donz0r's blog

How does querying work with the existing VSO interface?

Querying a VSO server with the VSO interface of SunPy can look like this:

>>> from sunpy.net import vso
>>> client = vso.VSOClient()
>>> qr = client.query(vso.attrs.Time('2001/1/1', '2001/1/2'), vso.attrs.Instrument('eit'))

The query method of VSOClient receives any number of attributes and conjugates them. So if you read the exemplary query above, it reads like “query all the data that was observed from 2001/1/1 to 2001/1/2 and was observed using the instrument EIT”. query returns an instance of sunpy.net.vso.vso.QueryResponse which can be used to download the queried data.

How should querying the database look like?

To keep the usage consistent with the query interface of the VSO package described above, I plan to make the database query interface usable like this:

>>> from sunpy.net.vso import attrs as vso_attrs
>>> from sunpy.database import Database
>>> from sunpy.database import attrs as db_attrs
>>> database = Database('sqlite:///:memory:')
>>> entries = database.query(vso_attrs.Instrument('eit'), db_attrs.Tag('sun'), db_attrs.Starred(True))

The class Database gets a new method called query which accepts any number of attributes and returns a generator of DatabaseEntry instances. I hope you can see the similarities to the VSO query interface. Apart from the VSO attributes from the module sunpy.net.vso.attrs, this query method will also support database-specific attributes such as Tag or Starred. Equivalent to the VSO query method, this one also conjugates the passed attributes before they are processed, so the query above can be read as “give me all entries that were observed with an EIT instrument and are tagged 'sun' and are starred”.

Edit: After a night of good sleep, I think I have come to a better API for the Starred attribute. Or, more general, for any boolean attribute:

>>> starred_entries = database.query(db_attrs.Starred())
>>> unstarred_entries = database.query(~db_attrs.Starred())

The iterator starred_entries will contain all entries that have been marked as starred, whereas unstarred_entries will contain all non-starred entries.