tkinter missing TkDefaultFont

[jvalenzuela]

Bug report

Bug description:

Encountering problems looking up TkDefaultFont with Python 3.14.7 on Windows 10(x64), possibly due to the upgrade to Tcl/Tk 9.0.4 that occurred in the 3.14.7 Windows release ref.

The example script below demonstrates the problem:

import tkinter as tk
from tkinter import ttk
import tkinter.font as tkfont

app = tk.Tk()
style = ttk.Style()
font_name = style.lookup("TButton", "font")
font = tkfont.nametofont(font_name)
print(f"{font_name} is {font}")
app.mainloop()

This fails with the following traceback:

Traceback (most recent call last):
  File "C:\Users\jason\Documents\atform\master\font.py", line 8, in <module>
    font = tkfont.nametofont(font_name)
  File "C:\Users\jason\AppData\Local\Programs\Python\Python314\Lib\tkinter\font.py", line 25, in nametofont
    return Font(name=name, exists=True, root=root)
  File "C:\Users\jason\AppData\Local\Programs\Python\Python314\Lib\tkinter\font.py", line 96, in __init__
    raise tkinter._tkinter.TclError(
        "named font %s does not already exist" % (self.name,))
_tkinter.TclError: named font TkDefaultFont does not already exist

This example runs normally on Python versions up to and including 3.14.6, but fails on 3.14.7.

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[terryjreedy]

Since the failure is in nametofont, the last 2 lines are not needed. Verified in 3.14.7, 3.15.0rc2, and 3.16.0-.

IDLE still successfully finds TkDefaultFont in idlelib.help line 177:
normalfont = self.findfont(['TkDefaultFont', 'arial', 'helvetica'])
self is an instance of a Text subclass with this method:

    def findfont(self, names):
        "Return name of first font family derived from names."
        for name in names:
            if name.lower() in (x.lower() for x in tkfont.names(root=self)):
                font = tkfont.Font(name=name, exists=True, root=self)
                return font.actual()['family']
            elif name.lower() in (x.lower()
                                  for x in tkfont.families(root=self)):
                return name

I am pretty sure I did not write this.

[zware]

It turns out that this does seem to be a Tcl/Tk 9 change; style.lookup("TButton", "font") no longer returns a str, but a Tcl_Obj wrapping a Tk font object. The good news is that there's a simple workaround; call nametofont(str(font_name)).

It might be worth fixing nametofont to accept a Tcl_Obj whose typename is font (or otherwise just call str on name), but I think we need @serhiy-storchaka's opinion there.

[jvalenzuela]

Perhaps it may be better to apply the conversion to the return value of style.lookup() so other uses of that function continue to receive a string. Additionally, the name of the function nametofont() does somewhat imply conversion of a name(str) to a font, not a font to a font.

[zware]

See also gh-153513, where Serhiy specifically left font objects as Tcl_Obj in the changes for that issue.