You Can Do Really Cool Things With Functions In Python

Here are a few not-so-common things you can do with functions in Python, including closures and partial function application. Functions are incredibly powerful and you can use them to write code that's really clean and often a lot shorter than when relying on classes and object-oriented programming.
The code I worked on in this video is available here: github.com/ArjanCodes/2022-fu....
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👀 Code reviewers:
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🎥 Video edited by Mark Bacskai: / bacskaimark
🔖 Chapters:
0:00 Intro
1:32 Explaining the code example
3:56 About the Strategy pattern
4:29 Replacing the class structure by functions
9:24 Passing extra parameters using closures
13:37 Using partial functions
#arjancodes #softwaredesign #python
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[@ArjanCodes]

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

[@davia.sampaio8633]

It's really cool to watch your videos, because they open our minds to other possibilities of how to build the same code. You teach things that are not easy to find anywhere else.

[@ExplicableCashew]

I guess it matters how you define the function of the bread. I may be partial to German bread and will happily eat it by itself, but there's an argument to be made that, say, certain French breads shine as part of a breakfast. With American bread, islice it and toast it and it slaps. Italian bread is in a class of its own. I haven't tried Dutch bread, I gotta find some, I wonder if they import it here. And to give y'all some closure, I'm sure any bread from any country can be great if combined with the right ingredients

[@theMuritz]

Dutch bread, it’s simply a slice of very soft very limp toast called Roti … maybe that’s why a Dutch can’t understand other countries being fond of their bread … I’ve been living in Indonesia and they had adopted Dutch bread-making habits, while Vietnam for example offer baguette due to the French, and none of them compares to German variety of course …. 😜

[@alira7296]

Little Vim tip: full stop (.) repeats your last action. This includes entering insert mode and typing things. e.g. at 6:10 you type out "_avg" twice. You could instead just type it once, navigate to the next function, then press . and that'll type out "_avg" again for you. Same with _minmax a few seconds later! Edit: Just watched the part about partial functions. Can't believe I've never seen them before, they seem like such a useful tool!

[@ArjanCodes]

You’re absolutely right. I still have a lot to unlearn from not editing with Vim for about 35 years.

[@astronemir]

Wow. I really need to learn Vim properly and use it.

[@joeymea]

But also in vscode you can edit hundreds of lines at once (and I have done so in a productive way before)... I take people who talk about vim/emacs being best in the same way I take people talking up the King James Version. Sure, it's the Bible, but the language is archaic and outdated. Vim is great, but so is vscode. And this is coming from someone who is efficient in both. I definitely prefer vscode.

[@lawrencedoliveiro9104]

People think Emacs is old, but they forget that it was built by a whole bunch of smart hackers who lived and breathed coding, knew what they were doing, and were not beholden to marketing managers at any company. For example, they used an advanced programming language (LISP), which can still scare the pants off the Java/C♯/PHP crowd today. Does the automation language in _your_ favourite editor understand closures and lexical binding? (Interestingly, the sort of concepts discussed in this very video.)

[@ArpadHorvathSzfvar]

I like Vim too but it's not good at bigger refactors. So I use PyCharm for bigger projects, and sometimes I just switch on the Vim plugin to do some stuffs. For smaller things (indenting the whole file, removing indentation, adding comma to the and of lines, join all the lines, removing every fifth lines with macro) I just start a Vim in command line.

[@fschlz]

You sir, are putting out the best Python content on the internet. Thank you!

[@Jorrit_200]

I've temporarily moved to Finland from the Netherlands, and I can confirm: the bread is better in the Netherlands. There is a even bigger difference in cheese though: the cheese is horrible here.

[@TheStickofWar]

I feel this way about cheese when moving to Norway

[@ratfuk9340]

Finnish rye bread is unbeatable.

[@MisterMobius]

wow. I am currently working on a project where I use functions that return customized functions, just as in your example. So partial just blew my mind a little. Thank you so much for introducing this awesome tool to me :)

[@raeganb.7254]

This came in handy for my weekend project! I’ve watched through all your Python videos so far, and my code has started to improve dramatically as a result. Thanks for sharing!

