This Is Why Python Data Classes Are Awesome

[@ArjanCodes]

💡 Get the FREE 7-step guide to help you consistently design great software: arjancodes.com/designguide.

[@saketh_s]

One thing to note, when you pass data class "frozen=True", instance is immutable. So at 13:29 you are actually getting error for search_string which we are trying to change at _post_int_ at object creating not for person name (line 25) You can not change fields even inside the class even with setter methods.

[@sbeau]

Arjan, your ability to explain detail in a structured, easy to follow format is second to none. Thank you for another great tutorial.

[@VladimirSanchez]

He's in the same league as Nana! ;)

[@paulorsbrito]

What happened to “explicit is better than implicit” and to “there should be only one obvious way to do something”?

[@beershigachi4797]

Zen of Python. I love that. So when I saw dataclasses I decided to use Haskell as my primary language.

[@vedambala]

Arjan, You should seriously consider doing a full fledged Python course from beginner to advance level laden with Projects so we can learn Python the Pythonic way.

[@hematogen50g]

I came to Python from C# so I picked up coding quickly. But thinking in python way is harder, so such videos really help me develop python mindset.

[@auroraRealms]

Although I like the ability to learn and use Data Classes. All my years of database programming has taught me to use JSON, and Dictionaries as much as possible. Especially in Web Services. The caveat to using Data Classes is that; changing a column type, name or description, or adding or removing columns ends up as a use case/feature ticket for the programmer which can take a bit of time to run through the development cycle. These column changes are guaranteed to happen all the time, in the life cycle of the product. A second problem, is that the code must be completely thrown out, when starting a new product. Using Dictionaries very often can allow these column changes from config files without recompiling or restructuring the code base. Going back to Web Services. Dictionaries can be web requests and responses, in JSON, without translation. This makes for high performance, and easy maintenance in the life cycle of product. It also makes copying and pasting the code directly into other products a possibility.

[@duaneatnofroth]

I checked out your channel in hopes to see some examples of the techniques you mentioned in your reply above. I think you might get a good response if you ever decide to do some programming videos. I'd love to see some from you.

[@fabio.1]

👀

[@GankThat]

​​@@duaneatnofrothhe doesn't need to give examples when what he's stating is already best practice when working with data

[@techassets]

I have an observation. Even if you remove the person.name = "Arjan" and set the dataclass frozen=True you will have a FrozenInstanceError. Because you use the __post_init__ method and the frozen dataclass does NOT allow it ! So you must remove the __post_init__ method if you want to freeze the dataclass and prove that you cannot modify it after initialization. THANK YOU FOR YOUR AMAZING VIDEOS !

[@MiguelRozsas]

so, you are saying we not have both ? A frozen class and __post_init__ ? There is any workaround for this ?

[@jordansilke3629]

This is truly an excellent successor to your earlier video on data classes, bravo!

[@ArjanCodes]

Thanks Jordan, glad you liked it!

[@coupmd]

Enjoyed this video. Would enjoy more videos on intermediate/advanced python standard library tools and their examples of their intended use cases.

[@Volcan-kf3oz]

This guy really know what he is doing every video I've seen he able to break it down to where even a person new to python can understand "if you can't explain it simply, you don't understand it well enough." - Albert Einstein

[@ArjanCodes]

Thank you so much!

[@Cookie-mv2hg]

Since we're talking a lot about dataclasses, I wonder would it be great combining with sql database. If we could get a review that would be very helpful!

[@thomasoa]

The error you got one the frozen case was not when you set .name = “Foo”, but in the __post_init__ method, setting _query_string. Is there any way to do post_init when using a frozen dataclass?

[@yakkayakka360]

I noticed that too. Glad I'm not the only one. Did you ever get an answer?

[@neoporcupine]

Working my way through my third LinkedIn Learning (LIL) Python course, all of which claim to teach you the data structures available. Arjan's short video explains a dozen reasons why you want to use dataclass that the LIL courses didn't include. Thank you!

[@ritzg98]

This guy is excellent. I've only watched a couple of his videos so far. Planning on watching a few more today. Clear, concise, accessible. So far, great!

[@ArjanCodes]

Glad you like them!

[@bryan_hiebert]

Just like every video from you my knowledge of programming with python is advanced. I really appreciate the content you provide, thank you!

