API for tag selection

[braingram]

Let's say a user has an astropy.table.Table that when serialized could conform to a more generic table schema/tag.

af = AsdfFile({"my_table": generic_astropy_table})

On write, asdf will call the select_tag method of the table converter (if one is implemented) to determine what tag to use (and in turn which schema to use to check the serialized table). The current signature to select_tag provides the converter with:

• obj: in this case the astropy table
• ctx: the SerializationContext

Neither of which contain user-definable information about which tag to use in this specific case. It may be helpful to provide a way for a user to do something like:

af.use_tag(generic_astropy_table, "tag:stsci.edu:asdf/table/table-1*")

to inform asdf that the user would like to use the generic table tag for this object.

This may help to break the link between objects and tags for the python library. We should consider the side-effects this might produce.

Relation to astropy units

This could be helpful for other tags (like unit) where currently the asdf-astropy converter automatically selects a tag based on if the unit is vo-compatible or not. This has issues where a user may provide what they think is a valid astropy unit (but is not a vo-compatible unit) and then they see validation errors if the schema only allows the generic unit tag and not astropy units.

Generic options

An additional point to consider is if the per-object options (which already exist for array objects) could be standardized. So instead of the above use_tag perhaps something like:

af.set_object_option(generic_astropy_table, tag=""tag:stsci.edu:asdf/table/table-1*")

and to replace set_array_storage:

af.set_object_option(my_array, storage="inline")

and set_compression:

af.set_object_option(my_array, compression="bzp2")

and set_compression_kwargs:

af.set_object_option(my_array, compression_kwargs={"block_size": 10})

This last example highlights one concern with these generic options. The compression_kwargs are not saved to the file some consideration of how these options are round-tripped is worthwhile before developing this new API.