Don’t repeat the logic in the unit tests

There are multiple mistakes you can do, as software engineer, when you define and write the unit tests for your software.

One of the most common I saw it was to repeat the logic from the tested function/method in the unit test function

### contants.py
HARD_UPPER = 10
SOFT_LOWER = 5
INTERMEDIATE_VALUES = [5, 6, 7, 8, 9, 10]

### functions.py
from constants import HARD_UPPER, SOFT_LOWER, INTERMEDIATE_VALUES

def my_func(x):
	if x in INTERMEDIATE_VALUES:
		return x + 1

	if x > HARD_UPPER:
		return x + x

	if x < SOFT_LOWER:
		return x ** x

Let’s imagine we want to test this dummy function, my_func and we would do it in this way:

from functions import my_func

def test_myfunc():
    test_values = [4, 7, 8, 10]
    for x in test_values:
        if x in [5, 6, 7, 8, 9, 10]:
            expected = x + 1
        if x > 10:
            expected = x + x
        if x < 5:
            expected = x ** x

        assert expected == my_func(x)

It looks nice. It tests the boundaries, it seems to test all the intervals. But, let’s imagine someone is going into constans.py and does this change, increases the SOFT_LOWER with 1 and removes 5 from INTERMEDIATE_VALUES.

SOFT_LOWER = 6
INTERMEDIATE_VALUES = [6, 7, 8, 9, 10]

If we run our tests, everything is green, but some results are not the expected ones, for example, my_func(5), before 5 was in INTERMEDIATE_VALUES and the result was 6. Now, 5 is under the condition if x < 5 so, the result is 5 ^ 5 = 3125.

Ofcourse, abose it is just a silly example where I tried to copy/paste the logic from the target function to the test and the easier fix would be just to try to hardcode the boundaries and some itermediate values, like:

def test_myfunc():
    assert my_func(4) == 256
    assert my_func(5) == 6
    assert my_func(10) == 11
    assert my_func(11) == 22
    assert my_func(8) == 9

Now, we can see the test is failing for x=5

>       assert my_func(5) == 6
E       assert 3125 == 6
E        +  where 3125 = my_func(5)

This is the case when the border values really matters and we want to be sure the developer is councious of this change (they will see the tests failing). This such case can be for example the tax applied (we don’t want to change the VAT value for a country too often, right?) or the maximum number of connected devices (eg: Netflix).

If we can argue the values are not so sensitive we could import directly the constants and use them instead of hardcoding in the test (eg: the time when the weekely report email is sent to the team’s PM and someone change it by mistake from 5PM to 5AM).

Monitor an error log with python and RabbitMQ

Nowadays there are many professional solutions to monitor your application for the errors. Some web frameworks have even build-in tools or support plugins to catch the programming exceptions and act accordingly.

Anyway, I wanted just to build a simple proof of concept how to monitor the web server error file and, when an event occurs and the file is changed, the monitoring script should send out an email. To monitor the log file I used pyinotify python module. This is an implementation on top of inotify, offering an easy interface to interact with the changes of the filesystem.

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Switch between branches using Nginx

For some of my projects, most of the time I use two branches: master and dev and I work mostly in dev. If I need to send the url to some clients with some dev/beta features I should create another (sub)domain like dev.myproject.com and sometimes, if I forget to change some configurations about the domain name (specially with some “very intelligent” CMSes that keeps the configuration in the database) I will get an email back saying “the new feature is not working” 🙂

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How to use python sched in a daemon process

Let’s assume we want to download a file (or to do some tasks) to every 5 seconds, but the condition is to not do the same task twice or more times at the same moment, even if takes more than 5 seconds. For example, we have a to download a file and this will take 8 seconds. Also, if takes more than 5 seconds, it should not wait until the next iteration, to start again (3 seconds more), but will start the download immediately.

So, the traditional cronjob/lock file combination was not suitable for my case.

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