# A question on astropy logarithmic units behaviour

**URL:** <https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195>\
**Category:** Astropy\
**Tags:** astropy, question\
**Created:** [January 31, 2025, 1:11pm UTC](https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195 "2025-01-31T13:11:17Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Bikkerion](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/bikkerion/32/611_2.png) [@Bikkerion](https://community.openastronomy.org/u/Bikkerion)\
**Post date:** [January 31, 2025, 1:11pm UTC](https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195/1 "2025-01-31T13:11:17Z")

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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

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**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\
**Post date:** [January 31, 2025, 3:08pm UTC](https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195/2 "2025-01-31T15:08:55Z")

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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`:

```python
>>> 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:

```python
>>> (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:

```python
>>> (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`:

```python
>>> (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:

```python
>>> (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'

```

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<div class="post-metadata">

**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\
**Post date:** [January 31, 2025, 3:10pm UTC](https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195/3 "2025-01-31T15:10:16Z")

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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.

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**Author:** ![Bikkerion](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/bikkerion/32/611_2.png) [@Bikkerion](https://community.openastronomy.org/u/Bikkerion)\
**Post date:** [January 31, 2025, 3:33pm UTC](https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195/4 "2025-01-31T15:33:39Z")

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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.

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**Author:** ![ayshih](https://dub1.discourse-cdn.com/flex005/user_avatar/community.openastronomy.org/ayshih/32/84_2.png) [@ayshih](https://community.openastronomy.org/u/ayshih)\
**Post date:** [January 31, 2025, 3:47pm UTC](https://community.openastronomy.org/t/a-question-on-astropy-logarithmic-units-behaviour/1195/5 "2025-01-31T15:47:33Z")

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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`.