[@anthonvanderneut]

13:31 Would it not be more useful to freeze the dataclass after calling __post_init__ , so that it errors on assigning "Arjan" instead of setting the (internal attribute) _search_string?

[@ChrisBNisbet]

Hmm, freezing the data class resulted in an error when trying to set up _search_string, which is probably not what you want. Did I read the error output correctly?

[@Rebeljah]

New AC video! I am actually using DCs right now to define a common set of data for server/client communication. It's very nice because I can subclass a super dataclass in order to get functionality like serialization/deserialization fairly easily. Adding new data types just takes a few lines of code now!

[@centar1595]

So you're enjoying the AC/DC video? :)

[@Rebeljah]

@@centar1595 yes I am! Lol

[@MarkHimsley]

At 13:28 the exception is that the __post_init__ cannot assign to _search_string in your frozen data class, not that you were setting the name to Arjan after the data object was constructed. So it looks like using __post_init__ to create calculated fields from the initialised data is not going to work.

[@MrGeordiejon]

Another video explains this and how you can use ```object.setattr(self, value)``` I spotted that too and looked before commenting - 👍

[@mpotane]

Wow, the most complete tutorial I've seen. Not a single detail has been missed.

[@ArjanCodes]

Thanks, glad you liked it!

[@borisvujicic7817]

This is great. ArjanCodes, have you thought about doing a video about Python descriptors? I think it could be very useful, as there is no good ones on KZbin.

[@johntamplin]

Take a look at the videos by Corey Schaffer. A very different style, but equally good, in my opinion. I believe he did one on Python descriptors.

[@sambeard4428]

Geweldig en duidelijk overzicht van deze erg handige feature. Gisteren liep ik precies nog tegen deze use case aan, veel boillerplate code. Dit lost het mooi op. Thanks!

[@Saidffff]

Nice one Arjan! Also, __slots__ give faster attribute access AND space savings in memory due to switching to a more compact data structure. When you create many instances of the class, that is another advantage. Got that from reading 'Python distilled', written by the venerable David M. Beazly.

[@chswin]

Lol a python dev worried about memory allocations…

[@FrankenLab]

@ArjanCodes, great video. I consider myself a casual programmer and have been programming with Python for about 5yrs and absolutely love it. I started programming "casually" in the 80's with C, Pascal, Assembler, then Perl and now Python. I really enjoyed C and Perl, but Python is by far my favorite now. This is the first video I've seen of yours and based on that I just subscribed to your channel. I enjoy your style and explanations and I don't have to play the video at twice the speed waiting for you to get to the point. I'm looking forward to seeing more of your Python videos.

[@aminramazanifar9743]

The final note about slot was really great. Thanks

[@nedegt1877]

I think it's better to have more in depths Python video's like this one instead of trying to limit your options. I mean if you search for Python you will find 1000's of "Hello World" Tutorials, some "How To's" but little technical explanation of the language it self like your video. I believe that if you can learn to master the language you can also master the challenges a language gives you. But since most Python programmers are "Self taught programmers" their knowledge is mostly limited to the sources they managed to learn from. Knowing the functions or having a list of the language API it self is not enough. Some examples of use cases like your video are very welcome. Thanks!

[@songsft.googleassistant2542]

May you have tons of subs Arjan cause you're one of the few youtubers whose python codes aren't messed up and you're instructions are on the point and useful.

[@mampiisaotaku]

i just discovered your channel a few videos back. and what you share is pure gold

[@ArjanCodes]

Thank you so much, glad you like it!

[@naderbazyari2]

I am modifying my code in pycharm right now according to your tips. Awesome video. Much much appreciated.

[@ArjanCodes]

Nice work!

[@equu497]

I wasn't even trying to learn about dataclasses...but it came up on my recommended and I watched the entire thing- Got me thinking I should clean up some old code.

[@flynnowen2478]

Love your content Arjan! I've been making an effort to apply your teachings as much as possible in my code. Would you consider doing any future videos on unit testing? I think it would be super useful. I personally use pytest, but you might know better frameworks.

[@alfonsov3190]

I had a couple of questions during the video, but immediately after they surged, you addressed them. Thanks a lot, as usual!

