Python staticmethod and classmethod
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- čas přidán 26. 11. 2021
- What are Python's staticmethod and classmethod?
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More importantly, how do they work? What are they good for? Let's find out all about staticmethods and classmethods.
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One thing that probably ought to have been mentioned alongside class methods are class variables. If you're only accessing class variables with a method, it's probably better to make it a class method. Makes it clear what you're doing and prevents accidentally overwriting class variables with method ones. But class variables aren't all that common to begin with, so this is kind of niche. Still, this is the only major use case outside of alternate constructors (and maybe singlets?) I can think of.
Yes thanks for pointing this out! In reality I pretty much never see this, but it's good to be prepared!
@@mCoding The scenario where I often fall back to using class variables is when I want to bundle some functionality together, but actually never need an instance of the class. Alternatively one could create a module (this would be the functional programming approach), but if the bundle is rather small, it's just a bit less convenient when importing.
I use class attributes all the time. Say, I have a constellation of 5 identical satellites, and my class holds their data. I'll handle the data in a base class, and each satellite gets a class color, or matplotlib position, so I can plot data clearly...for every orbit. There are other satellite dependencies that get handled a class attributes, like "bad channels", since thing break in space. Also: launch date, nominal equator crossing time, etc, etc....
I've been working on a task that has to do with constructing a class instance with additional arguments and couldn't wrap my head around how to do that without breaking the existing API. This video couldn't have come more timely: now that I know that there's such thing as classmethod, I'll explore a bit more about that, and I think this will be handy in solving my problem. THANK YOU for the excellent content!
I have been doing python for years now for research projects and uni homeworks. Basically been using with it almost all the time that I worked. I never got to learn these concepts tho despite trying a couple of times - all of the tutorials are either too basic or too specific to follow. This channel really fills that gap with flying colors, and I cannot stress enough how great of a find it is for me. Thanks, keep and up the good work!
I was just using staticmethods on my project the other day. I just like having the extra namespace for being more explicit 🙂 Cheers!
Yeah I can tell by your name 😂
What a coincidence, I just learned about staticmethod the other day! Great video, great explanations. Thanks very much.
Very welcome!
One reason to use static method is that they are inherited and can be re-defined in sub-classes.
That’s true of all methods.
Oh, you mean in place of top-level functions?
Really helpful to learn what the use cases might be !
2:23 Been searching for a good explanation of why one typically uses classmethod instead of staticmethod for alternative constructors. This explained it perfectly. Thanks!
Thank you for such a helpful video James! , much appreciated brother 👍🏼👏🏽
These videos are amazing for intermediate level Python programmers and professional use. Keep making more!
That's the goal! See you in the next one! (Or maybe in a previous one first?)
For the matrices, I think it's better to do this:
class Matrix:
...
def can_multiply(self, other):
...
and use a.can_multply(b). This way, you don't have to make a map from classes to functions or use things like type(a).__module__.can_multiply(a, b) to check if you can multiply two variables of unknown type.
AND you still can call it Matrix.can_multiply(a, b)
A random use case for static methods I found was when parallelising code. If I call an instance method, the whole instance object would be passed to the child process - a massive overhead that caused pretty much negated the parallelisation gains. A static method allowed passing of only the required attributes to process
A good thing to realize is that editors like Pycharm indicate there is something wrong with a method when the method does not use any instance or class variables, and showing the message "could be static". This causes a lot of confusion for beginning programmers who then change all methods to @staticmethods.
Idk why but I actually like Visual Studio Code better for Python
Thanks for this. I have never used static methods myself and have sometimes wondered if I was missing an important use case. Guess not!
this was truly amazing. THANK YOU
Becoming a CZcams Python legend, one video at a time.
Great work. I'm, so glad I found your channel in it's "early" stage.
I dig you'r thumbnails too. Simple, but class Y:.
Glad to have you aboard! I'm amazed you call it the early stage! I remember when I had 30 videos and only 29 subscribers, that was the early stage for me!!
The black hole joke was funny
It wasn't mentioned, but I think another reason you might call a class method from an instance is if your instance was created from a factory based on an ABC or Protocol, so all of the classes have the same class method with potentially different content.
Although in that case maybe its still better to have an instance method which wraps the class method.
Really great stuff thank you so much for making this
Great video. Thank you
I'm a C# developer learning python and I was very confused because there aren't any similar things like class methods in C# or any other mainstream OOP language (Ts, java, etc). So this video helped me a lot. Thank you!
clear and straight to the point!
Great as always, thanks
One of the best videos of your I really like it.
It's just the arguments that change.
normal: self, *args
@classmethod: cls, *args
@staticmethod: *args
@property: self
Could you comment on what's the size of a class, like an empty class or the one containing some class variables. How does these differ, kind of memory allocation that happens during compilation?
My main use cases for static methods have been polymorphic work that doesn't need access to the cls or instance beyond variations in the type (rarest case), or serializer classes that use methods to (de)serialize a single field (i don't need the serializer class just the day passed in), or unit test classes
I got asked the difference between the two on an interview question last week!
love the full and simple explanaition. i use staticmethods when appling MVC structure to my api's
from the user_model UserModel class i need only sertain methods from the UserModel class. so i can call whathever query method i need in my controllers.
