</font></div>
</div>
<div>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace">-------------------------------------------</font></div>
<div><br>
</div>
<div><br>
</div>
<div>In my unittest I don't want to run the ping
command, (It might not be available on the build
system) I merely want to check that a call to
subprocess.Popen is made and that the parameters are
what I expect?</div>
<div><br>
</div>
<div>So far I have this, but it doesn't work and I
suspect it's way off!!</div>
<div><br>
</div>
<div><br>
</div>
<div><font face="courier new, monospace">-------------------------------------------<br>
</font></div>
<div>
<div><font face="courier new, monospace">import mock</font></div>
<div><font face="courier new, monospace">import
unittest</font></div>
<div><font face="courier new, monospace">from tutor_q
import Pinger</font></div>
<div>
<font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace">class
Test_Pinger(unittest.TestCase):</font></div>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace"> def
test_ping_host(self):</font></div>
<div><font face="courier new, monospace">
pinger = Pinger()</font></div>
<div><font face="courier new, monospace">
assert pinger</font></div>
<div><font face="courier new, monospace">
subprocess = mock.Mock()</font></div>
<div><font face="courier new, monospace">
subprocess.Popen.return_value = 0</font></div>
<div><font face="courier new, monospace">
subprocess.assert_called_once_with(['ping','localhost'])</font></div>
<div><font face="courier new, monospace">
pinger.ping_host('127.0.0.1')</font></div>
<div><font face="courier new, monospace"><br>
</font></div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
Often the trickiest thing about mocking is mocking the right name.
Here you've created a mock, and it's assigned to a local variable
named subprocess. This name is not related to the global subprocess
name in the tutor_q module, which is the name that will be used by
your pinger object.<br>
<br>
You have to assign to that global. A module's globals are also
available as attributes on the module object, so you can do it like
this:<br>
<br>
<font face="courier new, monospace"><br>
</font>
<div>
<div><font face="courier new, monospace">import mock</font></div>
<div><font face="courier new, monospace">import unittest</font></div>
<div><font face="courier new, monospace">import tutor_q<br>
</font></div>
<div>
<font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace">class
Test_Pinger(unittest.TestCase):</font></div>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace"> def
test_ping_host(self):</font></div>
<div><font face="courier new, monospace"> pinger =
tutor_q.Pinger()</font></div>
<div><font face="courier new, monospace"> assert pinger</font></div>
<div><font face="courier new, monospace"> subprocess =
mock.Mock()</font></div>
<div><font face="courier new, monospace">
subprocess.Popen.return_value = 0</font></div>
<div><font face="courier new, monospace">
tutor_q.subprocess = subprocess</font><font face="courier new,
monospace"><br>
pinger.ping_host('127.0.0.1')</font></div>
</div>
<font face="courier new, monospace"><font face="courier new,
monospace"> </font>subprocess.assert_called_once_with(['ping','localhost'])</font>
<br>
<br>
I've also moved the assert to after the code under test, since
that's when the calls will be available to make assertions about. (I
didn't actually test this code, if I got it wrong, someone on the
list will school me!)<br>
<br>
A problem with this code is that you replace tutor_q.subprocess with
a mock, but you don't restore it when the test is done. Mock has
tools that can do this for you:<br>
<br>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace"> def
test_ping_host(self):</font></div>
<div><font face="courier new, monospace"> pinger =
tutor_q.Pinger()</font></div>
<div><font face="courier new, monospace"> assert pinger</font></div>
<div><font face="courier new, monospace"> with
mock.patch("tutor_q.subprocess") as subprocess:<br>
subprocess = mock.Mock()</font></div>
<div><font face="courier new, monospace">
subprocess.Popen.return_value = 0</font></div>
<div><font face="courier new, monospace"> </font><font
face="courier new, monospace">pinger.ping_host('127.0.0.1')</font></div>
<font face="courier new, monospace"><font face="courier new,
monospace"> </font>subprocess.assert_called_once_with(['ping','localhost'])</font>
<br>
<br>
Note that there is an unfortunate coupling of your test code and
product code: the test had to know how subprocess was imported. In
tutor_q.py, if you had instead used "from subprocess import Popen",
then your mock would have to be similarly adjusted to mock
tutor_q.Popen instead of tutor_q.subprocess.<br>
<br>
--Ned.<br>
<blockquote
cite="mid:CAHvkzy=ghCOm-EV158ct0DhGZtprKy4h-93uMBhdt59ZF5yPAQ@mail.gmail.com"
type="cite">
<div dir="ltr">
<div class="gmail_quote">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>
<font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace">if __name__
== '__main__':</font></div>
<div><font face="courier new, monospace">
unittest.main()</font></div>
</div>
<div><font face="courier new, monospace"><br>
</font></div>
<div><font face="courier new, monospace">-------------------------------------------</font><br>
</div>
<div><br>
</div>
<div><br>
</div>
<div>Can anyone point me in the right direction on how
to mock up these subprocess calls?</div>
<div><br>
</div>
<div>Thanks</div>
<span class="HOEnZb"><font color="#888888">
<div><br>
</div>
<div>James</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
</div>
</font></span></div>
</div>
</div>
</div>
<br>
</div>
<br>
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</blockquote>
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