Add initial test suite

[aklajnert]

This is a very initial test suite that tests mostly the script's entrypoint.
The DownloadManager class is entirely mocked, so it not tested.
All the HTTP communication is mocked, the responses are stored within tests/cassettes directory (everything there is auto-generated).
I did also some minor changes in imports to make testing easier.

I can't see any good place to put a description of how to run tests, so I've created a README.md within the tests directory for now.

Given the fact that I've focused only to test a single module, the code coverage is decent:

----------- coverage: platform win32, python 3.8.3-final-0 -----------
Name                                  Stmts   Miss  Cover
---------------------------------------------------------
spotdl\__init__.py                        1      0   100%
spotdl\__main__.py                       41      4    90%
spotdl\download\__init__.py               1      0   100%
spotdl\download\downloader.py           107     88    18%
spotdl\download\progressHandlers.py      85     53    38%
spotdl\patches\__init__.py                1      1     0%
spotdl\patches\pyTube.py                 90     90     0%
spotdl\search\__init__.py                 1      0   100%
spotdl\search\provider.py               111     12    89%
spotdl\search\songObj.py                 70     25    64%
spotdl\search\spotifyClient.py           13      1    92%
spotdl\search\utils.py                   38     10    74%
---------------------------------------------------------
TOTAL                                   559    284    49%

If you are OK with what's already done, I think it could be merged and then I'll work further on extending the suite.

[ghost]

Well, as embarassing as this is, I have never written up tests before. Any things unique about test code that i should know?

I'll look through your code on a general basis and let you know by the end of this week.

[aklajnert]

Most of the tests are quite similar. First I fake sys.argv to simulate different command-line arguments.
Then I call the entry point function and see what is happening. I test if the valid message is printed to the console and if the correct method from DownloadManager has been called. The download methods are not executed as I mock them within patch_dependencies() fixture.

The tests are written with pytest framework, which is the best framework available for python, and one of the best testing frameworks in general. I recommend reading a bit about it, especially about fixtures which are very useful while testing.

And, well, practice makes perfect - start small and grow :)

[ghost]

cool. Thanks for the "well, check this out and this too" - a little head start helps a lot. Thanks again.

[phcreery]

@aklajnert Thanks a ton for getting this in motion. I, like @Mikhail-Zex , have never officially used tests but I see the need for them. Also thanks for documenting use in the readme.

Question, would the monkey patches be needed for asyncio?

[aklajnert]

Question, would the monkey patches be needed for asyncio?

It depends on the test, what you want to achieve with it. Probably some mocks will be needed to make the tests faster.

If you merge this, I'll build on it to extend the tests and add some more. Preferably on the DownloadManager class after #994 will be merged.

[ghost]

We're definitely merging this. Hey @phcreery review this. I will too.

[phcreery]

Looks good to go for me.

[aklajnert]

Should I also change a branch here, or can it go to master? If yes, then I think it could go to both actually.

[ghost]

Change the branch. I'll merge it.

[aklajnert]

@Mikhail-Zex - I've just rebased the changes to the new branch, should be good to go

[ghost]

Cool.