How To Write Unit Tests For Existing Python Code // Part 1 of 2
Рет қаралды 182,277
Күн бұрынIn this video, I show you a practical example of adding unit tests to existing code. This first part focuses on adding tests while not changing the original code. In the second part (coming out next week), I'll show how refactoring the code simplifies test writing while also improving the design.
NOTES:
1) There are a few things in the test setup that are not ideal, like how dates are used in the test code, using a real API key, and doing actual card charges. I'll address these things in part 2.
2) patching means that you're replacing a dependency in a function or method with something else. The thing that you replace it with is called a mock (apologies, my use of the terms in the video is a bit messy).
The code I worked on in this episode is available here: github.com/ArjanCodes/2022-te....
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👀 Code reviewers:
- Yoriz
- Ryan Laursen
- Sybren A. Stüvel
- Dale Hagglund
🎥 Video edited by Mark Bacskai: / bacskaimark
🔖 Chapters:
0:00 Intro
1:05 Explaining the example
4:10 Analysis & Setup
6:05 Adding LineItem tests
8:40 Adding Order tests
10:56 Adding PaymentProcessor tests
15:50 Analysis of the test setup so far
17:05 Adding pay_order tests
#arjancodes #softwaredesign #python
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@ArjanCodes 9 ай бұрын
💡 Get my FREE 7-step guide to help you consistently design great software: arjancodes.com/designguide.
@grmancool 2 жыл бұрын
btw Arjan just wanted to mention that I literally got a job offer with twice my current salary by remembering things from your videos in the technical interview. most probably wouldn't have gotten it without you. thanks so much for what you do!
@ArjanCodes 2 жыл бұрын
That’s awesome, congratulations!
@lackofsubtlety6688 2 жыл бұрын
That basically means that you owe him 20 percent of that, right? :p Grats on the job man!
@pranavnyavanandi9710 2 жыл бұрын
Was that a course you took?
@Yeet_the_code 2 жыл бұрын
Well done and congratulations!!
@henrikholland1730 2 жыл бұрын
That’s amazing. Congrats.
@sillytechy 2 жыл бұрын
I ll be Frank, design patterns, unit tests for existing code. You are my mentor. And you put up videos which are useful in a production environment.
@ArjanCodes 2 жыл бұрын
Thank you so much, glad you like the videos!
@mytelevisionisdead 2 жыл бұрын
Hi Frank!
@chaddrobertson5805 2 жыл бұрын
I've been programming for a while using Python, and I'd plateaued quite badly. Your videos are fantastic, and they're constantly teaching me something new. Thank you!
@ArjanCodes 2 жыл бұрын
Thank you Chadd, glad you like the videos!
@astronemir 2 жыл бұрын
Thank you, this is exactly the nightmare I have to deal with. Instead of payment processors it’s expensive computations on gigabytes of data that need to be tested. Sigh. Can’t wait for part 2
@mikeciul8599 2 жыл бұрын
9:23 there are two functions called `test_order_total`. Python only sees the second one, and the first one doesn't run. This is a great example of how you can have 100% test coverage and still be missing tests! I have my vscode configured to run flake8 and mypy. I make this particular mistake all the time when writing tests, but I get a helpfully annoying red underline under the name of the first duplicated function name.
@transatlant1c 2 жыл бұрын
Came here to report the same thing! Nice spot 👏
@plato4ek 2 жыл бұрын
Actually pytest shows the coverage of 82% for `test_order.py`, which gives a hint. So it's not a 100% coverage.
@NostraDavid2 2 жыл бұрын
I'm so glad you've made this video, even though I don't need it anymore. I got a bit annoyed that there were so many Unit Test introductions, but nothing about what to do if you already have existing code, especially if said code contains a state, because all introductions have a nice, pure (meaning the function only has input and output - no sideeffects like printing text or saving files), function, which simply doesn't reflect reality.
