Astroquery API Specification

Service Class

The query tools will be implemented as class methods, so that the standard approach for a given web service (e.g., IRSA, UKIDSS, SIMBAD) will be

from astroquery.service import Service

result = Service.query_object('M 31')

for services that do not require login, and

from astroquery.service import Service

S = Service(user='username',password='password')
result = S.query_object('M 31')

for services that do.

Query Methods

The classes will have the following methods where appropriate:

query_object(objectname, ...)
query_region(coordinate, radius=, width=)
get_images(coordinate)

They may also have other methods for querying non-standard data types (e.g., ADS queries that may return a bibtex text block).

query_object

query_object is only needed for services that are capable of parsing an object name (e.g., SIMBAD, Vizier, NED), otherwise query_region is an adequate approach, as any name can be converted to a coordinate via the SIMBAD name parser.

query_region

Query a region around a coordinate.

One of these keywords must be specified (no default is assumed):

radius - an astropy Quantity object, or a string that can be parsed into one.
        e.g., '1 degree' or 1*u.degree.
        If radius is specified, the shape is assumed to be a circle
width - a Quantity.  Specifies the edge length of a square box
height - a Quantity.  Specifies the height of a rectangular box.  Must be passed with width.

Returns a Table.

get_images

Perform a coordinate-based query to acquire images.

Returns a list of HDUList objects.

Shape keywords are optional - some query services allow searches for images that overlap with a specified coordinate.

(query)_async

Includes get_images_async, query_region_async, query_object_async

Same as the above query tools, but returns a list of readable file objects instead of a parsed object so that the data is not downloaded until result.get_data() is run.

Common Keywords

These keywords are common to all query methods:

return_query_payload - Return the POST data that will be submitted as a dictionary
savename - [optional - see discussion below] File path to save the downloaded query to
timeout - timeout in seconds

Asynchronous Queries

Some services require asynchronous query submission & download, e.g. Besancon, the NRAO Archive, the Fermi archive, etc. The data needs to be “staged” on the remote server before it can be downloaded. For these queries, the approach is

result = Service.query_region_async(coordinate)

data = result.get_data()
# this will periodically check whether the data is available at the specified URL

Additionally, any service can be queried asynchronously - get_images_async will return readable objects that can be downloaded at a later time.

Outline of an Example Module

Directory Structure:

module/
module/__init__.py
module/core.py
module/tests/test_module.py

__init__.py contains:

from astropy import config as _config

class Conf(_config.ConfigNamespace):
    """
    Configuration parameters for `astroquery.template_module`.
    """
    server = _config.ConfigItem(
        ['http://dummy_server_mirror_1',
         'http://dummy_server_mirror_2',
         'http://dummy_server_mirror_n'],
        'Name of the template_module server to use.'
        )
    timeout = _config.ConfigItem(
        30,
        'Time limit for connecting to template_module server.'
        )

from .core import QueryClass

__all__ = ['QueryClass']

core.py contains:

from ..utils.class_or_instance import class_or_instance
from ..utils import commons, async_to_sync

__all__ = ['QueryClass']  # specifies what to import

@async_to_sync
class QueryClass(astroquery.BaseQuery):

    server = SERVER()

    def __init__(self, *args):
        """ set some parameters """
        # do login here
        pass

    @class_or_instance
    def query_region_async(self, *args, get_query_payload=False):

        request_payload = self._args_to_payload(*args)

        response = commons.send_request(self.server, request_payload, TIMEOUT)

        # primarily for debug purposes, but also useful if you want to send
        # someone a URL linking directly to the data
        if get_query_payload:
            return request_payload

        return response

    @class_or_instance
    def get_images_async(self, *args):
        image_urls = self.get_image_list(*args)
        return [get_readable_fileobj(U) for U in image_urls]
        # get_readable_fileobj returns need a "get_data()" method?

    @class_or_instance
    def get_image_list(self, *args):

        request_payload = self.args_to_payload(*args)

        result = requests.post(url, data=request_payload)

        return self.extract_image_urls(result)

    def _parse_result(self, result):
        # do something, probably with regexp's
        return astropy.table.Table(tabular_data)

    def _args_to_payload(self, *args):
        # convert arguments to a valid requests payload

        return dict

Parallel Queries

For multiple parallel queries logged in to the same object, you could do:

from astroquery.module import QueryClass

QC = QueryClass(login_information)

results = parallel_map(QC.query_object, ['m31', 'm51', 'm17'],
                       radius=['1"', '1"', '1"'])

results = [QC.query_object_async(obj, radius=r)
           for obj,r in zip(['m31', 'm51', 'm17'], ['1"', '1"', '1"'])]

Here parallel_map() is a parallel implementation of some map function.

Exceptions

  • What errors should be thrown if queries fail? Failed queries should raise a custom Exception that will include the full html (or xml) of the failure, but where possible should parse the web page’s error message into something useful.
  • How should timeouts be handled? Timeouts should raise a TimeoutError.

Examples

Standard usage should be along these lines:

from astroquery.simbad import Simbad

result = Simbad.query_object("M 31")
# returns astropy.Table object

from astroquery.irsa import Irsa

images = Irsa.get_images("M 31","5 arcmin")
# searches for images in a 5-arcminute circle around M 31
# returns list of HDU objects

images = Irsa.get_images("M 31")
# searches for images overlapping with the SIMBAD position of M 31, if supported by the service?
# returns list of HDU objects

from astroquery.ukidss import Ukidss

Ukidss.login(username, password)

result = Ukidss.query_region("5.0 0.0 gal", catalog='GPS')
# FAILS: no radius specified!
result = Ukidss.query_region("5.0 0.0 gal", catalog='GPS', radius=1)
# FAILS: no assumed units!
result = Ukidss.query_region("5.0 0.0 gal", catalog='GPS', radius='1 arcmin')
# SUCCEEDS!  returns an astropy.Table

from astropy.coordinates import SkyCoord
import astropy.units as u
result = Ukidss.query_region(
    SkyCoord(5,0,unit=('deg','deg'), frame='galactic'),
    catalog='GPS', region='circle', radius=5*u.arcmin)
# returns an astropy.Table

from astroquery.nist import Nist

hydrogen = Nist.query(4000*u.AA, 7000*u.AA, linename='H I', energy_level_unit='eV')
# returns an astropy.Table

For tools in which multiple catalogs can be queried, e.g. as in the UKIDSS examples, they must be specified. There should also be a list_catalogs function that returns a list of catalog name strings:

print(Ukidss.list_catalogs())

Unparseable Data

If data cannot be parsed into its expected form (Table, astropy.io.fits.PrimaryHDU), the raw unparsed data will be returned and a Warning issued.