[1.1] Unit conversion error with astropy 4.2

[olebole]

With the new Astropy 4.2, Specutils (1.1 from PyPI) starts to show a number of failures in test_smoothing.py that look strange to me:

__________________________ test_smooth_custom_kernel ___________________________

self = <Quantity [1131.01333332, 1366.39097043, 1380.73344458, 1315.70807832,
           1218.18529306, 1225.68833337, 1264.9...557.36062977, 1565.6571005 , 1519.2402814 ,
           1489.30273208, 1434.86908964, 1202.1970936 ,  874.83988599] mJy>
unit = Unit(dimensionless), equivalencies = []

    def to_value(self, unit=None, equivalencies=[]):
        """…"""
        if unit is None or unit is self.unit:
            value = self.view(np.ndarray)
        else:
            unit = Unit(unit)
            # We want a view if the unit does not change.  One could check
            # with "==", but that calculates the scale that we need anyway.
            # TODO: would be better for `unit.to` to have an in-place flag.
            try:
>               scale = self.unit._to(unit)

/usr/lib/python3/dist-packages/astropy/units/quantity.py:738: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = Unit("mJy"), other = Unit(dimensionless)

    def _to(self, other):
        """…"""
        # … 
        if self is other:
            return 1.0
    
        # …
        if isinstance(other, UnitBase):
            self_decomposed = self.decompose()
            other_decomposed = other.decompose()
    
            # …
            if(self_decomposed.powers == other_decomposed.powers and
               all(self_base is other_base for (self_base, other_base)
                   in zip(self_decomposed.bases, other_decomposed.bases))):
                return self_decomposed.scale / other_decomposed.scale
    
>       raise UnitConversionError(
            f"'{self!r}' is not a scaled version of '{other!r}'")
E       astropy.units.core.UnitConversionError: 'Unit("mJy")' is not a scaled version of 'Unit(dimensionless)'

/usr/lib/python3/dist-packages/astropy/units/core.py:950: UnitConversionError

During handling of the above exception, another exception occurred:

simulated_spectra = <specutils.tests.spectral_examples.SpectraExamples object at 0x7f151d2c5c40>

    def test_smooth_custom_kernel(simulated_spectra):
        """
        Test CustomKernel smoothing with correct parmaeters.
        """
    
        # Create the original spectrum
        spec1 = simulated_spectra.s1_um_mJy_e1
        flux_original = spec1.flux
    
        # Create a custom kernel (some weird asymmetric-ness)
        numpy_kernel = np.array([0.5, 1, 2, 0.5, 0.2])
        numpy_kernel = numpy_kernel / np.sum(numpy_kernel)
    
        custom_kernel = convolution.CustomKernel(numpy_kernel)
        flux_smoothed_astropy = convolution.convolve(flux_original, custom_kernel)
    
        # Calculate the custom smoothed
        spec1_smoothed = convolution_smooth(spec1, custom_kernel)
>       compare_flux(spec1_smoothed.flux.value, flux_smoothed_astropy, flux_original.value)

/usr/lib/python3/dist-packages/specutils/tests/test_smoothing.py:58: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/lib/python3/dist-packages/specutils/tests/test_smoothing.py:34: in compare_flux
    assert np.allclose(flux_smooth1, flux_smooth2)
<__array_function__ internals>:5: in allclose
    ???
/usr/lib/python3/dist-packages/astropy/units/quantity.py:1509: in __array_function__
    args, kwargs, unit, out = function_helper(*args, **kwargs)
/usr/lib/python3/dist-packages/astropy/units/quantity_helper/function_helpers.py:545: in close
    (a, b), unit = _quantities2arrays(a, b, unit_from_first=True)
/usr/lib/python3/dist-packages/astropy/units/quantity_helper/function_helpers.py:345: in _quantities2arrays
    arrays = tuple((q._to_own_unit(arg)) for arg in args)
/usr/lib/python3/dist-packages/astropy/units/quantity_helper/function_helpers.py:345: in <genexpr>
    arrays = tuple((q._to_own_unit(arg)) for arg in args)
/usr/lib/python3/dist-packages/astropy/units/quantity.py:1339: in _to_own_unit
    _value = value.to_value(self.unit)
/usr/lib/python3/dist-packages/astropy/units/quantity.py:741: in to_value
    value = self._to_value(unit, equivalencies)
/usr/lib/python3/dist-packages/astropy/units/quantity.py:660: in _to_value
    return self.unit.to(unit, self.view(np.ndarray),
/usr/lib/python3/dist-packages/astropy/units/core.py:987: in to
    return self._get_converter(other, equivalencies=equivalencies)(value)
/usr/lib/python3/dist-packages/astropy/units/core.py:918: in _get_converter
    raise exc
/usr/lib/python3/dist-packages/astropy/units/core.py:903: in _get_converter
    return self._apply_equivalencies(
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = Unit("mJy"), unit = Unit("mJy"), other = Unit(dimensionless)
equivalencies = []

    def _apply_equivalencies(self, unit, other, equivalencies):
        """
        Internal function (used from `_get_converter`) to apply
        equivalence pairs.
        """
        def make_converter(scale1, func, scale2):
            def convert(v):
                return func(_condition_arg(v) / scale1) * scale2
            return convert
    
        for funit, tunit, a, b in equivalencies:
            if tunit is None:
                try:
                    ratio_in_funit = (other.decompose() /
                                      unit.decompose()).decompose([funit])
                    return make_converter(ratio_in_funit.scale, a, 1.)
                except UnitsError:
                    pass
            else:
                try:
                    scale1 = funit._to(unit)
                    scale2 = tunit._to(other)
                    return make_converter(scale1, a, scale2)
                except UnitsError:
                    pass
                try:
                    scale1 = tunit._to(unit)
                    scale2 = funit._to(other)
                    return make_converter(scale1, b, scale2)
                except UnitsError:
                    pass
    
        def get_err_str(unit):
            unit_str = unit.to_string('unscaled')
            physical_type = unit.physical_type
            if physical_type != 'unknown':
                unit_str = "'{}' ({})".format(
                    unit_str, physical_type)
            else:
                unit_str = f"'{unit_str}'"
            return unit_str
    
        unit_str = get_err_str(unit)
        other_str = get_err_str(other)
    
>       raise UnitConversionError(
            "{} and {} are not convertible".format(
                unit_str, other_str))
E       astropy.units.core.UnitConversionError: 'mJy' (spectral flux density) and '' (dimensionless) are not convertible

/usr/lib/python3/dist-packages/astropy/units/core.py:886: UnitConversionError

Full test log is here. This happens in a number of test_smooth_* functions (test_smooth_box_good, test_smooth_box_good, test_smooth_gaussian_good, test_smooth_trapezoid_good).

I am not sure whether this is fixed in the current "master" branch; however I couldn't find a commit with an obvious name that fixes it.

[olebole]

May I ping here? The specutils were just removed from Debian Testing because of this issue, and I would at least need to know whether I can just disable this test for the moment, or if this points to something really serious.

[pllim]

astropy 4.x is no longer being developed. Is this still a problem in astropy 5.x?

[olebole]

I can confirm that this one is fixed now. I don't see exactly whether this was astropy 5.0 or specutils (specifically #833)