@zactamzhermin1434 2 жыл бұрын
Love your intermediate tutorials with examples + refactoring! Would love more TDD and software design patterns guides (for dummies + for non-dummies) as well as I can write code in a pretty straightforward manner but not so much with the mindset of a production setting with best practices. Also really nice setup :D
@vikingthedude 2 жыл бұрын
You're quickly becoming one of my favourite programming tutorial channels. KZfaq has so much content which, on one end is all beginner programming concepts, and at the other end it's all frameworks and fancy programming features. I think you fill that gap in between where we take basic programming features and compose them in ways that make software development scalable and maintainable.
@jevandezande 9 ай бұрын
When writing code with pytest.raises, I recommend only putting the line that you expect to raise in the block, otherwise it might accidentally catch a failure that is happening elsewhere.
@daymaker_bybit Жыл бұрын
After 3 hours of pain googling and youtubing, finally only your video made it clear how to run test from "tests" folder and make import work. FINALLY! THANK YOU SO MUCH
@ArjanCodes Жыл бұрын
Glad it helped!
@M_7844 2 ай бұрын
Same here. I find it quite complicated to make import work correctly. I would like to ask, is it right I can't run test_line_item.py as separate file getting import error "No module named 'pay'? So far when I wanted to test my code I just run separate parts of it, but with import in siblings folders it's not possible.
@Melindrea 2 жыл бұрын
Nice timing! I'm currently doing something like an internship where one of the things I'll be doing is to help betterify code written by my supervisor. One of the first things I'm going to be doing will be to set up tests and automate the various "did you write it appropriately". His code is quite good in general, but he has no tests, and when I asked if he used PEP-8 or another styleguide ... the answer was "I try to make sure to keep consistent tabs". I might not be able to help make the code itself better/faster (don't know yet!), but i will for sure be able to help make sure it's easier to refactor it and show that it's *working as it should*
@12valmirjr 2 жыл бұрын
Arjan, you always exceed the expectations. Thank you once more for the content. The quality of the tips and the video itself are amazing!
@Betacak3 2 жыл бұрын
In my experience, old code that doesn't have tests is often nearly untestable. I recently rewrote a (very old) program that dealt with databases. The UI and the rest of the code are so interwoven that you can't even let it generate an SQL string without first creating a UI component that's responsible for it. That's why I didn't even bother with it. I just rewrote it from scratch.
@beaumcdermott8742 7 ай бұрын
phenomenal editing job. Thanks for creating this. All content, no fluff, right to the point, no hemming/hawing, quick pace but calm delivery. I tip my hat to y'all. Subscribed and liked. Thank you for a top tier tutorial.
@ArjanCodes 7 ай бұрын
Thank you so much for the kind words, Beau!
@coldhardwick 2 жыл бұрын
you an mCoding are some of the most valuable resources I've seen on youtube in a looong time....
@amardeep.sahota 2 жыл бұрын
I learned all these techniques hard way and hours of hard work. I wish I had come across your channel before. If you are a python developer, you can’t appreciate Arjans content enough this content is literally the abstract of years of hard work.
@vbaclasses3553 2 жыл бұрын
When you know the content is going to be worth the watch, so you like the video even before you start watching. Vreeslik baie dankie.
@marcocofano2545 2 жыл бұрын
Great one. Good point that you can realize where the code needs refactoring from writing unit tests. The tests that check raise value error can actually Raise it from different calls (one the init of the object and the other from the call of the method) so those tests are actually not unit and are not testing in the way they are intended.
@rishabhahuja7413 2 жыл бұрын
Excited for the Part2!! Arjan please post it asap...
@JacinLowe 2 жыл бұрын
Long time watcher. This was the video i was waiting for. I'm learning how to do testing and didn't know where to start.
@Bentroen_ 2 жыл бұрын
I just started delving into unit testing this week, so this video came at a perfect time!! Thank you so much, looking forward for part 2 :)
@ArjanCodes 2 жыл бұрын
Coming up on Friday 😉.
@sarkhanmammadli9673 2 жыл бұрын
Your videos are fantastic, and each time I am learning something new.
@eyalbahar9291 2 жыл бұрын
Honestly im so greatful for your videos. Im learning so much, enjoying my time and gertting alot of motivation to improve as a datascientist/developer. Thank you so much!!!! Youre awesome!