[@ArjanCodes]

You're welcome :).

[@danieleatzeni2437]

Hey man, cool video! I saw that when you set frozen=True, you got an error in the __post_init__ (not in the main). Is there a way to use __post_init__ for frozen dataclasses?

[@funkmedaddy]

I ran into this issue some time ago, apparently u can't do it directly since that would require you to change the object which is prohibited by the frozen=True statement (according to documentation __setattr__ and __delattr__ methods are added which will throw the error). But u can get around that using object.__setattr__(self, 'field_name', field_value) which is whats happening in the __init__ anyways (i guess it circumvents the __setattr__ method defined in class and calls the definition directly from the object class)

[@LeteFox]

you can use super().__setattr__("_search_string", f"{self.name} {self.address}")

[@funkmedaddy]

@@LeteFox yes as long as the super class isn't frozen itself, altho if you're not inheriting from anything it's more concise

[@kunedroid3446]

I was just on this issue too... I don't know mate, it seems that python is trying too hard to add features which it was not well planned to and we are ending up with so many esoteric rules/details that learning python by anything other than reading books on "how to adhere to the pre-defined rules/standards of keywords" will soon be impossible..

[@ArjanCodes]

I would actually use a different approach altogether if you want a frozen dataclass, but still would like to have a search string like in the example in the video (didn't have time to cover this because the video was already quite long). I wouldn't store the search string in an instance variable, but use a property instead. It would mean that you need to compute the search string on the fly every time though, so it might not be the preferred solution if you need to access it often, but I think it is a lot cleaner than circumventing attribute assignment using __setattr__.

[@JoshSmeda]

I really love your videos on these subjects - keep it up!

[@ArjanCodes]

Glad to hear, Joshua - and I will ;).

[@AbhishekVaid]

The progression of introducing features and clarity of explanation is very high ! Great Job !

[@ArjanCodes]

Glad you liked the video!

[@fvbakel]

Great overview of the dataclass. I wonder what your view is on binding the dataclass to a SQL database. Is there an easy way? It seems that the Django and sqlalchemy methods are not compatible with the dataclass? Further more, what do you think about nested dataclass and the impact on hashing?

[@Rebeljah]

I am 100% gonna use the repr=False option, thank you!

[@arunny9430]

can you please give an example of when it would be a advisable to have kw_only = True? What advantages would it provide?

[@ArjanCodes]

I think it's helpful in particular if you have several arguments that are easily mixed up. For example, if you have a class EmailMessage(from: str, to: str, message: str) and you call it in the code without keywords, it's hard to make sense of what is the from address and what is the to address. Having to provide them as keyword arguments avoids that problem and ensures that the code is easier to read.

[@Kirac322]

Great video Arjan! Learned quite some new things even though I regularly use dataclasses. One suggestion which I encounter quite often in data science: Where and when to properly calculate additional properties (features for my models) after first initialization when there are dependencies between multiple objects, e.g, you have to wait for both to finish initialization, then calculate features. I'm currently working with a hirarchy of dataclasses and calculate these properties in the top level in __post_init__ but am unsure if that's the best way. Thanks!

[@ArjanCodes]

Thanks so much, glad it was helpful!

[@wtfKwaku]

OMG, I been working on this project and for like 3 days I been trying assign a class and return something! Great Video!

[@ArjanCodes]

Glad it was helpful!

[@jma42]

I also want to add that you can actually use asdict method from dataclasses and from_dict method from dacite library. It works as it sounds to be and even works in nested dicts/dataclasses!

[@christensencode7538]

Your videos on dataclasses are awesome! I've used them for a few projects and they really help.

[@WalterBoring]

Cool video. I didn't even know about the dataclasses lib until this. I'm putting it into my project now.

[@JohnnysaidWhat]

Great video really interesting topic had no idea the data class decorator existed. Just a small suggestion. The top end eq on your voice is a little shrill. DeEsser or small eq to lower the high freq bands. Thank you for this video all the same!

