[issue42750] tkinter.Variable equality consistency

[Ivo Shipkaliev]

Ivo Shipkaliev  added the comment:

2. (continued) .. This is including the case when both variables have the same 
internal name (the current implementation).

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[issue42750] tkinter.Variable equality consistency

[RhinosF1]

Change by RhinosF1 :


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nosy: +RhinosF1

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[issue42750] tkinter.Variable equality consistency

[Ivo Shipkaliev]

Ivo Shipkaliev  added the comment:

There are 2 good reasons for making Tk variables comparable by value:

   1. We honor the equality operator! The equality operator (==) has to compare 
two objects by value. The current implementation does not fulfil this. Yes, if 
two Tk variables have the same names in the Tcl interpreter, they are 
guaranteed to have the same value.

>>> a = tk.IntVar(name='a')
>>> b = tk.IntVar(name='a')
>>> assert a == b  # this is not enough; equality means "by value"
   # what about when they don't have the same name?

>>> assert a is not b  # this again does not make sense, since
   # both Python "a" and "b" identifiers lead to
   # the same "self._tk.globalgetvar(self._name)"
   # Tcl registered variable: name="a"

>>> a.set(42); assert b.get() == a.get() == 42  # yes!
# equality in names guarantees equality in value

Yes ... BUT, the negation is not true: if two Tk variables have different 
names, that does NOT mean that they have different values. This is where we 
fail the equality operator:

>>> c = tk.IntVar(name='c', value=42)
>>> assert a._name != c._name and a.get() != c.get(), \
... "Difference in names does not guarantee difference in value"

   2. When we're interested in Tk variable comparison: .get() becomes 
redundant. Every time two Tk variables are of the same type AND they refer to 
the same value, we can safely compare them as equal, regardless of how they are 
named in the Tcl interpreter.

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[issue42750] tkinter.Variable equality consistency

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

a = tk.IntVar(name='a')
b = tk.IntVar(name='a')
assert a == b  # they refers to the same variable
assert a is not b  # but they are different objects
a.set(42); assert b.get() == a.get() == 42  # yes, it is the same variable
c = tk.IntVar(name='c', value=42)
assert c != a  # they are different variables
assert c.get() == a.get() == 42  # although having equal values
a.set(43); assert c.get() != a.get() == 43  # and setting one does not affect 
other

I do not see good reasons for making Tk variables comparable by value instead 
of name.

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[issue42750] tkinter.Variable equality consistency

[Ivo Shipkaliev]

Ivo Shipkaliev  added the comment:

If it's gonna break existing code -- fine. I just wanted to point out that if 
two variables are of the same type and refer to the same value, they should be 
considered equal, even if they are not the same variable.

In the current implementation, two StrVars can hold the same strings, but are 
compared unequal, which doesn't seem right.

Considering that we are going through the Tcl interpreter, equality should 
compare by value, not by identity (regardless of the variable names).

Look at this, please.

>>> int_0 = tk.IntVar()
>>> int_0.set(51)

>>> int_1 = tk.IntVar()
>>> int_1.set(51)


How can:

>>> int_0.get() == int_1.get()
True

and

>>> type(int_0) == type(int_1)
True

..but:

>>> int_0 == int_1
False

This means that the equality operator is only showing the difference in their 
names, and the statement above loses meaning, as it can never return True.

I think "int_0 is int_1" should be False as it is now. "is" should take into 
account their names as defined in the Tcl interpreter.

But "int_0 == int_1" should be True. Same as a couple of identical Python lists 
with different names.

Thank you!

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[issue42750] tkinter.Variable equality consistency

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

It will break existing code which depends on the current behavior.

I don't see a problem with comparing variables by name.

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[issue42750] tkinter.Variable equality consistency

[Ivo Shipkaliev]

New submission from Ivo Shipkaliev :

Greetings!

I just noticed:

>>> import tkinter as tk
>>> root = tk.Tk()

>>> str_0 = tk.StringVar()
>>> str_0.set("same value")

>>> str_1 = tk.StringVar()
>>> str_1.set("same value")

So:

>>> str_0.get() == str_1.get()
True

But:

>>> str_0 == str_1
False

So, maybe a Variable should be compared by value, and not by identity (._name) 
as currently? (please view attached) Does it make sense?

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components: Tkinter
files: equality.diff
keywords: patch
messages: 383807
nosy: epaine, serhiy.storchaka, shippo_
priority: normal
severity: normal
status: open
title: tkinter.Variable equality consistency
type: behavior
versions: Python 3.10
Added file: https://bugs.python.org/file49698/equality.diff

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