@ArjanCodes 2 жыл бұрын
I'm glad to hear the videos are helpful!
@universecode1101 2 жыл бұрын
Now we're all leveling up guys, great 👏🏻😊
@rhernaus 2 жыл бұрын
Thank you for this valuable content. I have watched nearly all of your videos because I like the way you explain things. I'll start leaving comments on your videos to help the KZfaq algorithm even more 🙂
@ArjanCodes 2 жыл бұрын
Glad you like the content! And thanks for liking/commenting under the video. It’s indeed good to help the KZfaq algorithm, but I also find the feedback really helpful in improving the videos I post.
@pericofalcor 2 жыл бұрын
Awesome video, I really enjoyed it. It would be very interesting to see mocking applied to a database connection. I work with a 25 year old project in Perl, and a relatively younger Python code, and both have no tests at all. Both are inmense codebases and is usually seen as "imposible" to add unit testing to it. Everything is based on data stored in a mysql database, but I have no clue on how to even start testing when I have to mock so much data
@mauisam1 2 жыл бұрын
#11 here. Can't wait until next week. Great video again!
@rjgonzalez8108 2 жыл бұрын
Best python content here on YT. Also really like that shirt you're wearing. I would like to know where I can get one if you don't mind.
@jorgealarconalvarez 2 жыл бұрын
This tutorial is very complete, thanks man!
@samggez 2 жыл бұрын
Arjan you are a true gem, hope to learn more about python from you! Keep it going!
@ArjanCodes 2 жыл бұрын
Hey Samir thank you, and will do! 😉
@conconmc 2 жыл бұрын
This is fantastic, eagerly waiting for the next video!!
@seanoverton798 2 жыл бұрын
Great videos mate. Always learning something new from them.
@pramithijagdish2821 3 ай бұрын
This was super helpful! Thanks for sharing.
@ArjanCodes 3 ай бұрын
Glad the video was useful!
@jandom1988 2 жыл бұрын
This is a super mega important topic, thanks for the video! Test order total is defined twice, I think, so only one will be exercised
@jandom1988 2 жыл бұрын
Also monekypatch is a rather exotic path. Why not mock.patch with side-effect for the 3 different return values?
@jurybulich6324 2 жыл бұрын
Thank you Arjan, really glad that somehow KZfaq algorithms showed your channel) your content is very helpful! Nice
@ArjanCodes 2 жыл бұрын
Thank you Jury, glad the content is helpful!
@abdelghafourfid8216 2 жыл бұрын
very informative ! , I hope you'll cover also the type checking with mypy
@betterbrained 2 жыл бұрын
Thank you, Arjan, for your brilliant work and helping!
@ArjanCodes 2 жыл бұрын
Thanks so much, glad it was helpful!
@StellarEclipseStudio 2 жыл бұрын
I hope you bring Mock into the conversation! Easier to use than writing lambdas and mock functions. Mock(side_effect=inputs) would have been a simpler way to monkeypatch the input calls.
@StellarEclipseStudio 2 жыл бұрын
Mock is a class not well understood, I've had to teach other engineers about it many times. Well worth learning about if you're serious about writing test code! It's very powerful to use in conjunction with monkeypatch.
@Victorinoeng 2 жыл бұрын
Hey, @Joshua That is indeed an interesting functionality. Do you have any material to share? I have been using patch and magicMock, but feels so unnatural. Every time I have to Google/read up on some examples again
@_ipsissimus_ Жыл бұрын
Arjan, I always enjoy your videos. You really do a good job of pacing the complexity from simple to advanced that makes learning here so appealing. If I could request that you follow up these videos with a "how to write unit tests for existing python code using unittest and mock/patch" that would be great. my organization recently switched over from the simpler pytest to now using the more complex unittest lib as standard, and the abstractions for mock and mocked classes for testing really make it hard to grasp exactly whats going on sometimes. Great video anyways
@ArjanCodes Жыл бұрын
Thanks!
@user-fs5nx7bx3t 2 жыл бұрын
Just in ...Thanks Arjan for picking up this topic.
