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Mocking os.stat in Python
mocking, stat, mock, os.stat, unittest, unit test
8th of November 2009
I have some code that checks that a file is treated differently if some time has passed. In other words, it reacts if the modification time is more than it was before. The code uses os.stat(filename)[stat.ST_MTIME] to do this. The challenge was to mock os.stat so that I can pretend some time has passed without having to wait. Here was my first solution which made running many tests sloooow:
filename = 'foo.txt'
expect_timestamp = int(time.time())
...run the unit test with 'expect_timestamp'...
time.sleep(1)
expect_timestamp += 1
...run the unit test with 'expect_timestamp'...
So, here's how I mock os.stat to avoid having to do the time.sleep(1) in the test:
filename = 'foo.txt'
expect_timestamp = int(time.time())
...run the unit test with 'expect_timestamp'...
import os
from posix import stat_result
def fake_stat(arg):
if arg == filename:
faked = list(orig_os_stat(arg))
faked[stat.ST_MTIME] = faked[stat.ST_MTIME] + 1
return stat_result(faked)
else:
return orig_os_stat(arg)
orig_os_stat = os.stat
os.stat = fake_stat
expect_timestamp += 1
...run the unit test with 'expect_timestamp'...
I hope this helps someone else who's trying to do the same. It took me some time to figure out that os.stat is used by lots of various sub routines in the os module so it's important to only mock the relevant argument otherwise you might get unexpected problems.
Comment
We've hit things like this a lot. What we decided to do was make the os module a dependency of our object.
So:
def __init__(self, ..., os_mod=os):
...
self.os = os_mod
def method_that_uses_os:
self.os.stat(...)
This can then easily be injected when creating your test objects, and used in your tests (we use mocker for mocking, but the pattern applies pretty much universally).
def test_method_that_uses_os(self):
m_os = self.mocker.mock()
test_obj = SomeObj(..., os_mod=m_os)
m_os.stat(...)
self.mocker.replay()
test_obj.method_that_uses_os()
The only particular annoyance to this method is that the dependency list can get quite long. But it does keep things very explicit. :)
I ran into a need for this just today and I remembered I had read about your post. I thought you might be interested by the approach I took (I feel it's a little more generic and elegant than your solution):
https://hardcoded.svn.beanstalkapp.com/lib/hsutil/trunk/testcase.py
You make your testcases subclass this TestCase, then just call self.mock_osstat('mypath', st_mtime=42)
Let's assume you only want to find out the creation date...
Why not create a new method
get_cr_date(self, filename):
return os.stat(filename).st_crdate
Now you can easily mock that method if you use Mock for example:
@patch("yourmodule.yourclass.get_cr_date")
def test_should_do_sthg_special_when_x_minutes_passed(self, mock_get_cr_date):
mock_get_cr_date.return_value = whatever
do your test here...
I'm not entirely sure I understand but I'm sure there are other ways to do it. I think the trick I managed to point out was twofold. That you have to only do it for the file you expect to mock and that you need to use posix.stat_result()