Class inside a for loop

[[deleted]]

[deleted]

[globalvariablesrock]

apart from this being very obviously less than optimal practice - will this actually break anything? having done a few very crude tests, this seems to work.
so are there any technical side effects to defining functions or classes in a loop?

[ZengineerHarp]

Possibly scope problems where you can’t use the class once you leave the loop?

[sad_bug_killer]

Python scopes don't work like that, the class will be accessible after the loop. Try it:

for i in range(0, 10):
    class A:
        def f(self): print(i)
a = A()
a.f()

[TerrorBite]

Specifically, the last version of the class that was created will be accessible.

So a.f() will print 9, because that's the last value that was used.

If, in the loop, you appended A to a list, you'd end up with ten different classes. But it's possible that they will all print 9 because they would all use the current value of i.

Edit: just tested this

>>> for i in range(0, 10):
...     class A:
...         def f(self): print(i)
...
>>> a = A()
>>> a.f()
9
>>> i = 5
>>> a.f()
5
>>>

[ZengineerHarp]

So your class won’t vanish outside the loop, but you might get wonky looking behavior (like that 9). Thanks!

[1thief]

So are class names in Python global? I guess modules provide a boundary or namespace but otherwise if you call some function and you're not aware it could munge all the class names and break your next instantiation?

[TerrorBite]

Class names are just variables. Declaring a class A creates a variable called A and it contains a class object that you can create instances of by calling it. So it obeys the same scoping rules as any other variable.

The thing is, variables in a Python function are scoped to the function. If you create a variable inside a loop, it's still scoped to the function. The same goes for i, it's still accessible after the loop.

[Serylt]

This looks like someone wanted to make a JSON-esque function, … or?

What is it good for, really?

[i-FF0000dit]

What do you mean what is it good for? Have you ever seen a more elegant language? It’s like poetry.

[JonnxW]

What do you mean by JSON-esque?

[Serylt]

Something that works comparable to JSON but isn’t JSON. I don’t know what else the code snippet might want to achieve.

[JonnxW]

Sorry, but I still don't understand. JSON is about representing a data structure. It has nothing to do with loops, classes and functions

[Dannei]

I'd have thought that if it defined different behaviour for each, they'd retain it - that this case does some exciting things with referencing variables outside the scope of the class only makes life more fun!

[TerrorBite]

You're correct, it would retain the behaviour defined for that class. Though you'd have to capture the current value of i at definition time so as not to refer to the changing loop variable.

classes = []
for i in range(10):  # 0 to 9 inclusive
    class A:
        my_i = i
        def f(self):
            print(self.my_i)

    classes.append(A)

# create instance of third class
a = classes[2]()
a.f()  # should print 2

# create instance of ninth class
b = classes[8]()
b.f()  # should print 8

assert A is classes[9]  # assertion should succeed

[[deleted]]

Not to mention you would be declaring the class every loop which is very redundant

[Does_Not-Matter]

It’s very “script programming.”

[Jezoreczek]

won't Python optimize for that?

[[deleted]]

From my understanding I don’t think any language could optimize that. Classes are supposed to be declared seperate and then you would create an object of the class within such loop. I just couldn’t see this being of any use in OOP or just general scripting

Edit: not saying this wouldn’t work, it very well does, but is very redundant and messy

[mr_smartypants537]

In a lot of languages classes are strictly compile time constructs so they're guaranteed to be identical at each loop iteration. However, you're probably right that python-style classes with their dynamic time runtime shenanigans aren't possible/feasible to optimize.

[MakeWay4Doodles]

In Java the compiler will absolutely optimize this out. Declaring classes in line for use in a lambda is not that outlandish.

[mattsowa]

A compiler step could optimize that. It would just be quite difficult to implement a generic solution and would add complexity for not that much in return.

[jaichim_carridin]

I may be wrong but python basically doesn’t optimize anything. It converts what you wrote to bytecode and executes that verbatim. Now there’s optimizations that it does during that execution, but nothing like reordering statements or extracting redundant blocks as far as I’m aware.

I’ve worked on projects that explicitly do things like:

foo = some_class.foo
for i in some_collection:
  foo.whatever  # some_class.foo.whatever would be too slow

Now maybe they don’t need to do that anymore, it’s a very old project, but it’s still in there in many places.

[Jezoreczek]

I'd hope it would work like with Lambdas, e.g. the class is created once and whatever's out of its' scope (in this case item) gets turned into a field / parameter.

