from PyQt6 import QtCore

print('Qt Version', QtCore.QT_VERSION_STR)

loc_def = QtCore.QLocale('en')
loc_eur = QtCore.QLocale('de')

numbers = [123.456789, 123456.789, 123456789.]

for number in numbers:
    s1 = loc_def.toString(number, 'g')
    s2 = loc_eur.toString(number, 'g')
    print(f'{s1}     {s2}')

Qt6 versions up to 6.8.x seem to have a bug in capitalizing the exponent character even though 'g' (and not 'G') was specified.

Qt Version 6.2.3
123.457     123,457
123,457     123.457
1.23457E+08     1,23457E+08

Qt Version 6.8.2
123.457     123,457
123,457     123.457
1.23457E+08     1,23457E+08

Qt Version 6.9.0
123.457     123,457
123,457     123.457
1.23457e+08     1,23457e+08

Another issue exposed here is that there's also the https://doc.qt.io/qt-6/qlocale.html#groupSeparator that is locale dependent.

Relevant points:

1. from https://www.qt.io/blog/teaching-qlocale-more-about-number-formats (2020)
   Locale now [respects the case] of the exponent separator provided by CLDR; so, for instance, many locales now represent a million, in floating-point 'e' format, as 1E+06 instead of 1e+06.
2. fixed in https://code.qt.io/cgit/qt/qtbase.git/commit/?id=05c8a48612be38a3c50bb5ed9daf1adfc221b9f2 (2025-03)

The fix is also backported to 6.8 and 6.5, but that won't be in the pypi wheels.

@pijyoi , that's some thorough investigation for the CI-failure! I've updated the tests to expect failure for old versions of Qt.

Regarding group separator, I did consider it. But it's not supported in the standard Qt DoubleSpinBox, it wasn't implemented in the previous version of this SpinBox, and I felt it would be too complex to implement for a niche feature of a niche GUI-component.

import pyqtgraph as pg
from pyqtgraph.Qt import QtCore

def display():
    print(spinbox.value())

pg.mkQApp()
spinbox = pg.SpinBox(value=123456.789, step=1)
spinbox.setLocale(QtCore.QLocale('en'))
spinbox.sigValueChanged.connect(display)
spinbox.setMinimumHeight(40)
spinbox.show()
pg.exec()

On startup, the string "123,456" is displayed. (group separator has been added by Qt)
You can press the up and down arrows, it works fine.
Now let's say I edit the number, changing it from "123,456" to "123,499" and then press ENTER. The value changes to 12.0

So this means that the user, when entering numbers manually, must remove any group separators from the existing displayed string value.

Fixed the group separator issue by removing group separators from the number string produced by QLocale.toString() (the python float formatter didn't add these, so this was a new issue).
Also made tests run normally without xfail for qt < 6.
There was one CI failure for ubuntu-latest, pyside, python 3.10, but it seems unrelated to this PR:

FAILED pyqtgraph/examples/test_examples.py::testExamples[ MouseSelection.py - PySide2 ] - Failed:

Traceback (most recent call last):
File "/home/runner/work/pyqtgraph/pyqtgraph/pyqtgraph/Qt/OpenGLHelpers.py", line 109, in initializeGL
if not ctx.isOpenGLES() and ctx.format().version() >= (3, 1):
AttributeError: 'PySide2.QtGui.QOpenGLExtraFunctions' object has no attribute 'isOpenGLES'

Failed MouseSelection Example Test Located in MouseSelection.py

And now I screwed the whole PR up because I tried to merge this branch on my fork onto another branch on my fork, and there were conflicts.