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- Astroquery API Specification

# 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.'
            )

    conf = Conf()

    from .core import QueryClass

    __all__ = ['QueryClass', 'conf']

`core.py` contains:

    from ..utils.class_or_instance import class_or_instance
    from ..utils import async_to_sync
    from . import conf

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

    @async_to_sync
    class QueryClass(astroquery.query.BaseQuery):

        server = conf.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 = self._request(method="POST", url=self.server,
                                     data=request_payload, timeout=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)

            response = self._request(method="POST", url=self.server,
                                     data=request_payload, timeout=TIMEOUT)

            return self.extract_image_urls(response.text)

        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

## 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.ipac.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.