[@JorgeEscobarMX]

I'm re-watching this video after a year and now I understand the entire thing. Specially the slots parameter, that flew over my head a year ago.😊

[@barrykruyssen]

Hi Arjan. Great video, very well presented. I've just found your channel and will be watching all your videos as time permits. I'm a retired software engineer (started back in the early 80's). 5 weeks ago I decided, in my retirement, to switch back to linux (and python - which I have never used), just for fun, after 25 years in microsoft products (prior to that I was in the Unix world). I'll be changing 2 of my classes to data classes ASAP. I have just read your Software Design Guide and agree with much of what you've written. I think you have glossed over the most important aspect "Who’s it intended for?". Determining who is doing what, why they do it and what they expect from it and then managing the expectations is the key to a successful implementation. You do mention your “zoom out and zoom in” approach which probably encompasses this but in my planning, the people are the key and if there are problems in this area it can be very costly down the track (we qualified our prospective clients and if the people problem was too great we walked away from deals), as you say we write the code for the client, not for us. Thanks

[@davidlakomski3919]

I've tried 2 days ago to make a package to manage state of jobs with Pydantic, and even if it was less messy than what I did before, I realize now how easier and cleaner it would have been with dataclasses ! Thanks for the excellent content !

[@ArjanCodes]

Glad it was helpful, David!

[@zuydra]

Hi Arjan, your videos on Python are the best I have ever seen on KZbin. Please keep up the good work 👍

[@ArjanCodes]

Thanks so much! And will do 😊

[@mauisam1]

Again Arjan, another fantastic video. If I become half the programmer you are I will be lucky. But I just have the love of programming so I will continue to work on improving my knowledge and skill set. So many programmer say just read the Docs, but I learn from examples and the Docs usually have few if not poor examples. So a big part of my improvement will be in thanks to your love of sharing your knowledge. Thank you...

[@ChrisPatti]

I really love this video! It’s an excellent quick, guided tours to how powerful data classes, can be. Thanks for making it!

[@ArjanCodes]

Thank you Christopher. Glad it was helpful!

[@XCanG]

I use similar thing when working with fastapi, library named SQLModel, what it basicaly is very similar to dataclass, but it has validation from pydantic, so if you, say assign to person with a name type int, like Person(name=1), then you get ValidationError exception. It is more useful in fastapi as it may be used as a mechanism to prevent API being called and executed if provided arguments are not match. And you can have more validation methods in Field, like: le, ge, lt, gt; for list types you may provide min/max range, for strings you may add regex or even something more custom via function. I like it a bit more than just dataclass

[@DustinRodriguez1_0]

around 13:40 you introduced the 'frozen' decorator parameter. The example you were giving was, according to what you were saying, that you couldn't change the name after the instance was created... but that's not the error you got. The error you actually got was raised by your __post_init__ function which was assigning the _search_string field. So it seems that using frozen=True breaks your ability to assign fields in __post_init__? Is there a workaround for that or a different way you can have a functionally derived field like that in an immutable 'frozen' dataclass?

[@VinceKully]

There is a workaround. Frozen dataclass will add both a __setattr__() and __delattr__() methods to the class, and this can be used in the __post_init__ dunder method. e.g. object.__setattr__(self, '_search_string', f"{self.name} {self.address}")

[@cheesecake_mafia]

I think you might find this interesting if you don't already know, but you can define class variables while using @dataclass. You just need to import ClassVar from typing and then when you are defining the class variables, use ClassVar after the colon and then put square brackets and the datatype of the class variable. I think this feature was mentioned in the documentation and also Tech with Tim channel helped me realise this. Great Video! Cheers.

[@ArjanCodes]

Interesting, didn’t know that was possible. Thanks for sharing!

[@amosmunezero9958]

Great tutorial in Python, its been a while I havent done some Python, I didnt know much about DataClass. So thank you.

[@justinpeter5752]

Very good video. I think I learned a lot about classes in this video and I can see where I can use some of these in my code. Well I’m not quite there yet with building classes with multiple inheritance to worry about the issue with slots. But at least I’ll start using slots for performance improvements. Overall, bravo. I’m starting to understand more about Python from a systematical perspective.

[@hisamsowaib956]

Hope you can create a video on how we can use dataclasses with data coming from db. I think that is a more real life scenario. Thanks a lot.

[@marcioneto3016]

Amazing video, as usual. Here's a suggestion: how about a video on the infamous Visitor pattern next? Thanks for the amazing content. I really liked the Software Designer Mindset course, by the way.