@user-jt5mc2kr4m 2 жыл бұрын
What a great discovery your channel is! Thanks a lot for all videos you are doing, fabulous job! Many greetings from Ukraine 💛💙
@ArjanCodes 2 жыл бұрын
Thank you, happy you’re enjoying the content!
@sHillChannel 2 жыл бұрын
I swear you have a hidden video camera in my office. You keep making videos on exactly what I am working on.
@gayathris2108 4 ай бұрын
Found this to be simple and effective guide!!!
@ArjanCodes 4 ай бұрын
I'm glad to hear you enjoyed the video!
@PartScavenger 2 жыл бұрын
I must say that I'm a little disappointed that he starts by understanding the code, and the method isn't a thousand lines long. That said I did learn a lot and I love this channel thank you for all the amazing content!!! Thanks for testing videos!! Please more!!!
@frankie_goestohollywood 11 күн бұрын
Thank you Excellent, Arjan!
@ArjanCodes 11 күн бұрын
You are welcome!
@AgustinIgnacioGenoves 2 жыл бұрын
Thanks Arjan, thisvideo was super util for me.
@ArjanCodes 2 жыл бұрын
Glad to hear it was helpful!
@ericandes4288 Жыл бұрын
My professor assigned us to make unit test without ever telling us what a unit test is. Thank you for teaching me. I regret paying for college now lol
@JoshSmeda 2 жыл бұрын
You're a legend, Arjan 👍
@henryw.4953 2 жыл бұрын
etymology suggestion for monkey patch: 1) guerillia -> gorilla -> monkey; 2) monkeying about with code (en.wikipedia.org/wiki/Monkey_patch)
@dannijt69 2 жыл бұрын
super useful video, thanks again
@markasiala6355 2 жыл бұрын
The monkeypatch example was particularly interesting. I've only used it some to test a method that called input function. However, I modified sys.stdin to place my input strings rather than replacing the input function. Is there a preferred method or are they considered relatively equal?
@igorferreira9683 Жыл бұрын
Excellent video, I've been following it for some time and it's adding a lot of knowledge to me. A video on how to apply Mocks in situations involving API requests and database connection would be interesting.
@ArjanCodes Жыл бұрын
Great suggestion Igor, Thank you!
@KarlaMuller 2 жыл бұрын
I'd love to know where you got that cool geometric led lights behind you. Also, big fan of all your content!
@mikeciul8599 2 жыл бұрын
I'd like to know more about your choice of pytest over unittest. Are there particular factors that lead to a preference, or is it just what's more familiar? I've been working on some projects that mix bits from both pytest and unittest, and it's kind of a mess but I don't know which one to go with. There's things that annoy me about both. :D
@FabianSlonimczyk 2 жыл бұрын
Super interesting content. I have a question (open to anyone): I see that in testing with a "valid" card the expiry date is being hard coded. Would this not be a problem in the future, when the hard coded values become a "past date" and the tets are run again (and fail)? Thank you! PS (after watching part 2/2) My question is answered there!
@Izzat-bj1le Жыл бұрын
Amazing I love it, thanks for doing what u do
@RaghothamRao 8 ай бұрын
Thanks a lot! Very much helpful!
@ArjanCodes 8 ай бұрын
Glad you enjoyed the video :)
@syspogh1609 Жыл бұрын
Thanks Arjan for this good video.
@ArjanCodes Жыл бұрын
Glad you liked it!
@md.mohiuddin Жыл бұрын
Excellent tutorial.
@ArjanCodes Жыл бұрын
Glad you liked it!
@niconeumann2963 2 жыл бұрын
Really great video! I love to see things about testing :) Wouldn't it be better to put only the line which raises an error into pytest.raises (like in try/catch) or why is all the code inside the block?
@ArjanCodes 2 жыл бұрын
Thanks Nico! Yes, good point, it would be better to only put the line that actually raises the error in the with statement, as this focuses the error detection better.
@marcgierse1601 2 жыл бұрын
Very good video - as always :-). Just one remark: If you don't want to have some tests suddenly fail on 1/1/2025, you probably want to use either something like the freezegun module or set the card info dynamically :-).
