AVOID Making "THIS Stupid Mistake" In Python (It's bad)

Avoid making this stupid mistake in Python. It's can be really hard to catch when your projects become bigger, so it's good just to avoid it immediately.
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[@Indently]

As some of you kindly pointed out, I made a mistake in this video. Instead of checking: >> if not items Please use: >> if items is None The reason being that if you happen to pass in an empty list, the if block will still execute with "if not items" because an empty list is a "falsy value". While if you explicity check that the list is None, it will accurately create a new list if a list is not provided.

[@lyrikalinyoswiminpoo]

Boot camp or Self learning Python in 2023. Your advice?

[@creed404]

We still should check if we passed a list though 😅

[@dragweb7725]

At least if we put the guard "if not items" and we pass any falsy value the guard will still handle it and correct it (we could pass in a {}, a 0 or whatever without having trouble)

[@allo5668]

The problem with headlines like this is that when someone makes this mistake in my team it will be impossible to find this specific video when I search for it. I’ve had that problem before, where I searched “Indently import *” and nothing came up cuz the video was title “this is probably the worst mistake you can make”

[@Indently]

Thank you for you feedback and I understand that completely. I get that a lot with my Shorts. I will try to keep the titles interesting while making them more searchable in the future :)

[@SusanAmberBruce]

@@Indently Yes mate, you come across as a teacher, so we're not looking for clickbait init

[@thebigboi5357]

It also delegitimizes the video and makes it embarrassing to share with coworkers

[@creed404]

I don’t think so, cuz the title isn’t the only factor for videos recommendation, he still can put it in tags or the description

[@NoidoDev]

Pylint finds the error.

[@IlariVallivaara]

The "fixed" implementation seems to be broken too because it does not check for None explicitly, but also replaces an empty list with a new instance. This gives it different and unexpected behavior when passing an empty and populated list, which is not what we want (passing empty lists migh be easily happening in real world software). Example: def adder(x, items=None): if not items: items = [] items.append(x) return items a = [] b = adder(1, a) c = [1, 2, 3] d = adder(4, c) print("a:", a) print("b:", b) print("c:", c) print("d:", d) This prints out: a: [] b: [1] c: [1, 2, 3, 4] d: [1, 2, 3, 4] Edit: For some reason did not see the pinned post. Sorry for redundancy.

[@Indently]

Thanks for mentioning it and don't worry about the redundancy :)

[@arthurs3058]

Exactly

[@NickDanger3]

This looks more like a stupid feature of pythons implementation of default arguments, rather than a stupid mistake

[@chasedunagan]

Watched this video 6 months ago. Made the exact same mistake 6 months later. Remembered this video and came back to watch how to fix it. Thanks!

[@minhphamhoang2894]

It is better to use if items is None rather than if not items

[@Indently]

Very true, thank you.

[@stevenbrown9185]

I can't believe people are nitpicking calling it a mistake, even going so far as to say that an editor shouldn't even alert the user to it. This feature of Python, while perhaps one that has a purpose, is a pretty heinous violation of the Law of Least Astonishment, and that alone makes it something that should have a spotlight shined on it.

[@Indently]

A lot of people don't realise that it's the features of a language that usually lead to bugs, the features themselves are not mistakes, but the way we use them can easily turn into a mistake.

[@Indently]

and by a lot of people I mean like 3 people in the comment section 😂

[@anshg1104]

I have spent the last few days stuck, debugging my code on why the values in my function would still exist, after calling the function again later on. This all makes so much sense, thanks for the video!!!!

[@kruvik]

Thanks for the explanation, got this warning a couple of times in my code but never understood why.

[@5014eric]

I made this mistake yesterday. Surprised to find this video today. I was wondering whether missing parameters default to the value you gave it last time. I had to make a small test program to realise what was happening. VSCode, which warns me about a lot of errors, didn't help with with this one.

[@daniel_bailo]

When typehinting a variable that receives None, it's not better "items: list | None = None"?

[@Indently]

That's an approach I would admire. Otherwise I guess you should throw an error if it's not an optional. I don't always use optionals because they can really drag out the current line, and if it goes off screen it sucks. But I personally like the approach you provided and would use it whenever I can.