[@ArjanCodes]

You’re welcome - glad to hear that the videos help you improve your code.

[@XRay777]

functools is amazing. Besides partial there is also reduce, wraps and lru_cache which I use all the time. singledispatch can also be nice in very simple cases but it quickly hits a roadblock in more complex scenarios.

[@prestonfrasch9270]

Hi Arjan! Thanks for all your work teaching and coding! I came accross partial when I was working with python's multiprocessing library for performing a task with many files with some "standard" parameters that were the same for each and some "variable" parameters (such as the dataframe) that were unique to each process. This video gave me a better context of what partial actually is/does, and it's helping me with an NLP project refactor. 😄

[@robertbrummayer4908]

Great video, Arjan. I also use the functools a lot, e.g. partial. What I really love about Python is that it supports many different approaches and does not force you to use a single one. As a developer I can choose between many different approaches, and choose the one that I think is the best to solve my current problem. Python lets me express myself in code directly in various ways.

[@ArjanCodes]

Thank you Robert! It’s indeed one of the powers (and dangers) of Python that it allows for so much flexibility.

[@jordansilke3629]

Very informative! As someone who was previously unaware of partials, I have achieved similar functionality by setting constants and using them as default parameters (as you mentioned). This approach seems cleaner, though, so I will definitely give it a whirl in the future!

[@f0lkien]

Man, i'm programming for 15years or more. Mainly c,cpp, python only 3y, but ive newer knows this closure and partial mechanisms! Great!

[@rolandovillcaarias5112]

Thank you for your time and patient, the DOC contains a beautiful explanation in 7 steps about design better.

[@bartuslongus]

get_best_bread = partial(get_bread_from, country_code="de_DE") :) Great video! I was astounded seeing the closure part and then really impressed seing partial() in use. The flexibility and simplicity of providing values for named variables and using it on any function is amazing. My wife is gonna get jealous of the love I am developping for Python. Thanks for sharing this!

[@luandasilva4639]

Great video as always! Partials are nice. I wrote a validate(validate_func, data_type, data, ....) function that I could then call from a partial like validate_json = partial( validate, json.loads, "JSON") or validate_uuid = partial(validate, uuid.UUID4, "UUID").

[@alexanderzikal7244]

I like the last example with "partial functions". Still Composition, but very flexible. Thank You again for your teaching!

[@ArjanCodes]

Thank you for the support, Alexa! Glad you're enjoying the content!

[@flakobatako]

I'm in love with your content. I think I'll purchase your course the SW designer mindset. Keep it up my man!

[@HexenzirkelZuluhed]

I've been using partial() for similar applications before. Well explained.

[@eduardomoraes2819]

I’ve been living in the Netherlands for 3 years and you are the first dutch that I know that doesn’t like sliced bread 🤣 Very good video, as usual!

[@virtualraider]

This is one of your best videos yet (and they're all consistently great!) I had to post a comment just to give extra likes 👍👍👍👍👍 The bits about closures and partial are BOSS 🤓

[@ArjanCodes]

Thank you so much!

[@iliqnew]

Once again, Arjan! Super helpful. Thanks!

[@multigladiator384]

repeat and partial can come in really handy when it comes to multithreading and passing arguments to the executed function

[@nedegt1877]

Bedankt! Thank you, your video's are really helping me getting the most out of python and I really appreciate that as a self taught Pythonista :)

[@PanduPoluan]

My mother used to make bread with recipes that undoubtedly came from the Netherlands (as my country was a Dutch colony and before that we have no bread) and I'll say they're great! 👍🏽

[@cmdlp4178]

Very useful if you want to write to a file using print: with open("output.txt", "w") as f: fprint = partial(print, file=f) fprint("Hello world!") Same for printing to stderr import sys eprint = partial(print, file=sys.stderr) ... eprint(f"Error occurred: {error.description}")

[@smalltimer666]

I really like how your tutorials have never been completely OOPs. A functional-leaning python programmer is a rarity :D I wonder if you have a playlist of videos on functional-ish concepts. The way you describe things is perfectly applicable and would be a very clean and organised way to write julia code as well.