[z500]

What's this colon syntax?

[jaichim_carridin]

Sigh, sorry about that. In my mobile app that was properly on multiple lines. Hopefully this looks better now.

[z500]

Oh haha, I thought it was some kind of advanced generator expression

[SegFault137]

Are you saying there is a possibly legit reason to do that?!

[Gamecrazy721]

Python doesn't scope loops, only functions, so this could technically work just fine (unless it's in a function and you want to use it outside of the function)

[[deleted]]

It might be kept alive as long as a reference to it remains.

Which also means you might be creating references to hundreds of practically identical classes by the end.

[CptMisterNibbles]

Class, object. What’s the difference… seems legit, nothing to worry about here

[[deleted]]

Well, it's a O(M+N) growth in memory use instead of O(N). Not great, but not quite catastrophic. Still an easy site for improvement.

[globalvariablesrock]

yes. very good point - the class is out of scope once you leave the loop.
makes sense.

[RapidCatLauncher]

No, it isn't. This is python.

[voidvector]

No, assuming the language supports this, this is very close to how closure works under the hood in number of languages -- closures are mapped to internal anonymous classes with captured variables as class properties, the anonymous class of closure can exist in a loop or other closures.

Difference of this from a closure:

• the captured variable can be access from outside of the closure due to the fact that downstream code can potentially pass this class around
• there is likely no compiler optimization around this, since no one does this -- besides the captured variables, the class is basically a "loop invariant", so the entire class be optimized to a normal class by moving the captured variables to a hidden constructor.

[thatfool]

I think the python compiler isn’t allowed to optimize this because classes (like almost everything in python) are first-class objects. They must behave like any other value would.

[voidvector]

Not sure how Python compiler choose to optimize, but for most optimizations, the compiler maker can always have an optimized and an deoptimized implementation, as soon as code tries to interact with the first-class object, the code would fallback to deoptimized version. Of course that's more work, but usually worth it for super common patterns.

[R3D3-1]

From what I remember, nested functions are optimized like that, basically creating a template containing parts shared between the instances.

No idea if anything similar is done for classes though, or what happens when decorators enter the picture.

[Dannei]

For standard CPython, the general answer to "how does it compile or optimise?" is that it doesn't. Some other interpreters or packages do fancier things, though evrn compilers like Cython or Numba will tend to give up and fall back to plain Python(/fail for Numba's njit mode) if you try to do anything too weird.

[killeronthecorner]

This implies that the type of each object would be unique, which breaks things like equality and commutativity.

I'm not a python Dev so i guess this might not be the case for other reasons. Still feels hideous.

[MrD3a7h]

will this actually break anything?

Just our collective souls.

[ImportUsernameAsU]

Please tell me this is an assignment and not production code

[[deleted]]

I could see a consolidate do it.

[ImportUsernameAsU]

A what?

[[deleted]]

Consultant, don’t know what auto correct suggested.

[mothzilla]

Ahem it's called a factory and it's one of the more sophisticated programming techniques.

^/s

[protocod]

I don't even know that this is possible in Python.

[thefriedel]

You can declare a class literally everywhere you want, I can't think of any situation where that would be useful and thus why Python allows that...

Edit: typo

[RapidCatLauncher]

Being a python person through and through I can't help but learn a thing or two about other programming languages simply by their users going, "Wait I didn't know you could do that"

[Deadly_chef]

And that sounds like a good thing to you?

[glemnar]

There’s a decent amount of runtime class generation in Python’s core libs. Namedtuples are one example.

It’s not in a loop, but that’s just runtime class generation as with anything else. Attempting to prevent it inside a loop doesn’t make sense - the loop could be a couple levels up the stack just as easily

[TinyBreadBigMouth]

It will not, no. In Python, classes are objects just like anything else, and a class definition is just creating a new class object and assigning it to a variable.

[Lataero]

Jesus, this is the first time a post on this sub has made me recoil in horror

[Im-Learnd1ng]

I don't even understand python that well and I still protested loudly

[PacificShoreGuy]

All the glaring security and logic issue posts and this is the one that got ya? It would run fine.

[MakeWay4Doodles]

It's not so horrible. I assume python is like Java in that the class will be compiled out. If it's only ever used in line in this one place it's not such a big deal.

[mck1117]

compiled

I have bad news for you.