[@JusticeNDOU]

in the case above just call add_item and have the function return the list you cant declare a list then give it two different names which is effectively what you did there, you did not create two lists you gave the same list declare on the function two names list a and list b

[@vpr17]

Im a newbie here. How can the function access the same memory location even after its termination? If the list 'items' is a reference to an address, i would assume it looses that address information after function termination. Or does 'items' become a global variable? Or does function call stack work completely differently in python and c++.

[@Nerdimo]

In this example if a user passed an empty list “a” into the function, wouldn’t the returned list be different? It would make sense if you didn’t want to mutate the list passed, but in the case the list is non-empty, it would mutate it. I’d argue you should do if items is not None, not if not items.

[@bogdanborta8861]

Good job man, I like your videos so much,they are so useful ❤❤

[@HarryC4615]

Useful video, Thanks

[@ricgondo]

I shall save this one to my playlist as well!

[@Sinke_100]

This video helped my friend, it is most valued one you made

[@OhertRasmus]

Why did you add `items: list = []` after `if not items:`? Wouldn't it be just fine to do `items = []`? Or is this just a pythonic thing?

[@louisdrouard9211]

It is a type hint for his editor, it does nothing else.

[@nelmatrix3942]

Actually this feature is very useful. I have made use of this particular feature to implement a recursive algorithm that needs to keep track and update the same list object. It all comes down to your use case of certain feature, and how effectively you use it (that is, if you know what you are doing), therefore I won't consider it a useless feature.

[@farisfaikar_r]

Are you using Pycharm for this video? If that's the case, how do you make it minimalistic like that? It's so cool

[@Kommentierer]

It's the Beta UI like in Fleet

[@Randych]

Me at 3:44: "pffffff, what you're gonna do, some guard clause?" Boy...

[@bassycounter]

i feel like I get smarter when I consume your content, appreciate the stuff

[@xzex2609]

It's not stupid mistake if you design it to be , even if you assign it with walrus operator it never be the same list obviously. list_a = add_item('a',list_a :=[]) list_a = add_item('a',list_a :=[]) list_b = add_item('b',list_a ) unless we use it like this it will consider the list_a as a unique list , and the list_b is not a shallow copy of list _a as we may expect it to be

[@xzex2609]

it should behave this way , when you make list_a and list_b you just pass the string and the default value was set in your definition of your own function , its not a class that pass an instance (self) to make two different list , you write it to work like this . no interpreter should give you any warning , ... . try even two nameless list , and you will get dofferent id list_a = add_item('a',[]) list_b = add_item('b',[]) the solution is that you pass the list argument that you provide for your function and you yourself gave it a default value , so the interpreter is not giving you any error , cause you , yourself make a single list , if you pass two nameless list when you declaring your list_a and list_b , then every thing just works fine ,

[@mehtubbhai9709]

yes good explanation. The problem arises because the default empty list is created by reference, not by value

[@anto_fire8534]

why not do something like items = (not items is None and items) or [] instead of a if statement

[@Indently]

Because very few people can read that

[@anto_fire8534]

@@Indently Fair enough however it stops indentation and its not that hard to read if you learn how operators like or and and work in python really

[@Red-bg8xg]

a more readable way if you want to avoid the and & or operators: items = items if items is not None else []

[@Alanpoeta]

Why isn't the empty list in the if-statement also the same list always?

[@allo5668]

You create a new list there each time. But the list in the arguments is only created once. Quirk of the python parser

[@NoidoDev]

Just tested if Pylint warns about this, and yes, it does.

[@deathdogg0]

Please don't get me wrong, I love python, but I recently realized the only reason I do now is because I have worked in many statically typed languages. As a result I don't think I would ever make this mistake now. all that is to say though that it just goes to show that not because python is easy to learn and write means beginners should learn it. I actually think python is awful for beginners because of easy mistakes like these. After working with typed languages with optionals, the None solution is obvious, but again only if you come from a statically typed, verbose language. I think learning how memory works behind the scenes and how other more traditional languages work is another great way to avoid this. It would be nice if python added language level type checking, but I guess we can't have it all. Ok back to binging your channel now

[@kamurashev]

«If not list» is not that good also. Eg empty list is also evaluates for true.

