Table.read on FITS file triggers PyYAML import #7783
Description
In Gammapy some of our tests are failing now in the build where we check basic functionality without optional dependencies like PyYAML installed:
https://travis-ci.org/gammapy/gammapy/jobs/422455259#L2801
The problem is that Table.read on a FITS file will try to import yaml and throw an ImportError, even if I mention format="fits".
Minimal example that fails with Astropy master as of today( 9a99f5e ).
>>> from astropy.table import Table
>>> Table.read('/Users/deil/work/code/gammapy-extra/datasets/hess-dl3-dr1/obs-index.fits.gz', format='fits')
Traceback (most recent call last):
File "/Users/deil/work/code/astropy/astropy/table/meta.py", line 321, in get_header_from_yaml
import yaml
ModuleNotFoundError: No module named 'yaml'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/deil/work/code/astropy/astropy/table/table.py", line 2554, in read
out = io_registry.read(cls, *args, **kwargs)
File "/Users/deil/work/code/astropy/astropy/io/registry.py", line 517, in read
data = reader(*args, **kwargs)
File "/Users/deil/work/code/astropy/astropy/io/fits/connect.py", line 200, in read_table_fits
astropy_native=astropy_native)
File "/Users/deil/work/code/astropy/astropy/io/fits/connect.py", line 277, in read_table_fits
t = _decode_mixins(t)
File "/Users/deil/work/code/astropy/astropy/io/fits/connect.py", line 97, in _decode_mixins
info = meta.get_header_from_yaml(lines)
File "/Users/deil/work/code/astropy/astropy/table/meta.py", line 323, in get_header_from_yaml
raise ImportError('`import yaml` failed, PyYAML package '
ImportError: `import yaml` failed, PyYAML package is required for serializing mixin columns
I think given that PyYAML is an optional dependency, and FITS support is built into Astropy, this should be considered a bug?
cc @saimn @taldcroft