[issue41851] tkinter: add font equal methods

[Tal Einat]

Change by Tal Einat :


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[issue41851] tkinter: add font equal methods

[E. Paine]

Change by E. Paine :


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pull_requests: +21465
pull_request: https://github.com/python/cpython/pull/22434

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[issue41851] tkinter: add font equal methods

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I concur with Tal that negating size would be useless feature, and that we do 
not need an alternate equal() method. When we want to test that two strings are 
equal ignoring case we do not call a special method equalignorecase(), we just 
test that s1.casefold() == s2.casefold().

But I disagree that Font.__eq__() should be changed. The Font object is just a 
reference to the Tk font as the Path object is a reference to a file on 
filesystem. When we compare two Path objects we do not compare file sizes, 
modification times, permission bits and content. We just compare two full 
names. If two Path objects has the same full name, they are equal. If they full 
names are different, they are different, even if the corresponding files has 
the same content. The same is with Font objects.

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[issue41851] tkinter: add font equal methods

[Tal Einat]

Tal Einat  added the comment:

P.P.S. Re-reading that piece of code for IDLE's font config dialog, that 
actually looks to be intentional and correct.

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[issue41851] tkinter: add font equal methods

[Tal Einat]

Tal Einat  added the comment:

> > I also think using [.equal] wouldn't add anything

> I disagree and will stand by this one!

Please do note that the full quote was "I also think using it wouldn't add 
anything, and would be less clear, compared to font1.actual() == 
font2.actual()".

> I think it would be helpful to have a convenience method to check if two 
> fonts represent the same thing that is displayed to the user.

More convenient than font1.actual() == font2.actual() ?

Since this will be used rather rarely, IMO the more significant problem is 
discoverability - I've done a *lot* of work with Tkinter and had never heard of 
the .actual() method. However, adding another comparison method won't help with 
that by itself.

> I am reluctant to touch __eq__. Any ideas?

You wrote that "Checking to see if the fonts are the same Tk object is also 
very helpful (in other, granted fewer, contexts)": Could you elaborate on that?

I still think we should consider overriding __eq__. But I need to check how 
consistent Tktinter is regarding __eq__ checking whether two objects have the 
same name.


[goes to check the docs, code, and do some interactive investigation...]


Okay, so the Font class's .name attribute is just a name the user gives to a 
certain font configuration, which is supposed to be unique. Our docs are pretty 
clear about this:

"The Font class represents a named font. Font instances are given unique names 
and can be specified by their family, size, and style configuration. Named 
fonts are Tk's method of creating and identifying fonts as a single object, 
rather than specifying a font by its attributes with each occurrence." So in 
this case, tkinter is letting a particularly confusing Tk detail show through.

Note that providing a name isn't required, and how confusing the result is when 
not providing font names:

>>> Font(family='DejaVu Sans', size=12) == Font(family='DejaVu Sans', size=12)
False

Also, if you happen to try to create a font with a name which has already been 
used:

>>> Font(name='default font', family='DejaVu Sans', size=12)

>>> Font(name='default font', family='DejaVu Sans', size=12)
Traceback [...]
_tkinter.TclError: named font "default font" already exists

One can avoid this error by passing exists=True, but using that when the font 
doesn't exist raises an exception:

>>> Font(name='other font', family='DejaVu Sans', size=10, exists=True)
Traceback [...]
_tkinter.TclError: named font other font does not already exist


My point is: Using named fonts in tkinter seems incredibly annoying. Using 
fonts without names seems to work better. But without using names, font 
comparison is meaningless.

It seems to me that we could change __eq__ to:
1. Keep the current behavior if both font objects have explicitly set names or 
if their automatically generated names are equal (i.e. `self is other`).
2. Otherwise, compare the the results of calling their .actual() methods.

This would require adding a simple internal flag in __init__ to remember 
whether the name was set explicitly, since __init__ sets a unique name if not 
provided with one.



P.S. The "name" constructor parameter is confusing! Here even Terry J. Reedy 
got it wrong in IDLE font configuration dialog's code:
https://github.com/python/cpython/blob/d9ab95ff1fe81efdf70e545d536d9f6927d1ba81/Lib/idlelib/config.py#L745)

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[issue41851] tkinter: add font equal methods

[E. Paine]

E. Paine  added the comment:

> Why would negating the font size convert it from points to pixels??

Oh... I apologise: I clearly didn't think this through. It doesn't "convert" in 
any sense, it just negates the font size so 16 points becomes 16 pixels. 
Therefore, it is practically useless (besides, a new font object would not help 
the user a converted value anyway) - I therefore do not wish to proceed this 
this part of the issue (I have removed it from the title and will remove it 
from the PR).

> I also think using [.equal] wouldn't add anything

I disagree and will stand by this one! While the user could easily write it 
themselves, I think it would be helpful to have a convenience method to check 
if two fonts represent the same thing that is displayed to the user. When first 
creating this issue, I didn't want to touch __eq__ for two reasons:
1. I didn't know how much code changing this would break (it is very hard to 
check for code usage of dunder methods - if you know any ways please let me 
know!)
2. Checking to see if the fonts are the same Tk object is also very helpful (in 
other, granted fewer, contexts)

For example, something like this could be used in IDLE so that if the user sets 
their font back to the equivalent of the default ("TkFixedFont"), the value 
isn't kept in the user's local config (similar to how all the other options do).

Therefore, while I would like this somehow added, I agree that adding an 
`equal` method that does not do the same thing as `==` is confusing (you just 
have to look at Java and all the confusion over using `==` on strings - a 
'gotcha' for every new Java learner). However, as I said above, I am reluctant 
to touch __eq__. Any ideas?

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title: tkinter: add font neg and equal methods -> tkinter: add font equal 
methods

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