Excellent video
The ending brought so much nastalgia (e.g. binding). Thank you!
So when constructing a pandas DataFrame with the read_excel, read_csv or from_records etc, those are classmethods?
Actually pandas makes them all freestanding functions. I guess they expect that you don't ever inherit from a DataFrame.
I'd like to see a video about the 'global' and 'nonlocal' statements in python where you detail legitimate cases for when they can sensibly be used
Very nice video. I sometimes think staticmethod was added to please programmerd comming from Java.
3:43 any thoughts on
type(self).classattr
vs
self.__class__.classattr
?
Thanks for the video. Although I am gonna have to watch it a couple of times to let it all sink in.
would be nice to mention on typing more. Using mypy can_multiply(a: Matrix, b: Matrix) would have a nice context, imho.
And is it always factory class methods, returning instance of class to keep inheritance, should become Generic[T] where T Calendar subclass, like ‘def from_json(cls, …) -> ???:’ ?
Thanks
Hi James this is a very good channel
I was wondering can you please make a 30 minute python crashcourse covering the basics of python
I’m new to coding and wish to watch a short chars course from your channel as it’s very clear and concise simple to follow
Please consider my request
Thanks
Hey, I suggest watching Corey Schafer he has some good tutorials for beginners. However they are longer than 30 mins!
I really enjoy learning new stuff in Python on this channel, but I wonder how usefull all of this is. Are there any enterprises operating on Python? :D
1:36 In certain countries, the weekend is Thursday and Friday.
There is at least one country 🇲🇾, where certain states observe this rule, while others observe a Saturday/Sunday weekend.
Now imagine the fun of different branches of some organization, operating in different states, trying to decide when you can make a call to another branch and expect somebody to answer ...
I think that writing a datetime library would probably be the hardest library to write that seems like it would be easy at first glance.
Amazing video! But why is Sunday given as "U" in your is_weekend() method?
because then there would be 2 "S" days?
@@homelikebrick42 Thursday is also often an R when using single letter abbreviations of weekdays.
WOW 😳 amazing information.
Love 💕 from Pakistan
Thanks for watching!
Great info, thanks. I use static methods all the time, but I never knew that a class method was a thing!
Glad it was helpful!
5:40 what is "context" exact meaning in Python with the static method
He just meant the location the function is defined and its surroundings - nothing to do with the Python concept of context (manager).
On its own the name `can_multiply()` may mean anything, but defining it as either a static method in some `Matrix` class, or a function in some `matrix` module, makes it clear that the function is meant to be used with matrices.
@@terencetsang9518 Thanks, I supposed to get it.
But wouldn't the class always be the same because the method is defined in a class? If cls is identical to type(self) then how is a class method useful?
I got a notification of a reply to a comment of mine, but now it seems to be gone. Was it deleted? If so, why?
Wasn't me, probably yt auto spam filter.
@@mCoding oh well! Stuff happens 🤷
at line 12, why Sunday is encoded as 'U'?
Because sunday and saturday both start with S, it is common to represent sunday with U.
@@mCoding I see, and 'T' 's are OK as they are not next to each other, aren't they?
@@mCoding By the way, thanks, I did not know that
2:23 ... and that exactly what happens with any immutable class, that has something like __add__, because you can not make method both *class* and *instance*
Also you can not inherit from int for same reasons
0:57 "But sometimes when you write a method you don't really care about the specific instance of a class or maybe you don't have a specific instance yet. That's a good signal that what you are doing is..." applying OOP when you really shouldn't.
Sometimes, maybe! But let's not confuse using classes with using OOP.
2:16 Should answer your argument.
Would class methods cease to be a thing if there were the possibility of multiple constructors?
creating alternative constructors is precisely why classmethods exists.
@@alexd7466 then why not just have multiple constructors?
@@masheroz We have 3 ways to create alternative constructors already, and you still need more?
Since python is not static typed, a C.method() is not better than method(). But in static typed lang, such as c++, C.method() is much easier to remember and write, all you need to do is to remember C.me*, write C. and wait for the compiler and find the function start with me. In python, this trick is not stable even if you work with pycharm.
Im in the middle of a python course and have been watching plenty of videos and a show about programming, how long did anyone else take to catch on?
Keep at it! It can take years to see the importance of some of my videos, don't let them drag you down. Just keep coding and build stuff that interests you. The stuff that matters will become apparent through experience over time.
@@mCoding I'm def not expecting to get a job anytime soon but I have learned a lot and only a 3rd of the way through the course!
Hi James, Good day to you.. I have been following your channel for quite some time. It was very helpful to understand unfamiliar python concepts. Could you pls make a video on UDP/TCP packets logging and manipulation with python. Love from India
Suggestion noted! Though if i were to cover tcp or udp it would probably be a C or C++ video as python would struggle with such primitives.
definitely use class methods as constructor helpers: do not do work in dunder init! It should just initialize instance attributes.
3:09 This can only work in a language where classes are first-class objects!