[@ArjanCodes]

Thanks for the suggestion, Márcio, and glad to hear you enjoyed the course!

[@qrubmeeaz]

... aaand he's done it again!! Brilliant! Thank you.

[@robboerman9378]

I love your concise and clear explanations. Going to look into your course!

[@NdamuleloNemakh]

Exactly what I've been thinking all along, "strict mode" is a feature that must be seriously considered

[@nyghl]

It was fun to follow along and learn! Thank you ArjanCodes.

[@ninadloke785]

Watching your video for first time. Deserves subscribe!! Great work and thanks!!

[@jkrigelman]

Have you discussed documentation yet? Doc strings and generation tools like Sphinx?

[@christopheanfry2425]

Thank you so much for this video and for the design guide I’ve just download, it’s literally gold 🙏

[@ArjanCodes]

You’re welcome! Glad you find it helpful.

[@amadzarak7746]

Wow! How did I JUST find this channel. You are awesome man!

[@ArjanCodes]

Welcome aboard! :)

[@nicolasRebagliati]

if you use init=False, the repr=False should be implicit. Because if not the repr generated is invalid, because repr should generate an object equal to the original if you copy and paste it. Also you forgot to mention that if you use slots you cant not add attributes in runtime to your object

[@DrSpooglemon]

This does seem like a massive bug. I wonder if it has been/will be cleaned up by the Python maintainers.

[@emory_blame]

Hey. Nice video) Everything were fine and helpful until the 21:00 where you suggest to not using the inherent. For newcomers to python I can suggest video called super - is super. It were very helpful to me.

[@jaa928]

Thank you for the clear and concise explanation. As an experienced programmer, but newcomer to python this was an excellent, example-driven discussion.

[@ArjanCodes]

Happy you enjoyed the content!

[@broken_arrow1813]

Informative presentation. It would be nice if you clear the shell periodically so that outputs can be clearly visible after multiple runs.

[@ArjanCodes]

Will do in the future!

[@matthewnuzzaco2849]

This was super helpful, I used some of these tips almost immediately.

[@Ziggity]

Excellent video, but ever since your video about Pydantic I just don't use dataclasses anymore. Would love to see a follow up for Pydantic as well =]

[@pallu83]

me too, Pydantic is even more amazing and works in older Python versions too.

[@MichaelMorenoPhilosophy]

I think 5:15 is wrong as those are not class variables, they're member annotations as per PEP 526. Initializing them on the same line would make it them class variables though.

[@dmytroparfeniuk2670]

A question about integration tests. What are you doing with the database when you write your tests? Create a mock of the database or use the same database as on the production but running locally (we also have to to adjust the CI to with the external database somewhere), or run create test connection with sqlite that will be used for testing (also simplify the previous approach but only if you don't use any Postgres-specific stuff in your application)?

[@Andremzsptm]

Excellent video as always @ArjanCodes!

[@ArjanCodes]

Thanks Andre, glad you liked it!

[@nusermane1076]

Incredibly good explanation, will try this for myself tomorrow 👍👍👍

[@aadithyavarma]

At 13:27, the error is that, it cannot assign the value to _search_string which we made in __post_init__(), so how do we assign values in __post_init__ when the dataclass is Frozen? Do we not declare it in the beginning and create a new member named _search_string only on __post_init__() ?

[@korn6657]

I solved it by using Object.__setattr__ on the instance. I would say that it's hacky way to do it, though. Since it means you are doing something wrong if you have to do things like this.

[@aadithyavarma]

@@korn6657 Thanks for this tip. And I agree, this really shouldn't be encouraged just because it is possible.

[@8bitscoding220]

Cool video, thanks for it. I think that I would argue that if there are valid arguments against multiple inheritance (and valid arguments in favor of it), the limitations of an oriented object language should not be considered one. To make Python better we have to stop defending the poor design choices that were made in the past and move forward (I'm all in favor of breaking things if I can finally write def myfunc(myarg=[]) like in any other language, for example).

[@SiarheiAkhramenia]

My God, what an incredible sound quality!

[@ArjanCodes]

Thanks!

[@rrestituti]

Amazing video. I didn't know dataclasses and I found it amazing. Thanks, Arjan . Got a new subscriber.

[@ArjanCodes]

We're glad to have you, welcome aboard!

