Quantity constructor changes logarithmic units

[bwinkel]

I like the astropy units module a lot. Thanks for this great piece of work!

I think, I encountered a bug related to the Quantities class. When I use logarithmic units, the constructor seems to change the type of the unit. This does not happen, when I simply multiply with the unit:

# astropy.__version__ == '3.0.dev19502'

import numpy as np
from astropy import units as u
from astropy.units import Quantity

# define units
dimless = u.dimensionless_unscaled
dB = u.dB(dimless)

arr = np.array([1, 3, 5])

q1 = (arr * dB)         # q1.unit == Unit("dB(1)")
q2 = Quantity(arr, dB)  # q2.unit == Unit("dB")

type(q1.unit)  # astropy.units.function.logarithmic.DecibelUnit
type(q2.unit)  # astropy.units.function.mixin.RegularFunctionUnit

This is a problem for some of my use cases:

q1.to(dimless)  # <Quantity [ 1.25892541, 1.99526231, 3.16227766]>
q2.to(dimless)  # UnitConversionError: 'dB' and '' (dimensionless) are not
                # convertible

@u.quantity_input(a=dB)
def my_func(a):
    print(a)

my_func(q1)  # [ 1.  3.  5.] dB
my_func(q2)  # UnitsError: Argument 'a' to function 'my_func' must be in
             # units convertible to 'dB'.

I can work-around the first problem:

u.add_enabled_equivalencies(u.logarithmic())

q1.to(dimless)  # <Quantity [ 1.25892541, 1.99526231, 3.16227766]>
q2.to(dimless)  # <Quantity [ 1.25892541, 1.99526231, 3.16227766]>

But now the decorated function fails in both cases (with a different error!)

my_func(q1)  # ValueError: Unit  is not a physical unit.
my_func(q2)  # ValueError: Unit  is not a physical unit.

[mhvk]

Hmm, this is similar to #5851, where the question was why Quantity constructor did not return a function quantity such as Magnitude when passed a magnitude unit. As discussed there, in general, one wants to have a class initializer return an instance of that class. So, here, the initializer returns a Quantity with units of u.dB since that is the same thing as u.dB(u.one) - it fails it you do u.Quantity(1., u.dB(u.mW)). Following #5851, we decided that we would at least allow this to work if one passed in subok=True (this works since u.Dex is a subclass of Quantity). So, I think that answers part of your question - the main question here is whether we could perhaps document this better? (though it is in the help for Quantity in the latest version)

However, you also raise a new issue here, that quantity_input does not work in your example. I do think this would have to be a=u.dB(u.dimensionless_unscaled), but that doesn't work either. It should be possible to make this work, perhaps with subok=True used by default, or by allowing to request a specific quantity subclass. cc @adrn, @Cadair who wrote quantity_input.

[bwinkel]

Thanks a lot. Using subok=True indeed fixes most of my problems, except the one with quantity_input. Perhaps, as you say, the subok=True should be made default? I'm developing a new package and therefore, not only astropy docs need to clarify this, but I'd also need to do it in my docs, which would add a lot of overhead.

[mhvk]

@Cadair - as you have just been looking at quantity_input, what do you think of using subok=True by default in the Quantity constructor, so that things work for logarithmic units as well? (this issue)