[@siddsp02]

I wouldn't expect a lot of those types of videos due to Python's fairly limited support of functional programming. A few functional modules that are nice to use in the standard library (which might be covered later on) are itertools, operator, and the map, any, all, and filter functions.

[@smalltimer666]

@@siddsp02 Thanks!

[@DistortedV12]

Would also appreciate a treatment of the functional approach and its strengths. Also, mentioning how to write good functions in practice.

[@DerekHohls]

Actual of lot of folks who are not programmers but who use Python for basic data processing typically would never use a class at all.

[@jb_lofi]

@@DerekHohls Yes. I think this tends to get understated -- I've done a lot of Python and PowerShell "programming" for a lot of real-world enterprise uses, and I very, very rarely have to use an OOP approach when coding something from scratch. I rarely have to define my own classes at all; I wouldn't say most/all/anything superlative, but there's a lot of people who understand OOP but just never need to use it Python in that way.

[@Whatthetrash]

While I don't follow everything you do, I like to watch your videos because it exposes me to new ways of thinking as well as showing me the practical reality of what is typed to accomplish an objective. Very cool indeed. :)

[@exganza]

man your channel is underrated!! you are really provide a high quality contents with a great knowledge and experience, partial function are awesome i need to use it more, thank you.

[@ArjanCodes]

Thanks Ziad, happy you’re enjoying the content and it was helpful!

[@FirstLookVaper]

you told me to buy Dutch bread after getting the free guide, but I see no link in video description for where to buy it, so I guess I will just keep living in my "bread fantasy world". Love your vids. They are always very informative. Thank you!

[@markgacoka9704]

You have the cleanest code I’ve ever seen in my life!

[@rtiodev]

Great video as always. Keep going!

[@wernerlippert5499]

No, no, no - you definitely did not fall victim to the Dunning-Kruger effect (about bread), you just have a very good sense of humor.

[@lawrencedoliveiro9104]

12:35 But you could replace it with a docstring. Which could be dynamically generated, to incorporate the actual limit, and assigned to the ＿＿doc＿＿ property of the function object being returned.

[@jeancharlesmourey]

French bread is of course better by far than any other bread, but I guess it wouldn’t be fair to include it in the competition as there wouldn’t be any controversy.

[@jasonmcclatchie6877]

I am English and I'm still willing to back you up on the superiority of French bread!

[@obed818]

@@jasonmcclatchie6877 thanks hehe

[@HellsJayBells]

Good stuff here. I hadn't come across partial before either. I feel like you could have squeezed in a mention on decorators. Yes the closure construct is a little clumsy, but using the decorator syntax, it's a little less clunky. It seems to me that partial is used when you need more granular customization, where as a decorator helps with a more general replacement. Good video, thank you!

[@AnotherAvaibleName]

Thank you for providing the github repo of the example. Nice!

[@ArjanCodes]

You’re welcome!

[@littlegiant4400]

Hi, Your videos are awesome... one question though. How do you place that run button beside the split editor right icon in vs code, are you using any plugins/extensions?

[@Betacak3]

I'm German and I'm eating delicious bread as I'm watching this and I'm highly offended.

[@vladyn2522]

Thank you for another great video Arjan! also, your anecdote reminded me of a similar pun "no matter how kind your kids are, German kids will always be kinder"

[@orenozeri]

Hi Arjan, Your content is great, found that one amusing in particular, since my wife is Dutch living in Israel for almost 20 years, while still complaining over the local bread taste and praising the Dutch version 🤣hilarious !!! Thanks for the effort you put into the content. 🙏

[@LuddeWessen]

Excellent, as usual! The partial function will be used tomorrow, replacing a (very cumbersome) workaround. Thanks👌

[@ArjanCodes]

Glad to hear it was helpful, Johan!

[@oncedidactic]

I constantly write code that starts dead simple and expands in options progressively, ending up with very customizable functions. I’m gonna use partials to take care of this now!

[@SUGATORAY]

So what would be the difference between using closure and functools.partial? When would you prefer to use closure over partial?

