Save Units of columns into files with ascii.write()

I've noticed that there's no straightforward way to write out the units of a column in a generic way using astropy.table or astropy.io.ascii.

When you print the table on the terminal the Units appear correctly under the col-name but there's no way to save them into a file.

Similar to #4112

What if you use my_table.write(my_filename, format='ipac')? You should get something like this:

|wavelength|                  flux|
|    double|                double|
|    micron|erg / (Angstrom cm2 s)|
|      null|                  null|
     0.0805      2.91125425657e-17 
     0.0815      3.05136210514e-17 
     0.0825      3.22549746131e-17 
     0.0835      2.78810781295e-17 
     ...              ...

Yes but that only work if you need an IPAC table, not with any of the other formats available and for example the 'fixed_width' (in any of it's 3 forms) won't work...
So you are unable to get something like this:

         Name | Morphology  |        ra_u |       dec_u  | ...
              | InNS+B+MOI  | HH:MM:SS.SSS| ±DD:MM:SS:SSS| ...
 =============| ==========  | =========== | ============ | ...
 1_Cam_A      |    I        |04:32:01.845 | +53:54:39.03 | ...

and sadly the IPAC format isn't always the best format, that's why I want to make this Feature Request.

The ecsv format is the recommended generic way to write out to an ASCII file with units and all other metadata encapsulated. That format will round-trip and is readable by most generic CSV readers.

@Emberck

@Emberck , does ecsv as suggested by @taldcroft work for you? IMHO writing out units in fixed width format is not always desirable as some unit names can be a lot longer than your data.

@pllim

I will try to play a little with ecsv and tell you how it went
And @pllim I'm basically are with you but it's a request from the Boss...
Thanks

@Emberck

See also #2869.

@Emberck - note that the example you show is not including "units", but rather some other metadata which is more like "description". In astropy, "unit" has a specific meaning as a physical unit.

@taldcroft

@taldcroft well in my example you can see that I have a mix of metadata and physical units (such as Hours for the coordinates) and in the other cols (in this case it's a very long table) I have mags , seconds and deg which are included in astropy.units and have actual physical meaning.

Anyway, here is a workaround that is reasonably close to your desired output:

In [5]: t
Out[5]: 
<Table length=3>
  a      b     c  
int64 float64 str1
----- ------- ----
    1     1.0    c
    2     2.0    d
    3     3.0    e

In [6]: t['a'].unit = 'whatever'

In [7]: t['b'].unit = 'seconds'

In [8]: t['c'].unit = 'HH:MM:DDD'

In [9]: lines = t.pformat(max_width=-1, max_lines=-1, show_dtype=False)

In [11]: with open('myfile.dat', 'w') as fh:
   ....:     fh.write('\n'.join(lines))

In [12]: cat myfile.dat
   a        b        c    
whatever seconds HH:MM:DDD
-------- ------- ---------
       1     1.0         c
       2     2.0         d
       3     3.0         e

@taldcroft

Thanks @taldcroft
Yep that should work, now I'm doing something similar and there's also an option with an ipac table after saving the table you remove the null and Data Types rows...

In any case I think that would be nice to have the option of adding the 'Units row' within the ascii.write the same way that you can add the 'Names row' without workarounds...

That's why I send this Feature Request, and since write already save the row if the ipac format is used I guess it should't be much trouble...

Again thanks

P.D.: I'm been playing around with the ecsv output but so far I don't think it would work for me... still I will keep trying for a while

Do you have a requirement to be able to read such files back?

Yes, I need to be able to read the files in Python, IDL and by a human that's why I'm having such troubles finding just the right format

This is a duplicate of #756. afaict, it's exactly the same. So I suppose we're now at the timescale of looking at this issue approximately once every three years.

Closing this as a duplicate of #756.

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Save Units of columns into files with ascii.write() #4743

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Save Units of columns into files with ascii.write() #4743

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pllim commented on Mar 28, 2016

Emberck commented on Mar 28, 2016

taldcroft commented on Mar 28, 2016

pllim commented on Mar 28, 2016

Emberck commented on Mar 28, 2016

taldcroft commented on Mar 28, 2016

Emberck commented on Mar 28, 2016

taldcroft commented on Mar 28, 2016

Emberck commented on Mar 28, 2016

taldcroft commented on Mar 28, 2016

Emberck commented on Mar 28, 2016

keflavich commented on Nov 24, 2019

taldcroft commented on Nov 25, 2019

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