Where is cget in membership test in tkinter?

I spent quite a while looking at the line 17 (see setaview() below) and trying to comprehend where were the int, string and concatenation.

if clsname in vcache22:  # line 17

The error

TypeError: can only concatenate str (not "int") to str

was introduced in the line 21: The right line should be:

vcache22[clsname] = aview = cls()

The terminal session:

$ python setaview.py
setaview(Track)
Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python3.9/tkinter/__init__.py", line 1892, in __call__
    return self.func(*args)
  File "/home/vlz/Documents/ARCHIVE/musicbox/setaview.py", line 36, in <lambda>
    command=lambda x=t: setaview(x))
  File "/home/vlz/Documents/ARCHIVE/musicbox/setaview.py", line 17, in setaview
    if clsname in vcache22:
  File "/usr/lib/python3.9/tkinter/__init__.py", line 1652, in cget
    return self.tk.call(self._w, 'cget', '-' + key)
TypeError: can only concatenate str (not "int") to str

The function with the error:

def setaview(clsname):
    global vcache22, aview
    if clsname in vcache22:  # line 17
        aview = vcache22[clsname]
    else:
        cls = getattr(sys.modules['__main__'], clsname)
        vcache22 = aview = cls()  # line21
    # To indicate that smth has done
    print(f'setaview({clsname})')

This is the contrived example to isolate the problem I found in other longer code. The full script is below:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import sys
import tkinter as tk
from tkinter import ttk
aview = None
vcache22 = {}


class Track(tk.Listbox): pass  # noqa
class Album(tk.Listbox): pass  # noqa
class Playlist(tk.Listbox): pass  # noqa


def setaview(clsname):
    global vcache22, aview
    if clsname in vcache22:
        aview = vcache22[clsname]
    else:
        cls = getattr(sys.modules['__main__'], clsname)
        vcache22 = aview = cls()
    # To indicate that smth has done
    print(f'setaview({clsname})')


def main():
    root = tk.Tk()
    box = tk.Frame(root)
    box.grid(column=0, row=0)
    selected = tk.StringVar()
    i = 0
    for t in 'Track', 'Album', 'Playlist':
        rb = ttk.Radiobutton(
            box, text=t, value=t, variable=selected,
            command=lambda x=t: setaview(x))
        rb.grid(column=i, row=0)
        i += 1
        if t == 'Track':
            rb.invoke()
    tk.mainloop()


if __name__ == '__main__':
    main()

Using:
Python 3.9.2
Debian GNU/Linux 11 (bullseye)

> Where is cget in membership test in tkinter?

In python, when you do x in y, internally python will call the method __getitem__. So, when you do if clsname in vcache22 it's calling __getitem__ of the object, which in this case is a subclass of Listbox. For tkinter widgets, __getitem__ is aliased to cget. That is why cget is being called in this case.

The error message is pointing you to line 1652 of tkinter/init.py. If you look at that line, you might see just a couple lines further down this line:

__getitem__ = cget

This is in the definition of the base class for all widgets.