Tkinter misleading error in `Tk.__getitem__`

[TheLizzard]

Bug report

Bug description:

import tkinter as tk
root = tk.Tk()
print("background" in root)

Prints the following stack trace:

Traceback (most recent call last):
  File "/example.py", line 3, in <module>
    print("background" in root)
          ^^^^^^^^^^^^^^^^^^^^
  File "/tkinter/__init__.py", line 1727, in cget
    return self.tk.call(self._w, 'cget', '-' + key)
                                         ~~~~^~~~~
TypeError: can only concatenate str (not "int") to str

The TypeError referencing concatenation of an int to a str is very misleading. Error messages that don't make sense can be very confusing especially for beginners who don't know about magic methods like __iter__ and __getitem__.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-140481: avoid misleading exception in tk.Tk.__getitem__ #140483
• gh-140481: Improve error message when trying to iterate a Tk widget, image or font #140501

[TheLizzard]

I am working on a PR that will change the problamatic '-' + key in tkinter/__init__.py as well as create a __contain__ method.

[serhiy-storchaka]

The in operator tries to call the __contains__() special method if it exists. Otherwise it tries to iterate the object -- tries to the __iter__() special method if it exists, otherwise calls the __getitem__() special method with increasing integer index starting from 0, unless it fails.

Misc.__getitem__() is an alias of Misc.cget() which only supports string keys. It produces the above exception.

We can improve this by adding the __contains__() or __iter__() methods which always raise exception with more sensible error message or return some reasonable result, or by adding a type check in Misc.cget() (but in the latter case we cannot know that it was called in the in operator).

I do not think that supporting the in operator or iteration makes sense for Tk widgets. They are not general containers, subscription is only a convenient syntax sugar for cget(). Tk widgets can contain other widgets, defining __contains__() or __iter__() with other semantic would be confusing. So, adding __iter__() which always fail is a better option. But this would make Tk widgets supporting the collections.abc.Iterable protocol, which is undesirable.

We need a way to make a class non-iterable, even if it has the __getitem__() method, without making it a virtual subclass of collections.abc.Iterable. And there is such way -- set __iter__ = None.