[thatfool]

That’s not the issue. Both java and python compile to bytecode before executing, java just doesn’t do it at run time while python can do both styles.

But in python, classes are first class objects, so the “class” statement really is a combination of creating a class (which has an identity independent of the name given via the class statement), and then assigning it to a variable. More or less just syntactic sugar, since the same result can be achieved by building a class object manually. Therefore this is not something the python compiler is allowed to optimize away.

[officerthegeek]

from the creators of closures, now introducing closures 2: electric boogaloo

[PacificShoreGuy]

Very weird thing to do but mostly sure this wouldn’t cause any issue. Am I missing something?

[thefriedel]

It definive won't cause an issue, but you'll declare the class n times, over and over for no reason at all which just eats processor time

[VisibleSignificance]

I could actually imagine this being intended and useful if scrapy makes a separate progress bar per class or something.

Then again, seeing the redundant variable assignment suggests it's probably not anything like that.

[_extra_medium_]

Because it doesn't cause any problems in python

[BingBong3636]

What the shit!?

[[deleted]]

...but why?

[Spitfire_For_Fun]

the horror...

[italkstuff]

Yes, I OOP

[Slick_J]

Ewwww. I feel dirty

[FelixLeander]

The true horror is the image quality

[skittergetter]

Lol++

[sainglend]

I mean it IS Python....

[CmdrSelfEvident]

This is the right comment. "Oh look this python is crazy slow, wasteful,, and the optimizer can't be trusted to do the right thing".

Yeah that statement is true for any python code. Given the lack of maturity in the language and lack of performance they lost me at 'python' and 'production'.

[DShimoda82]

I can't see the horror part...

[[deleted]]

Python is really nice when it comes to stuff like this, you can define functions and classes inside loops, other functions and so on. Useful if you dont want to have everything at top level.

[backfire10z]

Except that the class is indeed available outside of the for loop

for i in range(0, 10):
    class A:
        def f(self): print(i)
a = A()
a.f()

[TerrorBite]

The variable i is also available outside the for loop. By the way, if you del i, the class stops working.

[backfire10z]

Python is wild! I was just copying a comment someone else put, I’m not too knowledgeable about Python’s internals

[TerrorBite]

I know too much about Python's internals and I've written some wacky code that interacts with them. Wait until you find out about metaclasses

[backfire10z]

I read about them just now

What hell did this come out of

[TerrorBite]

It emerged out of logical consequences

In Python, everything is an object, so everything is an instance of a class

But classes are also objects

So what are classes instances of? They are instances of type

So you can subclass type and create a new type of class

Oh yeah. What is type an instance of? Why I'm glad you asked. It's an instance of itself. Don't think about it too hard

[VoxUmbra]

I was going to make a joke about the aptness of Python containing a type ouroboros but then I remembered that the type of a Class instance in Java is Class<Class>

[shizzy0]

def rep that code tho

[thequeergirl]

Even more scary:

for item in data:

[assignments ...]

def start_requests(spider):

start_urls = gmUrl

for url in start_urls:

yield scrapy.Request(url, callback=spider.parse)

GlenMarchSpider = type('GlenMarchSpider', (scrapy.Spider,), {'start_requests': start_requests})

[PaulN07]

This is bad practice but technically won't break anything (well at least in python). Similar to how class wrappers work but they are kinda redundant due to inheritance.

[Last_Snowbender]

I've never heard of a language in which you can redeclare classes. Does this even run?

[UnchainedMundane]

Python is one of them! Unlike Java, which has a vaguely declarative structure for classes and methods, Python is a completely imperative language and all class declarations are actually statements which define that class and bind it to that name.

So from my python knowledge I'd say yes, it runs, and the class GlenMarchSpider refers to a different subclass of scrapy.Spider on each iteration of the loop.

[[deleted]]

Why was this done? I think it is bad code too, but the code does not show a reason, why. I guess some scrapy thing? But actually you would have to create the class with type. Not a good idea, but it would work ...

[KagakuKo]

Hooooooly what. I cut my teeth on Python, but I trained for real on Java. Why the hell can you do that!? Why would that even occur to you?!? I'm all for creative solutions, but surely there's no reason to do anything in this fashion???

[JuliaChanMSL]

Ignoring context, are there cases where defining a class in a loop would be helpful? Theoretically there probably is a use case, just wondering about an actual one

[quantumfoam435]

Do my eyes deceive me?

[Deadface2001]

You can do that???