A question on astropy logarithmic units behaviour

Astropy
,
1

Hi all,

I’m posting this one because I’ve encountered strange behaviours in logarithmic astropy units. It looks like with the most recent version of astropy (conda create -n test_env astropy) the issue still persists. I’m not sure if it’s a bug, so I first wanted to check if that’s an intended behaviour of astropy.units.dB

Let’s take the base example:

from astropy import units as u
gain = 5 * u.dB(1)

This produces a nice 5 dB value. However, let’s in case if we have some calculations done before:

gain = ((15*u.m)/(3*u.m)) * u.dB(1)
or
gain = ((15*u.m)/(3*u.m)).to(u.dB(1))

The result is 6.989700043360188 dB, which is somewhat unexpected as it looks like “* u.dB(1)” automatically applies the 10*log10 operation.

Another example is

power = 5*u.dB(u.W)

Which produces a nice 5 dBW value. However, the same u.dB(u.W) can’t be applied to

power = ((15*u.m)/(3*u.m)) * u.dB(u.W)

Which will just throw an error that they are of different types and can’t be multiplied.

I’m aware of u.Decibel and u.DecibelUnit, but I wonder if u.dB was intended to operate this way.

Many thanks in advance!

Kind and best regards,
Boris

2

I don’t have answers for you, but I figured I’d make your report a bit more stark:

The product of a number and DecibelUnit creates a Decibel:

>>> 5*u.dB(1)
<Decibel 5. dB>
>>> 5*u.dB(u.W)
<Decibel 5. dB(W)>

The product of a Quantity and a dimensionless Decibel gives “natural” results:

>>> (5*u.one) * (1*u.dB(1))
<Decibel 5. dB>
>>> (5*u.m) * (1*u.dB(1))
<Quantity 5. dB m>

The product of a Quantity and a dimensioned Decibel is not allowed:

>>> (5*u.one) * (1*u.dB(u.W))
...
UnitTypeError: Cannot multiply function quantities which are not dimensionless with anything.

The product of a dimensionless Quantity and a dimensionless DecibelUnit triggers a logarithmic conversion of the Quantity:

>>> (5*u.one) * u.dB(1)
<Decibel 6.98970004 dB>

But the product of a Quantity and a DecibelUnit raises an error if either are dimensioned:

>>> (5*u.m) * u.dB(1)
...
TypeError: unsupported operand type(s) for *: 'Quantity' and 'DecibelUnit'
>>> (5*u.one) * u.dB(u.W)
...
TypeError: unsupported operand type(s) for *: 'Quantity' and 'DecibelUnit'
3

As a workaround, if your calculation is guaranteed to be dimensionless, you can call .to_value(u.one) to turn it back to a simple number before attaching the decibel unit.

4

Hi ayshih, many thanks for your replies! Regarding the workaround - indeed, that’s very similar to what I did in my project. I just added .value to get the raw number.

5

Sure, calling .value instead of .to_value(u.one) can be fine as long as you’re certain that that the units exactly cancel out, including not having stuff like u.m / u.mm.

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