@ArjanCodes 2 жыл бұрын
Hi Marc, I’ll address this issue in part 2 this Friday.
@codeaxen Жыл бұрын
Hello thanks for this video really helped... but is it possible to do a real payout system tutorial with stripe putting all these tests together ?
@shaheerzaman620 2 жыл бұрын
Fantastic stuff!
@ArjanCodes 2 жыл бұрын
Thanks, glad you liked it!
@Phaust94 2 жыл бұрын
Awesome stuff man, as always!
@ArjanCodes 2 жыл бұрын
Thank you!
@kosmonautofficial296 2 жыл бұрын
great video thanks!
@DarknezRocks 2 жыл бұрын
Thanks for the video, wasn't aware of monkeypatch, looks much easier to use than the other patches available to pytest and unittests. Had just one remark, be careful with only checking the exception type, especially if the function being tested has multiple of the same exception types, you had 2x ValueError in the same function, but could've triggered the wrong if statement without knowing. I prefer to also check the actual error message within an assert, to make sure that I'm triggering the correct ValueError.
@mikeciul8599 2 жыл бұрын
That's a great reason to use custom exceptions, and create a different one for each fail case!
@okdokie278 2 жыл бұрын
does anyone know which VSCode appearance theme is being used? :O
@gw032sysint9 2 жыл бұрын
Can you please suggest what is the right way to put constants (like error msgs, log messages, other strings used)? What is the best practice to use constants in python : (1) a separate .py file for storing strings (2) Enums (3) Dictionary (4) Named Tuples (5) Data classes (6) anything else
@renatocustodio1000 2 жыл бұрын
It seems that monkeypatch is even more powerful than Java's mockito in some aspects. Nice video.
@InspektorDreyfus 2 жыл бұрын
Will the branch coverage (additional to line coverage) be in part 2? Can the coverage metric also be shown directly in the editor?
@cordempire4789 2 жыл бұрын
Hey man - moved house and I need to redecorate. Where did you get that thing on your wall? I love it!
@YagiTheDog Жыл бұрын
Great video!
@ArjanCodes Жыл бұрын
Thanks!
@vicentcaselles4807 8 ай бұрын
How do you import pay from the tests subfolder? It never works for me and I have to import it via relative imports which are so crappy
@Klej0aka0Klej 2 жыл бұрын
I hope the code quality of tests in part2 will be much better, for example many many cases in here should use fixtures to not repeat code or create classes itself inside every test.
@ArjanCodes 2 жыл бұрын
Yes - I’m covering fixtures in part 2.
@jackanderson9258 2 жыл бұрын
When should I use assert vs self.assertEqual?
@taylormonacelli 2 жыл бұрын
You're all dressed up these days. I need hacker hoodie back once in a while
@jerepovedamartinez4574 2 жыл бұрын
Hi Arjan, would be nice to know how to test very complex clases like the ones that usually appears in Deep Learning. For example, pytorch_lightning.LightningDataModule... things like the shape of tensors in several configurations, training loops, etc. I don't really know if it's worth to unit test somehow these kind of things. Currently, I'm placing asserts in my the code itself but not unit testing systematically, would be nice to know at least your point of view about this topic!
@williamwei7899 9 ай бұрын
why would we always want to test the invalid situation? is it just to see if it raises the Error we expected?
@sriramjaisankar9121 2 жыл бұрын
Hi Arjan, Are these tests possible when my application built on streamlit?
@Kliamframe Жыл бұрын
Sir, You're a genius!
@ArjanCodes Жыл бұрын
Thanks, happy you’re enjoying the content!
@jiazeli5419 2 жыл бұрын
Hi, why you write "if __name__ == __main__" in main script instead of module scripts?
@rodrigo2112- 2 жыл бұрын
Great video! I'm catching up with your videos, so maybe you've done that already, but it would be a nice follow up to show how to create the same payment processing system and its tests if we were writing it using TDD from scratch, probably using dependency injection for the input and the payment service and therefore not need to monkeypatch
@ArjanCodes 2 жыл бұрын
Thanks and love that suggestion!
@jpactor155 2 жыл бұрын
These videos are great.
