# Python (relative) Imports and Unittests - Derp!

Note: This is old! Probably use pytest, probably also pip install -e . is a helpful idea :) Keeping this up for posterity though!

Been far too long since I wrote something. Spent way longer than I feel good about talking about derping around getting my ass kicked by some obvious shit so figured I should write it up as a reminder for me and to hopefully save some anonymous Internet person some heartache! Anyway, I’ve been meaning to put on my big boy pants and start actually writing test cases for all the Python code I write… I’ve been terrible at this and figured it’s time to get my life together with it. Rather than have a bunch of code that won’t really matter, I’ll just use ridiculously dumb examples here to get the issue and the “fix” (not being dumb!) across.

A pretty standard structure for a python project would look something like this:

mycoolproject/
├── mycoolproject
│   ├── __init__.py
│   ├── my_module_1.py
│   └── my_module_2.py
└── tests
├── __init__.py
└── test_basics.py


You can see that we have our “project” – in this case named “mycoolproject” and within that directory we have a folder for our tests and a folder for the actual project code. In the root directory for our project we would probably have other stuff like a setup.py or a requirements.txt or whatever, but those things don’t matter for us at the moment. One last bit – we need those __init__.py files in there to make this a “project” – from the real Python docs:

“The __init__.py files are required to make Python treat the directories as containing packages; this is done to prevent directories with a common name, such as string, from unintentionally hiding valid modules that occur later on the module search path. In the simplest case, __init__.py can just be an empty file, but it can also execute initialization code for the package or set the __all__variable, described later.”

You can find that doc here if you want to read some more about it.

Ok so now that we have the overall gist of things down, let’s take a look at our ridiculously over simplified “module”. Here is the contents of our file my_module_1.py:

my_string = 'whoa, this is so kewl'
print(my_string)


Pretty serious code :)

Our other script “my_module_2.py” is also pretty simple, but this one refers back to the first module to use the variable “my_string” – we’ll get to why this tripped me up in a bit:

import my_module_1
my_new_string = f'carl said: {my_module_1.my_string}'
print(f'carl said: {my_module_1.my_string}')


Alrighty, so if we run “my_module_1.py” it will simply print “whoa, this is so kewl” – no surprise there. If we run “my_module_2.py” it will also print “whoa, this is so kewl” because it is importing/loading the first script, and then of course it will also print “carl said: whoa, this is so kewl” as expected. Great, so our super fancy project works as desired. Now, because we are trying to be better about testing, let’s write a super simple test to validate this code works as expected.

In our test folder, we’ll create a new script called “test_basics.py” that looks like this:

import unittest
import my_module_1

class TestMe(unittest.TestCase):
def test_stuff(self):
assert my_module_1.my_string == 'whoa, this is so kewl'

if __name__ == '__main__':
unittest.main()


At the top we’ll import the unittest library to use for our testing, and we’ll also import our script “my_module_1” so that we can validate (assert) that our variable “my_string” is actual equal to what we think it should be (“whoa, this is so kewl”).

So let’s go ahead and run our test suite and see what happens:

Carls-MacBook-Pro-2:mycoolproject Carl$python3 -m unittest tests/test_basics.py E ====================================================================== ERROR: test_basics (unittest.loader._FailedTest) ---------------------------------------------------------------------- ImportError: Failed to import test module: test_basics Traceback (most recent call last): File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/unittest/loader.py", line 153, in loadTestsFromName module = __import__(module_name) File "/Users/Carl/Desktop/mycoolproject/tests/test_basics.py", line 2, in &lt;module&gt; import my_module_1 ModuleNotFoundError: No module named 'my_module_1' ---------------------------------------------------------------------- Ran 1 test in 0.000s  Well… not ideal eh? Obviously we have some kind of import error since Python is complaining it can’t find our module “my_module_1”. What gives? Well, we know that Python is mad at the line where we import “my_module_1” so obviously we need to start there. We also know that our modules actually run fine within their directory – they do exactly what we think they should. So with this information we can understand that Python – when ran from the tests directory has no idea how and where to find the module we are trying to run. This makes sense because when you think about it Python will search for modules in the local folder and the system path(s) – we can see where Python is looking by importing sys and printing out the path, let’s see what that looks like in our tests folder: Carls-MacBook-Pro-2:tests Carl$ python3
Python 3.6.4 (v3.6.4:d48ecebad5, Dec 18 2017, 21:07:28)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
>>> import sys
>>> sys.path
>>>


Ok, so we know it’s looking in the normal paths, and that very first entry ('') shows us its going to look for stuff locally too, but obviously nowhere to be seen is the module we’ve been building. So somehow we need to tell Python to look there, what happens if we ask Python to import “my_module_1” from “mycoolproject” like so:

from mycoolproject import my_module_1


Let’s run it and see what happens:

Carls-MacBook-Pro-2:mycoolproject Carl\$ python3 -m unittest tests/test_basics.py
whoa, this is so kewl
.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK


Hey that seems a lot better huh? Up to this point everything has been super straight forward and if you’ve done any amount of Python stuff you’ll be more than familiar with import errors as you’ve undoubtedly forgotten to import something and had this happen to you. The next bit is where I got tripped up… let’s add a quick test case to test our other Python file:

import unittest
from mycoolproject import my_module_1
from mycoolproject import my_module_2

class TestMe(unittest.TestCase):
def test_stuff(self):
assert my_module_1.my_string == 'whoa, this is so kewl'

def test_other_stuff(self):
assert my_module_2.my_new_string == 'carl said: whoa, this is so kewl'

if __name__ == '__main__':
unittest.main()


Pretty straight forward stuff here too – we simply imported the other module and added a test case to assert that the string is what we think it should be. So what happens if we run our unit tests again?

whoa, this is so kewl
E
======================================================================
----------------------------------------------------------------------
ImportError: Failed to import test module: test_basics
Traceback (most recent call last):
module = __import__(module_name)
File "/Users/Carl/Desktop/mycoolproject/tests/test_basics.py", line 3, in &lt;module&gt;
from mycoolproject import my_module_2
File "/Users/Carl/Desktop/mycoolproject/mycoolproject/my_module_2.py", line 1, in &lt;module&gt;
import my_module_1
ModuleNotFoundError: No module named 'my_module_1'

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)


Annnnnnd we’re back to not working. So what gives? We get kinda the same error as before – complaining about import errors and whatnot. Not cool, especially since we basically did the same thing we did for “my_module_2” as we already did (and got working) for “my_module_1”. This time we can see that Python is upset about not finding a module called “my_module_1” – pretty ridiculous given the fact that importing that module is the second line of our test file right? BUT this is not failing in our test file – and here is what tripped me up. The issue is (and I’ll probably butcher the exact technical reasoning for this but you can check out this super handy SO post) that Python is confused about where that module is because the path for the execution (via unittest) is not in the same folder as the modules themselves. So we can address this by ensuring that the imports in our modules are not relative, but instead fully qualified if you will. Changing our “my_module_2” file to look like this:

from mycoolproject import my_module_1

my_new_string = f'carl said: {my_module_1.my_string}'
print(my_new_string)


Instead of importing from our local file we are now specifying the project that we are importing from, running our test again we get the following (good) results:

whoa, this is so kewl
carl said: whoa, this is so kewl
..
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK


TL;DR – pay attention to your imports, easy thing to fix but easy to miss it, run it locally and have everything run great and then be dumb like me and get angry at tests for not behaving the way you think they should :)