setting a locale that uses comma as decimal separator breaks tkinter.DoubleVar

[thawn]

BPO	39827
Nosy	@serhiy-storchaka, @thawn, @E-Paine, @akulakov
Files	test_doublevar.py: minimal code example

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-03-02.15:11:30.139>
labels = ['3.7', 'type-bug', 'expert-tkinter']
title = 'setting a locale that uses comma as decimal separator breaks tkinter.DoubleVar'
updated_at = <Date 2021-08-21.08:08:14.008>
user = 'https://github.com/thawn'

bugs.python.org fields:

activity = <Date 2021-08-21.08:08:14.008>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tkinter']
creation = <Date 2020-03-02.15:11:30.139>
creator = 'thawn'
dependencies = []
files = ['48943']
hgrepos = []
issue_num = 39827
keywords = []
message_count = 5.0
messages = ['363184', '363371', '371394', '400005', '400023']
nosy_count = 4.0
nosy_names = ['serhiy.storchaka', 'thawn', 'epaine', 'andrei.avk']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39827'
versions = ['Python 3.7']

[thawn]

This issue occurs when a locale is set that uses comma as decimal separator (e.g. locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')).
I have a tkinter.Spinbox with increment=0.1 connected to a tkinter.DoubleVar.
When I change the value of the Spinbox using the arrow buttons and subsequently try to read out the variable with tkinter.DoubleVar.get(), my code throws the follwoing error:
_tkinter.TclError: expected floating-point number but got "0,1".

Here is a minimal code example:

-------------

import tkinter
import locale

locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')


class TestDoubleVar():
    def __init__(self):
        root = tkinter.Tk()
        self.var = tkinter.DoubleVar()
        self.var.set(0.8)
        number = tkinter.Spinbox(
            root,
            from_=0, to=1, increment=0.1,
            textvariable=self.var,
            command=self.update,
            width=4
        )
        number.pack(side=tkinter.LEFT)
        root.mainloop()

    def update(self, *args):
        print(float(self.var.get()))


if __name__ == '__main__':
    TestDoubleVar()

---

Actual result: the code throws an error

Expected result: the code should print the values of the DoubleVar even with a locale set that uses comma as the decimal separator.

n.b. the problem also occurs with tkinter.Scale

[thawn]

I just found the following code in lines 314-320 of tkinter/ttk.py, which are clearly not localization-aware:

def _to_number(x):
    if isinstance(x, str):
        if '.' in x:
            x = float(x)
        else:
            x = int(x)
    return x

I'll keep looking for similar stuff and add a pull request once I think I have nailed down the issue
I'll look for something similar in

[E-Paine]

Is this a problem exclusive to tkinter? I ran the following code, and it appears to be an issue with the builtin 'float' method:

>>> import locale
>>> locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')
'de_DE.utf8'
>>> float("1,2")
ValueError: could not convert string to float: '1,2'

I have somehow managed to corrupt my locales so I would appreciate if you could try on your machine as well (is it just me?!).

[akulakov]

I wrote a longer explanation but BPO and Chrome ate it.

The issue is that DoubleVar is not locale aware. I don't know what should be the actual fix but the following may be a useful workaround (It works on my system):

import tkinter
import locale
import sys

locale.setlocale(locale.LC_NUMERIC, 'de_DE')
class Var(tkinter.DoubleVar):
    def get(self):
        return locale.atof(self._tk.globalgetvar(self._name))

class TestDoubleVar():
    def __init__(self):
        root = tkinter.Tk()
        self.var = Var()
        self.var.set(0.8)
        number = tkinter.Spinbox(
            root,
            from_=0, to=1, increment=0.1,
            textvariable=self.var,
            command=self.update,
            width=4
        )
        number.pack(side=tkinter.LEFT)
        root.mainloop()
    def update(self, *args):
        print(self.var.get())
if __name__ == '__main__':
    TestDoubleVar()

[serhiy-storchaka]

See also bpo-12558 which exposes the other side of the same issue.

Las time I looked at it, it was inherently a Tcl issue. In some places it uses locale-dependent formatting of floating point numbers, but locale-unaware parsing, or vice versa.

[slawomirmarczynski]

Setting the locale results in buggy behaviour of spin boxes with non-integer content::

    locale.setlocale(locale.LC_ALL, 'pl_PL')  # any locale with the comma as a decimal separator
    variable = tk.DoubleVar(0.5)
    spinbox = ttk.Spinbox(self, from_=0, to=10, increment=0.1, format = "%.1f", textvariable = variable)

The first click on the up/down arrows works correctly (?), any futher manipulation lock the spin box between 0,0 and 0,1 (comma as the decimal separator).

The simple workaround:

    locale.setlocale(locale.LC_ALL, 'pl_PL')  # any locale with the comma as a decimal separator
    locale.setlocale(locale.LC_NUMERIC, 'en_US')  # the dot is the decimal separator again

makes spin boxes fully useable, but... this is no solution when l18n must be used.

The suggested subclassing DoubleVar:

class Var(tkinter.DoubleVar):
    def get(self):
        return locale.atof(self._tk.globalgetvar(self._name))

does not work in my program (CPython 3.9).

[StanFromIreland]

This is a duplicate of #56767 as it is the same problem just in a different place, maybe they should be merged?