In linux, copying a tkinter font doubles its size when using Tk 8.6 and a hi-res screen.

[culler]

Bug report

Bug description:

>>> import tkinter
>>> from tkinter.font import Font
>>> root = tkinter.Tk()
>>> f1 = Font(name='TkDefaultFont', exists=True)
>>> f1.actual()
{'family': 'Noto Sans', 'size': 20, 'weight': 'normal', 'slant': 'roman', 'underline': 0, 'overstrike': 0}
>>> f2 = f1.copy()
>>> f2.actual()
{'family': 'Noto Sans', 'size': 40, 'weight': 'normal', 'slant': 'roman', 'underline': 0, 'overstrike': 0}
>>> f3 = f2.copy()
>>> f3.actual()
{'family': 'Noto Sans', 'size': 80, 'weight': 'normal', 'slant': 'roman', 'underline': 0, 'overstrike': 0}
>>> f4 = f3.copy()
>>> f4.actual()
{'family': 'Noto Sans', 'size': 160, 'weight': 'normal', 'slant': 'roman', 'underline': 0, 'overstrike': 0}

The example above is from Ubuntu 24.04 running on an Asus Zenbook with EVO screen. The display scaling from the system settings is the default, namely 200%. This does not happen when the display scaling is 100%.

Note that font f1 is already (more than) doubled in size, compared to what wish8.6 (built from 8.6.15 source) reports:

% info patchlevel
8.6.15
% font actual TkDefaultFont
-family {Noto Sans} -size 9 -weight normal -slant roman -underline 0 -overstrike 0
% font config TkDefaultFont
-family sans-serif -size -12 -weight normal -slant roman -underline 0 -overstrike 0

The negative value -12 means that the size is 12 pixels, as opposed to 12 points. I think the intitial expansion being slightly more than a factor of 2 may be caused by rounding when converting pixels to points.

Note that the behavior of Tk 9 is different. Using wish9.0 on the same system produces:

% info patchlevel
9.0.0
% font actual TkDefaultFont
-family {Noto Sans} -size 10 -weight normal -slant roman -underline 0 -overstrike 0
% font config TkDefaultFont
-family sans-serif -size 10 -weight normal -slant roman -underline 0 -overstrike 0

Also, the pending new version of tkinter from #124156 does not have this doubling behavior when used with Tk 9.0. (I have not tested the new tkinter with Tk 8.6, but I expect it to have the broken behavior since I don't think that the PR changes Font.copy.)

CPython versions tested on:

3.12

Operating systems tested on:

Linux

[NathanDunfield]

I think the doubling behavior is because tkinter's Font.copy relies on Font.actual:

def copy(self):
        "Return a distinct copy of the current font"
        return Font(self._tk, **self.actual())

On a high-res Linux screen with Ubuntu 24.04, the size and self.actual()['size'] differ, apparently by the scaling factor:

>>> import tkinter
>>> root = tkinter.Tk()
>>> f1 = Font(font='TkFixedFont')
>>> f1['size']
20
>>> f1.actual()['size']
40

This seems consistent with the Tk docs which say that font actual:

Returns information about the actual attributes that are obtained when font is used on window's display; the actual attributes obtained may differ from the attributes requested due to platform-dependant limitations, such as the availability of font families and pointsizes.

[NathanDunfield]

Also, on stock Ubuntu 24.04, I see a very different result from @culler's built-from-source Tk 8.6:

$ wish
% info patchlevel
8.6.14
% font actual TkDefaultFont
-family {Noto Sans} -size 20 -weight normal -slant roman -underline 0 -overstrike 0
% font config TkDefaultFont
-family sans-serif -size 10 -weight normal -slant roman -underline 0 -overstrike 0

Note everything here is in points, not a mix of points and pixels. The same difference between config and actual shows up in user-defined fonts:

% font create MyFont -family {Noto Sans} -size 8
MyFont
% font actual MyFont
-family {Noto Sans} -size 16 -weight normal -slant roman -underline 0 -overstrike 0
% font config MyFont
-family {Noto Sans} -size 8 -weight normal -slant roman -underline 0 -overstrike 0

[culler]

This is probably a difference between 8.6.14 and 8.6.15. There is now a release of 8.6.16. I will report if it differs from 8.6.15.

[NathanDunfield]

@culler: I think this Tk ticket is related.

[culler]

Indeed! It looks like they ended up fixing a related Tk bug but never got to the tkinter issue raised in the original report, which seems to be the same as this one..

[NathanDunfield]

I installed the proposed Ubuntu package libtk 8.6.16-1 by downloading the binary from launchpad. Now I get:

% font actual TkDefaultFont
-family {Noto Sans} -size 10 -weight normal -slant roman -underline 0 -overstrike 0
% font config TkDefaultFont
-family sans-serif -size 10 -weight normal -slant roman -underline 0 -overstrike 0
% font create MyFont -family {Noto Sans} -size 8
MyFont
% font actual MyFont
-family {Noto Sans} -size 8 -weight normal -slant roman -underline 0 -overstrike 0
% font config MyFont
-family {Noto Sans} -size 8 -weight normal -slant roman -underline 0 -overstrike 0

[NathanDunfield]

I'm thinking maybe there's nothing to do here for tkinter. I suspect it would have been a better design to have Font.copy() invoke config instead of actual as this seems like a poor copy:

>>> f1 = Font(family='sans-serif', size='10')
>>> f1['family']
'sans-serif'
>>> f1.copy()['family']
'Noto Sans '

However, if Tk itself is working right, they should always display the same. Moreover, Font.__init__ invokes actual which is why one sees:

>>> Font(font='TkDefaultFont').config()['family']
'Noto Sans`

whereas wish would say sans-serif. But presumably changing from actual to config would cause minor havoc and be more trouble than it's worth.