I'll try to write a CONTRIBUTING.md later today, and create some "good first issue" issues to help you (and maybe other) get started

Sheesh I'm in trouble now. It's hard to figure out, in what do I need help. I grew up in a family, where if I want something to be done, I had to do it myself, and so it's difficult for me to find what others can help with (and I'm maximalist and I feel that only I can do things in the way I want it to be 😏. Nonetheless I'll try to give some description about each task, so you can see what I have in my mind. Btw, sorry about that @Moosems, when I scrapped half of your tooltip code, because I had a better idea).

However I always come up with new ideas, of what I should add to tukaan, and how it should be implemented. I have dozens of notes on my phone, that I made on the train, where I have enormous amount of time, but no WiFi :), but unfortunately I don't really have time to implement them after all.
So I'm just dumping here all the notes I find, and try to write a little context to them. If one of you would like to pick up one of them, feel free to open an issue if you want to discuss it.

Let's start with the simpler ones:

• LabeledView widget
  a.k.a labelframe. It could be really similar to the Frame (probably they should even be in the same file), but with a title attribute (a TextProp from _props.py. It should also inherit Frame.
• ButtonGroup widget
  A group of buttons, similar to RadioGroup. The button labels and callbacks could be specified in a dict, like items={"Text": do_something}. The leftmost button should have the Left.TButton style, the rightmost Right.TButton, and all the others Middle.TButton (similar stuff when the orientation is vertical), this way it's possible to do, that only the outer buttons have round corners. (I think my forest theme has this feature, but it might be only in one of my local branches)
• Switch widget
  Really just a checkbutton with different style applied. Should check if the Switch layout is available in the current theme, and use TCheckbutton otherwise
• Add window icon
  Currently it's not possible to add any window icon. It should be a property in wm.py (idk what that class is called), that accepts either a pathlib.Path object, or a tukaan.Icon object, and sets the window icon. Cross-platform twist: it should be possible to set a png file on every system, an ico Windows, and icns on macOS
• Add button variants
  variant option for the Button widget to set styles like "outlined" (Outline.TButton internally), "filled" (Filled.TButton) and "link" (Link.TButton)
• Add button color variants
  color option for the Button widget to set styles like "primary" (Pimary.TButton internally), "secondary" (Secondary.TButton), "normal" (TButton) and "danger" (Danger.TButton)

Sorry for any typo and grammar error, I'm dead tired 😴 😩 . I will continue this list tomorrow. I have a lot more items ;)

I think I might try working on the window icons, but im not sure how to allow pngs to be set on every system...

@Moosems I remember I promised you a tutorial on Tcl calls, I seem to have forgotten it :(

The Tcl interface

The Tcl class is a bridge between Python and Tcl. It can be used to initialize the interpreter, to run the mainloop, to load dll files to Tk, and to call Tcl commands from Python.

from tukaan._tcl import Tcl

Calling Tcl from Python

There are two methods to call Tcl code in Tukaan.

The first is the Tcl.call method. You can pass in arbitary number of argument of any type (see Tcl.to below), and it will evaluate it. The first argument is always the ruturn type of the call, if you don't expect any return it should be None

>>> Tcl.call(None, "grid", textbox_widget, *Tcl.to_tcl_args(column=3, rowspan=8))
None
>>> Tcl.call(str, textbox_widget, "get")
foobar

There's a utility method called to_tcl_args, which can be used to convert Python keyword arguments to Tcl keyword arguments (column=3 -> ("-column", "3")). It returns a tuple, which you have to unpack, when passing into Tcl.call (with the * syntax)

The second is the Tcl.eval method, which is very similar to Python's exec function, except in Tcl. It doesn't do any Py-to-Tcl conversion, so you can only pass in a single string, but this means that this function is also a lot faster. Similarly to Tcl.call the first argument defines the return type. The second argument is the script to evaluate.

>>> Tcl.eval(None, "grid .app.textbox_1 -column 3 -rowspan 8")
None
>>> Tcl.eval(str, ".app.textbox_1 get")
foobar

About converting things to Tcl

You might know, that in Tcl everything is a string. Therefore Tukaan needs to convert every Python object to a string, and the strings returned from Tcl back to Python objects (this is the return value I was babbling about above). However, it isn't simply done by calling the str() function, but finding a way, that it can be represented in Tcl. For example when a Python funciont is passed in, it creates a new Tcl procedure at C-level, which will call the Python function (if you're really interested in this part, it's here in _tkinter.c).
An object can also be converted to Tcl, if it has a _name attribute. Widgets use this attribute to store thier pathName, with which they are represented in Tk (e.g. .frame.notebook.tab_1.button).
If an object has a __to_tcl__ method, but no _name, Tcl.to will call it to represent the object to Tcl. This method is useful if you want to give a name to the object on the fly. For example, Tukaan adds a __to_tcl__ method to PIL images, which converts them to Tk images and then returns the image name, because by default PIL images aren't known by the Tcl interpreter.

If someone (@Moosems?, if you understand what the hack I'm talking about here) could revise this text, and it would be useful to have it in a For developers section in the documentation.