Discord!!!!
Discord gang
5:50 The fact that you aren't aware of something doesn't mean it doesn't exist. Static methods do have their valid usage, and although rare, it does occur and I've needed it one or two times in my programming career. It happens when you have e.g. a subclass B of class A, and it overrides the method m which doesn't depend on the specific instance--but of course it does depend on the class itself. For example, if you had IslamicCalendar, it would of course redefine what is_weekend means, but it would still be a static method.
Hi no need for the offensive tone on this channel. Your example is not one I would recommend using a staticmethod for.
@@mCoding "When you do use them, the justification for doing so is shaky at best" - do you consider that to be offensive tone?
Again, telling me you don't like what I told you doesn't solve anything. What _would_ you recommend?
Static variable
For implementing classmrthod, you can also just call the constructor of types.MethodType which I find to be cleaner and more readable than calling `__get__` on the underlying function
discord gang 🤙🤙
More like opened my CZcams now gang
🤝
this channel has only one purpose
it is to make overviews of python decorators
1:33 "assuming of course we're ignoring people that live close to black holes."
Physicists love this assumption
Sometimes I use static methods for “private” class methods. If there’s some function specific to the class that happens to be static but I don’t want to expose as part of the public api, I think it’s a good way to signal “this functionality shouldn’t be used elsewhere”
In python, one way to hint that a method is "private" is by using a single underscore.
def _private_foo(self):
pass
The notion is that, this method should not be used outside the class it is defined.
I think get what you're saying, but I don't think "private" is the correct term. And by public API, I assume you mean the module API? Static methods still exist in the class' public API. Like, if it was a module function, it implies that it can be used in any case where it applies, but if it is static, it implies that it should only be used with that class?
@@atrus3823 When you make a method, a staticmethod, it means that the method will not be using any other variables or methods of the class it is in, but still makes sense to be inside the class due to the staticmethod doing something related to what the class is supposed to do.
You can call the staticmethod with the class or the object of the class, outside the class and it still is valid and makes sense.
If you want to make the method seem "private" meaning it does not make sense to use it outside the class, then adding a single "_" at the beginning of the method is the pythonic way to do it.
In short, static and private are two independent features imo.
@@aadithyavarma Why are replying this to me? Did you mean to reply to the original post?
@@aadithyavarma i think he meant that (what i also do) that there are "helper" methods that are both private and static, when you need apply set of instructions on data, like formating, but don't want to write it several times.
assume that they do not live near black holes.
very important assumption indeed
static methods are great, you can import the class and then know that Matrix.can_multiply is a function from the matrix file, instead of importing * and having a random can_multiply function in your namespace
Static methods really are great, but 1) you're mixing classes and files, this is not Java where they are the same, 2) can_multiply is in fact an ordinary method, since a (and also b) is a Matrix. So yes, it is a good example of a method, but not of a static method.
Lately I've been reading big libraries and classmethods are used a lot. I actually haven't need them until now, but I hope I can get the opportunity to use them.
Good video as always. Thanks for the content!
;)
A hacker give this code to you
def password(str_input, str_key): -> int
Password_num = 0
str_key_len=len(str_key)
str_input_len=len(str_input)
for i in range(str_input_len):
Password_num += str_key.index(str_input[i])*str_key_len**(str_input_len-i-1)
return Password_num
This function can take a string and output the password needed to protect the string
Puzzle: Make a inverted function where argument are password and str_key
Input:
Line 1: password
Line 2: str_key
Output: A string
Constrains: str_key cannot have duplicate letter and must have at least character within the str_input
I always learned: If you think about using a static method, you probably don´t want that method in your class. Can anyone proofe me wrong here?
He explained it in the video with the "is_weekend" function.
Staticmethods can be useful to provide context to the programmer that the function belongs to a class. Even if it doesn't need to be in it.
I just like having things in a namespace. I also use it if the function is exclusively called by methods of a particular class. It's then a sign that says "You're not going to need this elsewhere, don't worry about it". Because when I read a module, I tend to read more carefully the standalone functions as things that I might want to call from outside the module. Burying something as an static method avoids that, and can mean that a method is defined close to where it's called, which further improves readability.
But yes, it's a minor stylistic thing, and it can "softlock" future widening of scope.
Confusing......
HELP
thanks. but not right for a beginner learner. It's like reading a file
static methods should just not be in the class since they don't use self.
Dont you want learn french language my friend ?
😣
this is not a good example to illustrate of. Confuses more than simplifies
eee sorry but not very good explanation, but thanks anyway
Hello fellow person with an m avatar 👀. Perhaps you would enjoy my video on descriptors instead?
Jeez, Python is becoming a patchwork. I find Java annotation-like syntax like this to be ugly. It doesn't fit Python at all.
Decorators are not “annotations”. They are actual function calls.
Your explanation is jumping everywhere and make me confuse
Why are you using pseudocode for teaching?
In my opinion this video needs executing and real code. it's not that hard. For me as a beginner it took а few hours to uderstand what's realy going on with my own code.
Please, add some more executing. It will make content so much easier to understand
boring.
lol i don't disagree
HELP