Loading Quantity from PyQt6 QSettings not working

[loenard97]

I would like to save a Quantity with the built-in QSettings tools of PyQt6 in a .ini file. QSettings automatically pickles and unpickles the Quantity, but calling a function wrapped by a @ureg.wraps decorator fails unexpectedly.

I have the following setting in the .ini file (located at QSettings().fileName()):

[General]
my_value_1=@Variant(\0\0\0\x7f\0\0\0\xePyQt_PyObject\0\0\0\0\xbd\x80\x4\x95\xb2\0\0\0\0\0\0\0\x8c\x4pint\x94\x8c\x12_unpickle_quantity\x94\x93\x94\x8c\x1apint.facets.plain.quantity\x94\x8c\rPlainQuantity\x94\x93\x94G?`bM\xd2\xf1\xa9\xfc\x8c\tpint.util\x94\x8c\xeUnitsContainer\x94\x93\x94)\x81\x94h\x6\x8c\x5udict\x94\x93\x94)\x81\x94\x8c\x6second\x94K\x1sK\x1\x8c\bbuiltins\x94\x8c\x5\x66loat\x94\x93\x94\x87\x94\x62\x87\x94R\x94.)

Here is a minimal example that tries to load this setting:

import sys
import pint
from pint import UnitRegistry
from PyQt6.QtCore import QSettings
from PyQt6.QtWidgets import QApplication

ureg = UnitRegistry()


# Some random function with units wrapped around it
@ureg.wraps(None, "second")
def wrapped_func(value):
    print(value)

# QSettings setup
app = QApplication(sys.argv)
app.setOrganizationName("Test")
app.setApplicationName("test")
QSettings.setDefaultFormat(QSettings.Format.IniFormat)
settings = QSettings()
print("QSettings file path", settings.fileName())

print("\nRegular quantity working as expected")
regular_quantity = 0.002 * ureg.second
print(regular_quantity)                                 # 0.002 second
print(type(regular_quantity))                           # <class 'pint.Quantity'>
print(isinstance(regular_quantity, pint.Quantity))      # True
print(isinstance(regular_quantity, ureg.Quantity))      # True
print(regular_quantity.__class__.__mro__)

print("\nLoading a quantity from QSettings")
unpickled_quantity = settings.value("my_value_1")
print(unpickled_quantity)                               # 0.002 second
print(type(unpickled_quantity))                         # <class 'pint.Quantity'>
print(isinstance(unpickled_quantity, pint.Quantity))    # True (as expected)
print(isinstance(unpickled_quantity, ureg.Quantity))    # False (for some reason?)
print(unpickled_quantity.__class__.__mro__)             # Exactly the same MRO as regular_quantity above

# The unit wrapper checks for isinstance(unpickled_quantity, ureg.Quantity)
# Because this is False the following function call throws a ValueError:
# ValueError: A wrapped function using strict=True requires quantity or a string for all arguments with not None units. (error found for second, 0.002 second)
wrapped_func(unpickled_quantity)

QSettings does unpickle the saved value correctly, but the check isinstance(unpickled_quantity, ureg.Quantity) returns False, even though the value is of type <class 'pint.Quantity'>. If I then try to use this value as an argument for a function with a @ureg.wraps decorator, the decorator checks for isinstance(unpickled_quantity, ureg.Quantity) in line 150 in registry_helpers.py, which returns False and therefore the decorator fails to accept my value as a Quantity, even though it is of type Quantity.

This if check in registry_helpers.py line 150 should return True, but does not:

        # third pass: convert other arguments
        for ndx in unit_args_ndx:
            if isinstance(values[ndx], ureg.Quantity):

I found out that UnitRegistry.Quantity is just a TypeAlias of Quantity. But that confused me even more, because I don't see why isinstance should return differently for pint.Quantity and ureg.Quantity if they are the same type. The Method Resolution Orders are even exactly the same for both regular_quantity and unpickled_quantity, but somehow the isinstance checks are different!

What makes this even more confusing is that I can't seem to reproduce the .ini file that causes this error. The .ini file I showed at the top is produced by a larger program that consistently produces this bug. But when I try to write a minimal program that saves a .ini file itself, the bug does not appear and I don't see any difference between the two files.

As a work around I can save my values without Quantities for now, but that kind of defeats the purpose why we are using pint in our original program.