Mailman 3 tkinter Variable implementation - Python-ideas

12 Dec 2020

      Hiya :)

I'm recently working with tkinter and I noticed that:
...
...
...
import tkinter as tk
string_var = tk.StringVar()
is throwing an AttributeError:
...
Traceback (most recent call last):
  . . .
  File "...\lib\tkinter\__init__.py", line 505, in __init__
    Variable.__init__(self, master, value, name)
  File "...\lib\tkinter\__init__.py", line 335, in __init__
    self._root = master._root()
AttributeError: 'NoneType' object has no attribute '_root'
... which is making me question myself:
-- "Why am I looking for a '_root' attribute on a NoneType object here?", or
-- "How did I end up with a NoneType, to be searched for a '_root'
attribute in the first place?"
I have done something wrong or I don't know something apparently.

Following the traceback, looking at "tkinter\__init__.py", lines 333-335
leads us to the implementation of StringVar's parent: class Variable:

333 >         if not master:
334 >             master = _default_root
335 >         self._root = master._root()

Class Variable constructor interface:

317 >     def __init__(self, master=None, value=None, name=None):

Clearly, on line 335, we are relying on the "master" to have a "_root"
callable. If "master" was not declared upon class instantiation (line 317),
a value can come from _default_root, which is a global module variable.
_default_root is holding a Tk instance, if one was instantiated. So, if
neither "master" nor _default_root was defined, line 335 will result in an
AttributeError.

Now, would it be a good idea if we keep the user better informed by making
the aforementioned lines look something like?:

333 >         if not master:
334 >             master = _default_root
335 >             if not master:
336 >                 raise RuntimeError(f"{self.__class__.__name__!r}:
'master' must be defined or a Tk instance must be present!")
337 >         self._root = master._root()

... and not relying on this inexplicable AttributeError.

Thank you for your time!

Regards
Ivo Shipkaliev

tkinter Variable implementation

Ivo Shipkaliev

William Pickard

Ivo Shipkaliev

Ronald Oussoren

Ivo Shipkaliev

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