@ArjanCodes 2 жыл бұрын
Thank you Jacob, glad you like them!
@gw032sysint9 2 жыл бұрын
Could you please tell (1) how to do Unit Testing for decorators - both methods and class based? (2) Flask's request module throws lot of context related errors while patching how to resolve those?
@andrewallbright 2 жыл бұрын
Excellent example software -- too often I see horrible example code when talking about testing (which leads me to conclusions I will not say). Question: what are your thoughts about starting with writing acceptance tests for brownfield projects?
@eduardovega8851 Жыл бұрын
How is he able to import pay.OBJECT inside the tes folder files? Why is it not working for me ? Someone help! i have all my __init__ files
2 жыл бұрын
Very interesting video, can't wait for part 2! You mentioned writing tests before the actual code. I wonder, how would this work for functions, that are computationally heavy, for example because they do some complex image processing, or perhaps create a 3D scene? How would you provide the correct result for the assertions? It's hard for me to imagine how this scenario would work. I'm only a beginner when it comes to programming, so I hope my question makes sense. :)
@ArjanCodes 2 жыл бұрын
Hi Dávid, if a function performs a very complex task, you would normally split up the function into several smaller functions that you can then test separately. For parts of your application that produce a complex result like a rendered image, a GUI, etc, a common technique used is snapshot testing. You generate the correct complex representation once and indicate this is the correct ‘snapshot’. When you change things in the code or add new features, the snapshot test verifies that the resulting output is still the same (or if it’s supposed to be different, you generate a new snapshot).
2 жыл бұрын
@@ArjanCodes Got it, thanks for the explanation Arjan!
@Otakutaru 2 жыл бұрын
I'm not doing this professionally, so I really doubt I will do many tests... But knowing how to is nice
@peterkruse8192 2 жыл бұрын
Depending on the size of your growing project tests will help you keep your own code stable while growing and extending it. If your feature set is very volatile tests will feel like a burden and overhead.
@splendorman7922 2 жыл бұрын
how do i test functions that returns unknown data? like fetching from an api? or functions that do really complex calculations and you cannot know the result before running it?
@ericsalesdeandrade9420 7 ай бұрын
Mocking for non-deterministic responses or validating the HTTP response code
@gh00n20 2 жыл бұрын
really good video. thx!
@ArjanCodes 2 жыл бұрын
Thanks, glad you liked it!
@MRBADGUY0027 8 ай бұрын
Hello Arjan, how does it look like if you've used "unittest" built-in lib? And why do you prefer pytest? Thnks
@JustMWest 4 ай бұрын
Excellent breakdown! I want to try it out but I can't for the life of me figure out how to import "pay" from within "tests". How does this work??
@Han-ve8uh 2 жыл бұрын
1. At 9:40, why did test_order.py have only 77%? I thought all tests/ folder files will be 100% since all tests will be run by default. Anyway is there any reason to be checking for 100% coverage in files under tests/? Intuitively I would only care about coverage of the files they are testing, and not coverage of the test script itself. 2. 14:20 def test_card_valid_date() contained no raises or assert. I thought every test would need one of these to communicate what is expected. Is this test function saying that if we can run without errors, then it passes? This feels wrong because even if it passes, it still doesn't clearly communicate the expected state, especially when there are many lines in the test function
@maikwiesmueller 2 жыл бұрын
why using monkeypatch over unittest.mock.patch ?
@UNgineering 2 жыл бұрын
tried to download the design guide, but when I enter my email it says "This value seems to be invalid"
@morkallearns781 Жыл бұрын
Any thoughts on pytest vs unittest?
@user-hk7wh9jm7l 2 жыл бұрын
Hi . Would be awesome if you could make a video about agile principles. Thanks in advance :)
@user-sm3qi1ku7r 8 ай бұрын
Hi Arjan It is a minor thing, but in 16:22, why do you prefer to use [int(d) for d in card_nr] rather than, e.g., list(map(int, list(d)))? Are there any benefits to this? Thanks
@oncedidactic 2 жыл бұрын
Cool video topic!
@ArjanCodes 2 жыл бұрын
Thanks!
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