Add docs for ALFA service about astroquery

[cdeil]

An independent issue I just noticed is that importing astroquery.alfa gives a SyntaxWarning:

In [2]: from astroquery import alfa
WARNING: SyntaxWarning: name 'ALFALFACAT' is used prior to global declaration [astroquery.alfa]

This happens at https://github.com/astropy/astroquery/blob/master/astroquery/alfa/core.py#L81 .

Can this be avoided e.g. by declaring

ALFALFACAT = None

at the top and then setting it here?

So ALFALFACAT is one product of ALFA? Maybe you can add a sentence describing this in the docs?

[keflavich]

Actually, I think the correct approach is to make ALFALFACAT a global configurable as in Vizier (see #101)

[mirochaj]

I'll add docs for alfalfa today - I think you're right that it should just
be "astroquery/alfalfa" -- alfa is the name of the receiver, and alfalfa is
the name of a survey.

As for the ALFALFACAT variable - I'm not sure I understand what's happening
in Vizier, but I only defined this variable so that in a given python
session a user would only download the entire catalog once (it takes
several seconds). Then, any subsequent cross-IDs with the catalog are very
fast. ALFALFACAT is identical to the result of "get_catalog" in
astroquery/alfa/core.py. To me, it seems simplest adopt Christoph's
suggestion and set 'ALFALFACAT = None' at the top.

-Jordan

On Tue, Jun 4, 2013 at 7:48 AM, Adam Ginsburg [email protected]:

Actually, I think the correct approach is to make ALFALFACAT a global
configurable as in Vizier (see #101#101
)

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/106#issuecomment-18909938
.

[keflavich]

Ah, sorry, I'd forgotten what that variable referred to. I think we need to have a better (more general/universal) caching scheme for this type of catalog (i.e., metadata catalogs / lists of URLs, etc.). Another Issue....

[keflavich]

@mirochaj Fix this too. Ask if you need help understanding the new module design we're using.

[keflavich]

ALFALFA has been partially refactored; closing. @cdeil, please re-raise any of the secondary issues above if they persist.