🔒 Type hinting: Classes and subclasses | Python | Page 1

silent hemlock Oct 30, 2024, 6:02 PM

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Suppose that I have classes Drawable, Placeable, and Draggable. The latter 2 are subclasses of the first. See bare implementation below

class Drawable:
    def __init__(self, parent: Union['Drawable', None], children: list['Drawable']):
        self.children = children
        self.parent = parent

class Placeable(Drawable):

    def __init__(self, rect: pygame.rect.Rect, parent: Union['Drawable', None]):
        super().__init__(rect, parent, [])
    
    def remove_child(self, target):
        self.children.remove(target)
    
    def add_child(self, target):
        self.children.append(target)


class Draggable(Drawable):

    def __init__(self, rect: pygame.rect.Rect, parent: Placeable):
        super().__init__(rect, parent, [])
    
    def release_click(self, pos: Tuple[int, int]):
      self.parent.remove_child(self)

I noticed that my IntelliSense doesn't realize that the Draggble.parent is supposed to be a Placeable but only sees it as a Drawable. Is it possible to type hint this better so that this wouldn't be an issue? I'm trying to learn how to type hint properly so that my code can be more readable in the future.

dense fogBOT Oct 30, 2024, 6:02 PM

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silent hemlock Suppose that I have classes `Drawable`, `Placeable`, and `Draggable`. The latter...

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@silent hemlock

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ancient cargo Oct 30, 2024, 6:06 PM

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you could type parent as Self | None

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(from typing import Self)

silent hemlock Oct 30, 2024, 6:17 PM

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but in this case, the parent is a Placeable, and self would be Draggable

undone tartan Oct 30, 2024, 6:27 PM

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sadly, if Drawable.parent: Drawable, and Draggable(Drawable), then Draggable.parent is also supposed to be Drawable, otherwise the liskov substituion principle isnt fullfilled
imagine something accepting a Drawable, it would try to set its .parent to some other Drawable, but your Draggable only works with a parent that is Placeable
if the parent was immutable it could be fine, me thinks (as using a Placeable as a Drawable is fine)
what is the actual hierarchy you want to build? how would you describe it without python specific stuff?

silent hemlock Oct 30, 2024, 6:43 PM

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A Placeable is also a Drawable

undone tartan Oct 30, 2024, 6:44 PM

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yes, but not all drawables are placeables

silent hemlock Oct 30, 2024, 6:44 PM

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I'll try to explain my end goal to see if i'm going about this hierarchy the right way

undone tartan Oct 30, 2024, 6:44 PM

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def f(x: Drawable, y: Drawable):
  x.parent = y

x: Draggable
y: Drawable
f(x, y)
# uh oh! Draggable.parent was only supposed to be a Placeable, but was set to a Drawable

silent hemlock Oct 30, 2024, 6:45 PM

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mmm

undone tartan Oct 30, 2024, 6:47 PM

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you could make Drawable generic over the parent type (bound by Drawable), and then say that Draggable(Drawable[Placeable])

class Drawable[Parent: Drawable]:
  parent: Parent

class Placeable(Drawable[Drawable]):
  ...

class Draggable(Drawable[Placeable]):
  ...

or make the parent attribute immutable, if thats fine with your design

silent hemlock Oct 30, 2024, 6:48 PM

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immutable just means that attribute can't be changed and is static, but can be reassigned, right?

undone tartan Oct 30, 2024, 6:48 PM

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no, cant be reassigned, e.g. via a @dataclass(frozen=True), only set on construction

silent hemlock Oct 30, 2024, 6:49 PM

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then I don't think that that would be feasible unfortunately

undone tartan Oct 30, 2024, 6:49 PM

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consider trying the generic approach then, though that can easily "spread" across the whole codebase

silent hemlock Oct 30, 2024, 6:49 PM

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What exactly does that mean?

undone tartan Oct 30, 2024, 6:50 PM

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are you familiar with how you can say "i have a list of some specific type T" with list[T]?

silent hemlock Oct 30, 2024, 6:50 PM

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Draggable(Drawable[Placeable])

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yes

undone tartan Oct 30, 2024, 6:50 PM

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thats because list is generic over 1 type parameter

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you could make your Drawable generic over 1 type parameter (bound by the Drawable type, probally, so e.g. the parent couldnt be.. an int) and use that as the type of the parent attribute, then, Drawable[Placeable] would mean "a drawable the parent of which is a placeable"

silent hemlock Oct 30, 2024, 6:52 PM

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aren't you unable to use methods of T in a case like that or am I remembering incorrectly

undone tartan Oct 30, 2024, 6:52 PM

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depends
it is parametric polymorphism, yes, but if you have it bound - you can use stuff thats in the bound

undone tartan Oct 30, 2024, 6:53 PM

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undone tartan you could make Drawable generic over the parent type (bound by Drawable), and th...

you could make stuff like

class Drawable[Parent: Drawable]:
    parent: Parent
    def draw(self) -> None:
        ...

class Placeable(Drawable[Drawable]):
    ...

class Draggable(Drawable[Placeable]):
    ...

def f[Parent: Drawable](drawable: Drawable[Parent], new_parent: Parent):
    drawable.parent = new_parent
    drawable.draw()

and that would be safe
(it would probally also make the most sense if Drawable was an ABC, im not sure what concrete implementation it could have)

silent hemlock Oct 30, 2024, 6:55 PM

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it's supposed to be one

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but the main thing is that Draggable is only supposed to have a parent that is a Placeable

undone tartan Oct 30, 2024, 6:56 PM

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yes, that is expressed here, with class Draggable(Drawable[Placeable])

silent hemlock Oct 30, 2024, 6:56 PM

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would it be able to call a Placeable method within that?

undone tartan Oct 30, 2024, 6:57 PM

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reveal_type(Draggable.parent) # Placeable

so yes

silent hemlock Oct 30, 2024, 6:57 PM

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wait I just realized yea

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before I start implementing these changes, I'd like to say what this is all for to see if this is the best way I would go about doing this

undone tartan Oct 30, 2024, 6:59 PM

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i think this approach is pretty clean

silent hemlock Oct 30, 2024, 6:59 PM

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I think so too but I still might be overcomplicating it

undone tartan Oct 30, 2024, 7:00 PM

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well, not seeing the "concrete" code, im not entirely sure myself, but the type logic seems fine here
often it happens that the concrete code you need for solving your actual problem is much simpler

silent hemlock Oct 30, 2024, 7:01 PM

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I'm trying to create a GUI that can assign players to a team. There are going to be 3 columns. One representing team 1, one representing team 2, and the other is unassigned. The gui is to support drag and drop on the players allowing them to be placed from one team to another. These are all abcs representing the gui representation of the teams (placeable) and the players (draggable)

undone tartan Oct 30, 2024, 7:02 PM

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if an ABC has only one concrete implementation, it is kind of a bad ABC
maybe just write the

class Team:
  ...
class Player:
  ...

and be done

silent hemlock Oct 30, 2024, 7:03 PM

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ig

undone tartan Oct 30, 2024, 7:03 PM

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i too have fallen into the over-abstraction and over-genericness trap many times
you'll know when you need it, and then designing it will be simpler
if you just have a "team and player" program - thats what should be in the program (unless its a library :d)

silent hemlock Oct 30, 2024, 7:04 PM

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fair enough

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Thank you

dense fogBOT Oct 30, 2024, 8:05 PM

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#🔒 Type hinting: Classes and subclasses