User defined units fail to parse when reading into QTable

I construct a QTable that takes in data as a user defined unit and output it to the current directory.

bandpass_sol_lum = u.def_unit('bandpass_sol_lum') 
lum_array = np.ones((5,5)) * bandpass_sol_lum

qt = QTable()
qt['Luminosity'] = QTable.Column(lum_array) 
qt.write('output_lum.fits',overwrite=True,format='fits')

When I read it back in:

qt_readIn = QTable()
qt_readIn['Luminosity'] = QTable.read('output_lum.fits',format='fits') # <--Exception thrown

I get:

YamlParseError: 'bandpass_sol_lum' did not parse as unit: At col 0, bandpass_sol_lum is not a valid unit. 

Is it possible at all to parse user defined units when reading them back in from a FITS file? @zpace

My environment: macOS 10.14.3 python 3.6.8 astropy 3.2 numpy 1.16.4

[Edit]: To address this issue, read the comments below and add the example to the documentation as discussed.

[CSwigg]

I could make the documentation changes if that’s alright.

[mhvk]

Fortunately, it is easier than that! The trick is to tell the whole system about the existence of this new unit using u.add_enabled_units(bandpass_sol_lum):

import numpy as np
from astropy import units as u
from astropy.table import QTable

bandpass_sol_lum = u.def_unit('bandpass_sol_lum') 
lum_array = np.ones((5,5)) * bandpass_sol_lum
qt = QTable()
qt['Luminosity'] = lum_array 
qt.write('output_lum.fits',overwrite=True,format='fits')

u.add_enabled_units(bandpass_sol_lum)
QTable.read('output_lum.fits', format='fits')
<QTable length=5>
 Luminosity [5] 
bandpass_sol_lum
    float64     
----------------
      1.0 .. 1.0
      1.0 .. 1.0
      1.0 .. 1.0
      1.0 .. 1.0
      1.0 .. 1.0

One thing that is odd - likely a bug - is that if I add the unit before writing the fits file, I get an error; e.g., if after the above I try rewriting:

WARNING: The unit 'bandpass_sol_lum' could not be saved to FITS format [astropy.io.fits.convenience]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-c1bb5ecc6cf2> in <module>()
----> 1 qt.write('output_lum.fits',overwrite=True,format='fits')

/data/mhvk/venv/numpy-dev/lib/python3.7/site-packages/numpy-1.17.0.dev0+0dcd300-py3.7-linux-x86_64.egg/astropy/table/connect.py in __call__(self, *args, **kwargs)
    112         instance = self._instance
    113         with serialize_method_as(instance, serialize_method):
--> 114             registry.write(instance, *args, **kwargs)

/data/mhvk/venv/numpy-dev/lib/python3.7/site-packages/numpy-1.17.0.dev0+0dcd300-py3.7-linux-x86_64.egg/astropy/io/registry.py in write(data, format, *args, **kwargs)
    564 
    565     writer = get_writer(format, data.__class__)
--> 566     writer(data, *args, **kwargs)
    567 
    568 

/data/mhvk/venv/numpy-dev/lib/python3.7/site-packages/numpy-1.17.0.dev0+0dcd300-py3.7-linux-x86_64.egg/astropy/io/fits/connect.py in write_table_fits(input, output, overwrite)
    385     input = _encode_mixins(input)
    386 
--> 387     table_hdu = table_to_hdu(input, character_as_bytes=True)
    388 
    389     # Check if output file already exists

/data/mhvk/venv/numpy-dev/lib/python3.7/site-packages/numpy-1.17.0.dev0+0dcd300-py3.7-linux-x86_64.egg/astropy/io/fits/convenience.py in table_to_hdu(table, character_as_bytes)
    530 
    531             # Try creating a Unit to issue a warning if the unit is not FITS compliant
--> 532             Unit(col.unit, format='fits', parse_strict='warn')
    533 
    534     # Column-specific override keywords for coordinate columns

/data/mhvk/venv/numpy-dev/lib/python3.7/site-packages/numpy-1.17.0.dev0+0dcd300-py3.7-linux-x86_64.egg/astropy/units/core.py in __call__(self, s, represents, format, namespace, doc, parse_strict)
   1843 
   1844         elif s is None:
-> 1845             raise TypeError("None is not a valid Unit")
   1846 
   1847         else:

TypeError: None is not a valid Unit

This is weird. And separate from your issue, so let’s make it a new one: #8887