[@user-tq9bu6ki2h]

It's better to use "if items is None" instead.

[@kamurashev]

@@user-tq9bu6ki2h yep, that’s what I meant. Я говорю по русски кстати 😉

[@Indently]

I'm sorry guys, this information is wrong, an empty list is a "falsy value" in Python.

[@Indently]

@Aleksandar, be aware that you must define the default value of the list to be "None" if you want to do that. And empty list is not the same as "None".

[@kamurashev]

@@Indently yep, but “not list” for empty list in “if not list” leads to executing the statements in if block as “not list” would be true. This means that if the empty list is passed as a parameter the new empty list would still be created. Basically my point was that this is a bad practice to do if not ‘smth’ if smth is not boolean. It often leads to subtle bugs. So it’s better to check explicitly the thing you’re actually checking for.

[@kunalsoni7681]

very smart 🤩

[@fafaratze]

haha!, its not a mistake nor a bad practice, its just how it works.

[@ttrev007]

their is a fine line between a feature and a bug. if it is not working the way you wanted it to it is indeed a mistake.

[@JusticeNDOU]

this is not a mistake you are just trying very hard to make it a mistake. functions and variables are the same things in python so if a functions returns a list or tuple or so on , there is no need to assign it to another variable as python will never create the same variable twice. so you have to think of functions as if they are not different to variables , you can still just treat function names as variable names they are all the same things.

[@Indently]

I promise you I'm not trying hard, this is what a lot of my students send me when they have a problem. I'm very aware it's a feature of Python and it belongs to how Python functions. But every bug is a mistake.

[@grayfoxa]

"The biggest stupid mistake" is to use "if" when solving this problem, it is enough to specify "items = items or []"

[@Indently]

If you read some of the other comments you might get an idea on why that isn't a working solution.

[@grayfoxa]

@@Indently why not? it's perfect works def add_item(item: str, items: list|None = None): items = items or [] items.append(item) print(f"{id(items)=}:", items) return items list_a = add_item('a') list_b = add_item('z', []) add_item('b', list_a) add_item('y', list_b) print(list_a) print(list_b)

[@IlariVallivaara]

@@grayfoxa : It is similarly broken as the original fix, as it fails when you give it an empty list instance. In this case it does not update that instance but creates a new instance. This makes it behave completely differently with empty and populated input, as demonstrated by this example below: def add_item(item: str, items: list|None = None): print(f"input {id(items)=}: {items}") items = items or [] items.append(item) print(f"output {id(items)=}: {items}") return items list_a = [] list_b = add_item("b", list_a) list_x = ["x", "y"] list_z = add_item("z", list_x) print("list_a:", list_a) print("list_b:", list_b) print("list_x:", list_x) print("list_z:", list_z) ==== This prints out: input id(items)=139785254467776: [] output id(items)=139785254486400: ['b'] input id(items)=139785254453376: ['x', 'y'] output id(items)=139785254453376: ['x', 'y', 'z'] list_a: [] list_b: ['b'] list_x: ['x', 'y', 'z'] list_z: ['x', 'y', 'z']

[@gardnmi]

I still make the mistake 10 years + of coding with python. The below showcases the concept pretty clearly. def add_item(item, items=[], check_local=False): item_id = f"item_id: {id(item)}" items_id = f"items_id: {id(items)}" items.append(items) if check_local: print("Local Variables") print(locals()['item_id']) print(locals()['items_id']) return item_id, items_id a = add_item('a', check_local=True) b = add_item('b') c = add_item('b', []) print(" Global Variables") print(globals()['a']) print(globals()['b']) print(globals()['c']) Output: Local Variables item_id: 9787744 items_id: 140481494365568 Global Variables ('item_id: 9787744', 'items_id: 140481494365568') ('item_id: 9803104', 'items_id: 140481494365568') ('item_id: 9803104', 'items_id: 140481494401856')