QuantityAttribute cannot have a unit and default=None

import astropy.units as u
from astropy.coordinates import (
    BaseCoordinateFrame,
    QuantityAttribute,
    frame_transform_graph,
    FunctionTransform,
    AltAz,
)


class MyFrame(BaseCoordinateFrame):
    a = QuantityAttribute(unit=u.m)


@frame_transform_graph.transform(FunctionTransform, MyFrame, AltAz)
def trans(my_frame_coord, altaz_frame):
    pass

This results in

Traceback (most recent call last):
  File "test_astropy.py", line 15, in <module>
    @frame_transform_graph.transform(FunctionTransform, MyFrame, AltAz)
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/transformations.py", line 659, in deco
    register_graph=self, **kwargs)
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/transformations.py", line 813, in __init__
    register_graph=register_graph)
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/transformations.py", line 709, in __init__
    for from_nm in fromsys.get_frame_attr_names():
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/baseframe.py", line 690, in get_frame_attr_names
    for name in cls.frame_attributes)
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/baseframe.py", line 690, in <genexpr>
    for name in cls.frame_attributes)
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/attributes.py", line 105, in __get__
    out, converted = self.convert_input(out)
  File "/home/maxnoe/.local/anaconda3/lib/python3.7/site-packages/astropy/coordinates/attributes.py", line 306, in convert_input
    raise TypeError('Tried to set a QuantityAttribute with '
TypeError: Tried to set a QuantityAttribute with something that does not have a unit.

Which is highly surprising behaviour to me.

[maxnoe]

I think I can see this potentially working (and might solve some of @Cadair’s problems?), but is there any chance you can link me to wherever you have this need, @MaxNoe? (i.e., is there a github repo?) I want to understand the details just a bit better.

It’s in https://github.com/cta-observatory/ctapipe

We are currently reworking the coordinates here: https://github.com/cta-observatory/ctapipe/pull/896

[eteq]

We have Attributes where if it is not set, we assume them to be the same in both frames.

I think I can see this potentially working (and might solve some of @Cadair’s problems?), but is there any chance you can link me to wherever you have this need, @MaxNoe? (i.e., is there a github repo?) I want to understand the details just a bit better.

The other question would be: why does registering a transformation even create an instance of a frame attribute?

In your example above, the a = QuantityAttribute(unit=u.m) line actually executes the constructor. QuantityAttribute is actually a descriptor - i.e., it’s the same sort of thing as a property. So creating the attribute is not the same as “populating it with data” (which happens in the MyFrame constructor rather than the QuantityAttribute constructor).

But registering the transformation then create a Quantity from the default because the get_frame_attr_names. I’m not entirely sure why (@adrn, do you?) but I think it was because SkyCoord uses the defaults some how? It doesn’t seem necessary, though, so I’ll see about fixing that in the PR to address the bug.

Regardless, I think the path forward is a quick PR immediately that addresses the “the current default doesn’t work” problem. That’s clearly a bug so should go in 3.1.x. Making None work the way described above is more of a feature and therefore may have to await 3.2 (but we can discuss it then independently of the above problem).