[@andywilson1162]

Great work Arjan, I would love to hear how you use the class when dealing with lots of data, do you use a dictionary of that class for example? Is that the best way, etc.

[@JoshPaulie]

If you're printing an object you'd expect the repr to be printed, not str, no? Thanks for videos! Enjoying this one on my lunch break 😋 Every few seconds I find myself yelling "wait what?!" Learned a lot!!

[@joxa6119]

Hi Arjan. This is a very excellent video. By the way, can I know what is the extension you used when you run the code (I mean when you click the | > button)

[@NicolasDumazet]

At 13:27 it looks like the error you get is not because of mutation of a final dataclass after init. The trace points to __post_init__ unhappiness during the initial Person() construction

[@marknielsen4494]

Super high quality content - thank you Arjan!!!

[@ArjanCodes]

Glad you enjoyed it, Mark!

[@davecobb5649]

Great video, very helpful. What editor are you using? Thanks

[@douglaspiresmartins2955]

Hey Arjan, excellent content. You mentioned Data-Oriented, do you think dataclasses can help too much with that? Help to split "data entities" and "code modules". Perhaps you could do a video about that. Many thanks!

[@ArjanCodes]

Thank you, and for sure that's possible! I like to use dataclasses for representing structured data, maybe add a few methods that operate on the data (but keep it limited), and then organize the rest of the code in functions and modules, and decouple them with abstractions (abcs or protocol classes).

[@shotihoch]

And again... VERY interesting. Started as video for tea-brake, finished in my obsidian, writing down some very interesting tips. Thanks.

[@ArjanCodes]

I'm glad you found the video useful! :)

[@franferri]

Very nice videos Arjan, I have improved my skills a lot with some of the info. May I ask you the brand of the glasses you are wearing? do you have the model.

[@aungkyawkyaw9114]

next level! Thanks Uncle Arjan.

[@alejandropereira5680]

Very useful explanation, to the point and with real use case, really liking these vids

[@ArjanCodes]

Thanks so much Alejandro, glad you liked it!

[@gergelygrosz3181]

Hello! Thanks for the video, it was very useful. Can I ask, what font do you use?

[@SystemSigma_]

Nice overview of dataclasses.. Next, we need an extensive video about the testing frameworks and mocking for TDD!

[@PetrSUsername]

(you know, it just saying for anyone else) Instead of writing the attributes individually in __str, you can use .__dict. :) I know you are explaining everything, though it might be worth to note that if you start writing __post_init, or too /many/complicated fields, it is in my opinion better to go back to regular class. And just write it yourself, it will be more readable probably. Anyway, good video.

[@thekishanpatel]

About the error that was raised after the "Frozen=True" decorator variable was introduced. A frozen dataclass no longer uses simple assignment. (e.g self._search_string = f"{self.Name}, {self.Address}"). Instead, it introduces __setattr__ . Hence when in the __post_init__ method, self._search_string = ... is done, it throws an error because it doesn't know of such an assignment method. So instead in the __post_init__ method, the correct way to set the _search_string would be: object.__setattrr__(self, '_search_string', f"{self.Name}, {self.Address}").

[@ArjanCodes]

Good to know, thanks!

[@zakseipel9494]

Talking about strict mode that was one of the reasons I started using Black and setting it to a very strict set of rules. It forces me to write leaner code and not be lazy. Comes at a price of extra dev time but worth it.

[@91F2Z]

Minor confusion...you use "slots = (min.timeit..." which seems that you are using the reserved word 'slots' as a variable name?

[@f4bglv]

Excellent video, very informative, concise and enjoyable... as always! Thanks again.

[@ArjanCodes]

Thanks so much Fabio, glad the content is helpful!

[@BryanChance]

Is this a new concept to Python? I like playing around with programming languages as a hobby. Is this like using delegates or overloading in other languages like C,Java, or C# but specifically for "data"?

[@timothyjames89]

This reminds me a lot of data models in an ORM, like SQLAlchemy. Would you ever use these two tools together? Thanks!

[@Michael00000001]

That's exactly the point. Do the data declaration where it belongs which is ORM, SQLAlchemy, DB, pandas etc.. IMHO Guido van Rossum would start to cry seeing how his whole data concept is misunderstood.