[@omerpriel5588]

Like many good things. It's really convenient and right when your team knows each other about it.

[@michaelbamiloshin1670]

Even if I'm not yet at the level where I have to implement most of the things you share, I still go ahead to watch your videos anyway. You teach well. That said, I thought I should point out something to you (and to those not familiar with this style). At minute 12:51, you add two additional zeroes, presumably to make the number 35,000. But that actually changes the number to 3,500,000. (And I confirmed this on my python shell.) 35_000_00 != 35,000 Instead, 35_000_00 == 3,500,000 35_000 == 35,000 3_4 == 34 (just to show those who might not know that you don't always have to end with zeroes) In essence, it seems to me you think adding two zeroes after the last underscore means python will treat the variable as a float. It won't. It'll just add two more zeroes to the integer. If you want a floating point number: 35_000.00 1_2.3 12_34_567_8009.027 are all different valid examples.

[@jjenn050]

Money is best dealt with in integers because floats will have errors dividing in binary. It is 35000.00. When returning values, you just have to deal with the extra 00 as cents.

[@elidc93]

I rarely comment on video tutorials, unless it gave me that "aha" moment, and this was one of those, thanks ArjanCodes for educating us! btw, have you profile which "way" is more faster, the OOP or FP style?

[@josgibbons6777]

Someone probably mentioned this already, but if you refactor the functions you're passing to partial so the arguments you're specifying therein are to the left of the one the new function still needs, you don't need to pass them by keyword.

[@ryanchou2846]

Is there any difference between using a partial function and a lambda function? For some f(x,y), shouldn't lambda x: f(x, y0) be equivalent to partial(f, y=y0) ?

[@davidbeauchesne7399]

Nice! Thanks for another great tutorial.

[@ShubhamMishra-gc5ow]

Hi, please make series of advance level python. Your way of explanation is very cool and the set of example you used is very cool.

[@rogifedi1]

Good stuff, thanks for this great video!

[@lawrencedoliveiro9104]

10:38 I like to use a name like “def_should_buy_avg”. The extra “def” on the front looks like the keyword for defining a function -- which is what the function does.

[@ravenecho2410]

Functional patterns are awesome and one of my favourite ways to structure code, u see it a lot in scala or heaven forbid R (their class like defns are terribad) But scalas map, filter, reduce, foldleft, currying or partial hell maybe i just used clojures and they got tail recursive. I had all of my steps be like functional but isolated environments be classes. I think both are cool

[@alfonsov3190]

I sure learn a lot from your videos. Thank you so much!

[@ArjanCodes]

You’re welcome, Alfonso!

[@petermleigh]

Great video. I've started to use partial more and more in my functional code. I was using lambda for the same result, but partial is a lot better.

[@martinleonhardt1541]

Interesting! What are the perks? In short perhaps...

[@petermleigh]

@@martinleonhardt1541 Partial will work around mutable types and closure better. Lambda got me in a pickle when I tried using it in a list comprehension.

[@elecdron]

Ok, dudes. I live in Russia and I can say that Russain bread has a taste of a cardboard.

[@THEMATT222]

Is it because of the sanctions?

[@elecdron]

@@THEMATT222 no, it always had been that way

[@THEMATT222]

@@elecdron Oofty doof oof oof

[@iansullivan9738]

@@THEMATT222 Google translate for the win

[@THEMATT222]

@@iansullivan9738 How so?

[@jonataseduardo]

Great video! Its begs the question of when a functional approach is better then a objected oriented one.

[@lawrencedoliveiro9104]

I have used both together.

[@davidlakomski3919]

Nothing really related to your video, but I wanted to thank you for your work. Not only it is very well explained but it really inspires me by debunking some dev "features" and mindset related to development. I don't really study your videos (sometimes still too advanced for me), but they show me every time the beauty of code and keeps me learning further. Furthermore, I tend to really enjoy your both light/humble and technical tone, I'm from marketing, and your channel is doing marketing right focusing on content value first. So again, thank you for your work :)

[@davidlakomski3919]

Just yesterday, it inspired me to have fun playing with inheritance and composition with PyMongo's source code (not touching at it, but making my way around it for some purpose). Trying to deep dive into code like you do, finding beautiful and fun features, far from just focused tech tutorials, more of a technical case study / learning experience. thx !

