asdf-astropy is intended to be an extension library for ASDF to enable support for the astropy package. It is intended to be used in conjunction with both the ASDF and astropy packages. To this end, the asdf-astropy package typically only needs to be installed in order to provide its functionality.

A quick example of ASDF

The ASDF format is a way of saving nested structures to yaml, where some of the stored data can be stored in a binary format. Thus, one typically structures an ASDF file as a dictionary, with key/value pairs.

For example if one wanted to store a Gaussian1D model in an ASDF file:

from asdf import AsdfFile
from astropy.modeling.models import Gaussian1D

# Create a Gaussian1D model
model = Gaussian1D(amplitude=10.4, mean=3.2, stddev=0.1)

# Create a tree structure for the ASDF file, and write it.
tree = {'gaussian_model': model}
ff = AsdfFile(tree)

# One can also create the file first, and then modify it directly.
ff = AsdfFile()
ff.tree['gaussian_model'] = model

To open the existing file one can use the top-level asdf.open method directly or as a context manager:

import asdf

ff = asdf.open("hello_world.asdf")

# As a context manager
with asdf.open("hello_world.asdf") as ff:

In either case the file ff will be an asdf.AsdfFile object which is accessible just like a Python dictionary. ASDF will fully realize all of the objects stored within the file automatically. For example, to access the model stored in the file ff['gaussian_model'] will be a Gaussian1D object exactly matching the model written originally.

One can also update an existing file. For example, if one wanted to update the hello_world.asdf file with a SkyCoord object:

import asdf
from astropy import units as u
from astropy.coordinates import SkyCoord

# Create a SkyCoord object
coord = SkyCoord(ra=10.625*u.degree, dec=41.2*u.degree, frame='icrs')

with asdf.open("hello_world.asdf", mode='rw') as af:
    af.tree['skycoord'] = coord

In the same way that the Gaussian1D model round-tripped, the SkyCoord object will also round-trip.

Further Reading

For further getting started material on ASDF see, Overview. If one wants to find more examples of how to write an astropy Table to ASDF files, see Using ASDF with Table I/O.