pulsar17's blog

Mocking and Patching

I also looked into Mocking and Patching this week. Basically, mocks are blank stubs that you can substitute for real objects while testing. They are an extremely handy tool. You can mock almost anything (almost... more on this later) - databases, external APIs, UFOs 😉... Mocks are part of the Python standard library, present in the unittest testing framework. So, mocks are stubs and you replace real objects with them using patching (also a part of unittest).

The need was to mock and patch datetime.now() for a test. This test was going through all the topics in a forum and filtering the topics which were created in last n number of days. Since test data is static, the tests run on data you created a month ago will show different results when you run datetime.now(), well, now. You might get a false sense of test correctness when you run them now but after a month, a year, those tests will suddenly start to fail. I hope I was able to convince you that the need for mocking dates in tests is real.

Turns out it is hard, real hard to mock datettime.now(), especially when using third party code like Django. I won't go into the details of why it is so, there are some great articles about that. I'll link to them in the footnotes.

With my novice Python skills, I came up with a solution of my own:

from contextlib import contextmanager

@contextmanager
def freeze(datetime_module, set_to):
        patcher = mock.patch.object(
        datetime_module, 'datetime', mock.Mock(wraps=datetime_module.datetime)
        )
        mocked_date = patcher.start()
        mocked_date.now.return_value = set_to
        yield
        patcher.stop()

Then you would use it as:

import datetime

def test_view_displays_topics_from_last_week():
        with freeze(datetime, set_to=datetime.datetime(2021, 6, 8):
                # do actual work
                pass

isinstance(value, datettime.datetime)

The problem is that the only way that the datetime.now() method can be patched is if we mock the parent datetime class. That is datetime.datetime. In doing so, we essentially replace datetime.datetime with an instance of Mock class.

So, now roughly

datetime.datetime == Mock()

Those isinstance checks will now fail when the value argument is a real datetime.datetime object. Since it would be asking if a date is an instance of a Mock instance (yes, an instance, not even a class!).

GSoC Mentors and Mentees Meet (G3M)

We had our first G3M (I gave it that name, it's not official) on 11th June. All the mentors introduced themselves and told us about their backgrounds and what part of Inkscape they work on. Then students told about what they're doing currently, which university/school they're from and what project they will be doing for GSoC (We also have one project from Outreachy). There are many interesting projects this year - on canvas bool ops, markers, alignment guides, actions improvements, etc.

In the meeting, Marc, one of the developers, suggested us to write blogs, and communicate regularly with our mentors about our work. This G3M was scheduled on a BBB instance. This is how all the developers usually communicate. That, the RocketChat instance and IRC are the modes at our disposal. We were also told that we will be having more such G3Ms in the future.

This was a fun experience. I got to know about the people who develop Inkscape. Together with the community they create an awesome piece of software that is Inkscape. I am glad I chose Inkscape as my org.

Final Thoughts

This week was a light one with respect to how much I could do. I learned many new things and got to know such great people. I hope I will be able to contribute more in the coming weeks.

grep -Rin "?next" .

./content/second.rst:121:        grep -Rin "?next" .
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./output/2021/06/the-beginnings.html:244:grep -Rin <span class="s2">&quot;?next&quot;</span> .
./output/2021/06/the-beginnings.html:255:grep -i <span class="s2">&quot;?next&quot;</span> <span class="k">$(</span>find . -print<span class="k">)</span> <span class="m">2</span>&gt;/dev/null

grep -i "?next" $(find . -print) 2>/dev/null