[@AleksanderFimreite]

The partial function calls looked really practical. Though I can't really recall seing anything like that in other coding languages. A more generic way of dealing with varying parameters would potentially be to pass in a generic payload class, then cast that to the expected version inside the function.

[@todorowael]

Thank you for the great tutorial and all the interesting Python content on your channel!

[@ArjanCodes]

Glad you like them!

[@pelissargiosergio]

Great content, I work in the Netherlands as a Python Backend Dev and and the bread is okish haha!

[@iklintsov]

Man you are the best! Could you make a video about design patterns of libraries like django or scrapy?

[@Megyez]

How would you do dependency injection for functions? For instance I have several functions where I want to use a database, for this I have a class or function which I want to re-use and mock for unit tests. If I use classes I would inject this dependency in the constructor. In the composition root I would assemble my objects. If I would do the same on function level for instance with te partial() it sounds like a lot of configuration, and a bit unclear for me how to structure the code then.

[@collegeinvestor7095]

17:55 where did the partial grab the prices from however if it was not told..?

[@ankurgaikwad7252]

What is the name of the theme you use in VS Code? I love the colour scheme of syntax highlighting in your videos.

[@sillytechy]

Hi Arjan, Please continue the Design Pattern Series, You can take one by one different design patterns with practical code for python. There is not much good content out in youtube.

[@nickhodgskin]

Love your videos Arjan!

[@ArjanCodes]

Thanks Nick, glad you like them!

[@fabhi]

Love your videos man

[@ArjanCodes]

Thank you!

[@georgetait864]

Do partials have any affect on time complexity of a function? Say you have a function where the part that is partially applied takes O(n^2), and the remaining part would be O(n). When calling the partially applied function, would this be O(n), because the O(n^2) part has already been handled or O(n^2) because it hasn't? This seems quite niche, but im currently working on graphical demonstration of time complexities and partials could be very helpful as I could only look at timing a specific part of a function. I watched this video a while back and remembered how useful I found it

[@javibits]

Always learning something from your videos. Great content

[@ArjanCodes]

Awesome! Thank you!

[@linuxmill]

I'm from the US, and I can vouch for the excellent NL accent! ...and I'll grant you the bread assertion

[@vardantheunderdog]

It's nice that you can replace interfaces and their implementations with partial functions. However I'm still concerned with what happens in cases where we need to do some error validation and type checking, any ideas?

[@P0lntL3sS]

I'm not sold on closures. Besides encapsulation what are they good for?

[@igorkuivjogifernandes3012]

Yeah, I'm liking your videos. I'm not a software engineering but I use python a lot to do data science.

[@grokes_workshop]

I totally agree about bread, I think the Polish one is the best ;) Anyway, another great tutorial, thanks!

[@katzy687]

Forget where I saw it on youtube, but someone commented that most classes are basically "collections of partially applied functions". In that scenario where I have similar methods, I would rather have user instantiate a class once than do the partial applications repetitively for the different functions / methods

[@hcubill]

Great video!

[@Julie9009]

Hi Arjan. Thank you for the instructions on passing extra params using closures. I bookmarked this video when I first watched it because I suspected that it would be useful, and today it was exactly what I needed to add functionality to the callback function passed to ftplib.retrbinary()

[@Erwipro]

I really like using functools.partial when using .apply on a pandas Series. It saves me the hassle of writing a lambda function. But now that I'm looking at it, .apply also can take the *args and **kwargs for that function so I don't even need a partial function :)

[@NicholasShanks]

I'm interested in what keys you're pressing. How do you: 6:06 forward-delete an argument including the following comma and space up to the next argument? 15:31 delete a whole line at a time incl. LF? 15:49 delete to the end of the line? 17:59 forward-delete up to the end of the argument list? Also British people know that our bread is the worst so we don't even pretend it's good.

