bug when serializing RectanglePixelRegion (probably any PixelRegion) to CRTF

[dmehring]

We have a developer trying to use this package. He has run up an issue trying to serialize a RectanglePixelRegion to CRTF

from astropy.coordinates import SkyCoord
from astropy import units as u
from regions import PixCoord
from regions import RectanglePixelRegion
region_pix = RectanglePixelRegion(center=PixCoord(x=42, y=43),
                                  width=3, height=4,
                                  angle=5 * u.deg)
region_pix.serialize(format='crtf')

fails with UnitConversionError: '' (dimensionless) and 'deg' (angle) are not convertible

The issue appears to arise at return serializer(regions, **kwargs) in serialize() in core/registry.py.
The issue is that kwargs is empty but _serialize_crtf(regions, coordsys='fk5', fmt='.6f', radunit='deg') which is called has a default unit of 'deg' which gets used in this case, even though it appears that the unit that should be used for the x value is empty/dimensionless.

[larrybradley]

Thanks, @dmehring. Yes, this is a bug in the API design and implementation of the CRTF serializer. It's on my radar to refactor the CRTF I/O functions. In the meantime, the workaround is to explicitly set the output coordinate frame to 'image', e.g,

from astropy.coordinates import SkyCoord
from astropy import units as u
from regions import PixCoord
from regions import RectanglePixelRegion
region_pix = RectanglePixelRegion(center=PixCoord(x=42, y=43),
                                  width=3, height=4,
                                  angle=5 * u.deg)
region_pix.serialize(format='crtf', coordsys='image')