Functional units in CDS tables #9946

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1. mhvk

   added

   Feature Request

   io.ascii

   units

   on Feb 13, 2020
2. DShivansh commented on Feb 13, 2020

   

   DShivansh

   on Feb 13, 2020

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   @mhvk can you please explain what is the difference between units and CDS units as I couldn't find any documentation. All I could find was #7751 and that too is not very clear.
3. mhvk commented on Feb 13, 2020

   

   mhvk

   on Feb 13, 2020

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   It's mostly the way to represent units in strings, which is mostly used in parsing and in to_string, and, implicitly, in things like reading a CDS table. For instance, at http://vizier.u-strasbg.fr/cgi-bin/VizieR-3?-source=J/A%2bA/557/A19&-out.max=50&-out.form=HTML%20Table&-out.add=_r&-out.add=_RAJ,_DEJ&-sort=_r&-oc.form=sexa, you'll see that the units of some columns have square brackets around them, but these are not recognized as meaning log10-base. Indeed, if I follow instructions on how to download this as an astropy table, I get:

   from astropy.io import ascii                                                                                                                              
   ascii.read('ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/557/A19/elm.dat', readme='ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/557/A19/ReadMe')
   WARNING: UnitsWarning: '[K]' did not parse as cds unit: Invalid character at col 0 If this is meant to be a custom unit, define it with 'u.def_unit'. To have it recognized inside a file reader or other code, enable it with 'u.add_enabled_units'. For details, see http://docs.astropy.org/en/latest/units/combining_and_defining.html [astropy.units.core]
   WARNING: UnitsWarning: '[cm/s2]' did not parse as cds unit: Invalid character at col 0 If this is meant to be a custom unit, define it with 'u.def_unit'. To have it recognized inside a file reader or other code, enable it with 'u.add_enabled_units'. For details, see http://docs.astropy.org/en/latest/units/combining_and_defining.html [astropy.units.core]
   <Table length=4183>
    logTe    logg     Mass      e_Mass       Age         e_Age   
     [K]   [cm/s2]    Msun       Msun        Myr          Myr    
   float64 float64  float64    float64     float64      float64  
   ------- ------- ---------- ---------- ------------ -----------
      3.85     3.0 0.24458909  0.0037877  34.94635274  8.27665511
      3.85     3.1 0.23590794  0.0036602  39.31200393   8.5529015
      3.85     3.2 0.22741011 0.00356954   41.6153104  8.72442808
      3.85     3.3 0.21925183 0.00350333  41.70069556   8.8584415
   
   

   Here, [K] should become dex(K) (i.e., u.dex(u.K)) and similarly for [cm/s2].

   To get this to work, one would need to add square brackets to the parser/lexer in the cds.py file linked at top and make those behave like the dex function in generic.py, and also adjust the CDS.to_string() method to convert logarithmic units to [...].
4. DShivansh commented on Feb 20, 2020

   

   DShivansh

   on Feb 20, 2020

   Last edited by DShivansh

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   @mhvk I have done the work of parsing and it is working fine for the above example(i.e. now it is not throwing any warning), can you please review the code. It's a very minor change. But can you suggest whether I am going right or not?
   Now I just have to implement the to_string functionality.
   P.S. sorry for the late reply, I found a lot of trouble studying the metaclasses :(
5. mhvk

   closed this as completedin #9971on Mar 4, 2020