[@comedyclub333]

While I use functools in many occasions I always wondered what the (technical) difference is between the partial function and creating a lambda (or even a function) simply reducing the arguments? I mean, i could also do buy_strategy = lambda prices: should_buy_average(prices, 4) While "partial" usually increases readability I wondered if there are any differences in terms of performance.

[@StanislavStratiev]

I am also interested in this.

[@marsma18]

Doing that, you use closure, but in simpler form.

[@comedyclub333]

@@marsma18 kind of, yes, but not in the way he showed closures in the video as my approach IS a function with a valid signature actually calling another function while the approach in the video RETURNS a newly generated function with a valid signature. Moreover I was interested in the technical difference than the logical difference.

[@RitchieDiamond]

@@comedyclub333 The only real difference that I came across was while implementing a naive save system in my game engine (naive in the sense that I just dump whole objects, including attached callback functions and everything to file using pickle). Now callback functions cannot typically have arguments, but often require a variety of them, so I end up using functools.partial fairly often - because lambdas are anonymous and cannot be pickled, while partial functions can.

[@comedyclub333]

@@RitchieDiamond That's actually something really interesting. I just tried it and you are right! It seems that lambdas really are not savable into serialized data. Crazy!

[@marcdelabarreraibardalet4754]

Nice video as usual! Question on partial. Before knowing about it, I used to do: buy_strategy = lambda prices: should_buy_average(prices, window_size=4) vs buy_strategy = partial(should_buy_average, window_size = 4) I think the result is the same. Are these methods equivalent, but partial with less code?

[@chriswunder5420]

There are a few differences I guess. For example with partial you can change the should_buy_average function (as long as it still takes the window_size) and need not change the partial statement, whereas you would need to change the lambda statement because it explicitly uses the parameters the should_buy_average function expects. You can also change the parameters' order of the function and the partial application still works.

[@alvaroe2704]

I have studied industrial engineering in Spain, with little curriculum in programming. Is it possible that I prefer functional programming due to my background in engineering? I.e. decompose the 'problem' in little steps via functions and then there is a main function that calls functions which in turn call lower level functions...and so on.

[@valseedian]

man, I wish so bad that c++ had something like "partial". it could make function pointer lists for weakly typed scripting much more manageable and most of the heavy lifting could be done by the compiler.

[@uniquenameification]

Nice! I’ve been writing Python for ages and didn’t realize partial was in there. Re:bread you need to try a good shokupan …

[@thomasferreiradelima2324]

Hey @ArjanCodes, unrelated question: What's this sweater you're wearing? Thanks!

[@aungkyawkyaw9114]

Really cool indeed!

[@DoubleT847]

I have just discovered the channel. This is Golden

[@ArjanCodes]

Thank you, glad you like the content!

[@ibrahimaba8966]

functools and itertools are realy cool modules to master.

[@ravenecho2410]

I implemented a basic linked list and a tree with closures in python, theyre super awesome but didnt know they were performant. I wish there was a better way to illustrate to the reader besides docstrings what a closure does... and if im not mistaking, its the only TRUE way to have a private variable (when supplied at run time) Definitely did not know the underscores with numbers, i just wish that there was like tail recursion call optimization in python. That hurts my soul, if we had that and a quick struct like rust i would go full functional in my patterns

[@MrTrebor2]

great job

[@barricuda5]

Thank you for this content

[@ArjanCodes]

You’re welcome, James!

[@Seawolf159]

You said 35_000_00 is thirty five thousand, but you also said that the underscores doesn't change anything about the integer value so it would be 3500000 so that wouldn't add 2 decimals randomly? So it should be 3_500_000 which is 3,5 million?

[@etiennebarre6226]

- netherlandic bread is the best. - hold my croissant.

[@amolsv]

What is "recording" on every line of your terminal?

[@michaelhaag3367]

Haha, loved the bit about the bread. I am from germany, and I think our bread is WAY better than the bread in every other country in the world ;) (maybe the netherlands can compare)

[@davia.sampaio8633]

Have I ever said how much I like your videos?

[@roshan8853]

Your bread tangent made me laugh a lot, thank you haha

[@Eriddoch]

Does partial() mess up autocompletion?