VOTable Handling (astropy.io.votable)#

Introduction#

The astropy.io.votable sub-package converts VOTable XML files to and from numpy record arrays.

Note

If you want to read or write a single table in VOTable format, the recommended method is via the High-level Unified File I/O interface. In particular, see the Unified I/O VO Tables section.

Getting Started#

Reading a VOTable File#

To read a VOTable file, pass a file path to parse:

from astropy.io.votable import parse
votable = parse("votable.xml")

votable is a VOTableFile object, which can be used to retrieve and manipulate the data and save it back out to disk.

Writing a VOTable File#

This section describes writing table data in the VOTable format using the votable package directly. For some cases, however, the high-level High-level Unified File I/O will often suffice and is somewhat more convenient to use. See the Unified I/O VOTable section for details.

To save a VOTable file, call the to_xml method. It accepts either a string or Unicode path, or a Python file-like object:

votable.to_xml('output.xml')

There are a number of data storage formats supported by astropy.io.votable. The TABLEDATA format is XML-based and stores values as strings representing numbers. The BINARY format is more compact, and stores numbers in base64-encoded binary. VOTable version 1.3 adds the BINARY2 format, which allows for masking of any data type, including integers and bit fields which cannot be masked in the older BINARY format. The storage format can be set on a per-table basis using the format attribute, or globally using the set_all_tables_format method:

votable.get_first_table().format = 'binary'
votable.set_all_tables_format('binary')
votable.to_xml('binary.xml')

The VOTable elements#

VOTables are built from nested elements. Let’s for example build a votable containing an INFO element:

>>> from astropy.io.votable.tree import VOTableFile, Info
>>> vot = VOTableFile()
>>> vot.infos.append(Info(name="date_obs", value="2025-01-01"))

These elements can be:

Here are some detailed explanations on some of these elements:

Using astropy.io.votable#

Standard Compliance#

astropy.io.votable.tree.TableElement supports the VOTable Format Definition Version 1.1, Version 1.2, Version 1.3, Version 1.4, and Version 1.5, Some flexibility is provided to support the 1.0 draft version and other nonstandard usage in the wild, see Verifying VOTables for more details.

Note

Each warning and VOTABLE-specific exception emitted has a number and is documented in more detail in Warnings and Exceptions.

Output always conforms to the 1.1, 1.2, 1.3, 1.4, 1.5 spec, depending on the input.

Verifying VOTables#

Many VOTable files in the wild do not conform to the VOTable specification. You can set what should happen when a violation is encountered with the verify keyword, which can take three values:

The verify keyword can be used with the parse() or parse_single_table() functions:

from astropy.io.votable import parse
votable = parse("votable.xml", verify='warn')

It is possible to change the default verify value through the astropy.io.votable.conf.verify item in the Configuration System (astropy.config).

Note that 'ignore' or 'warn' mean that astropy will attempt to parse the VOTable, but if the specification has been violated then success cannot be guaranteed.

It is good practice to report any errors to the author of the application that generated the VOTable file to bring the file into compliance with the specification.

Data Serialization Formats#

VOTable supports a number of different serialization formats.

  • TABLEDATA stores the data in pure XML, where the numerical values are written as human-readable strings.

  • BINARY is a binary representation of the data, stored in the XML as an opaque base64-encoded blob.

  • BINARY2 was added in VOTable 1.3, and is identical to “BINARY”, except that it explicitly records the position of missing values rather than identifying them by a special value.

  • FITS stores the data in an external FITS file. This serialization is not supported by the astropy.io.votable writer, since it requires writing multiple files.

  • PARQUET stores the data in an external PARQUET file, similar to FITS serialization. Reading and writing is fully supported by the astropy.io.votable writer and the astropy.io.votable.parse reader. The parquet file can be referenced with either absolute and relative paths. The parquet serialization can be used as part of the unified Table I/O (see next section), by setting the format argument to 'votable.parquet'.

The serialization format can be selected in two ways:

1) By setting the format attribute of a astropy.io.votable.tree.TableElement object:

votable.get_first_table().format = "binary"
votable.to_xml("new_votable.xml")

2) By overriding the format of all tables using the tabledata_format keyword argument when writing out a VOTable file:

votable.to_xml("new_votable.xml", tabledata_format="binary")

Converting to/from an astropy.table.Table#

The VOTable standard does not map conceptually to an astropy.table.Table. However, a single table within the VOTable file may be converted to and from an astropy.table.Table:

from astropy.io.votable import parse_single_table
table = parse_single_table("votable.xml").to_table()

As a convenience, there is also a function to create an entire VOTable file with just a single table:

from astropy.io.votable import from_table, writeto
votable = from_table(table)
writeto(votable, "output.xml")

Note

By default, to_table will use the ID attribute from the files to create the column names for the Table object. However, it may be that you want to use the name attributes instead. For this, set the use_names_over_ids keyword to True. Note that since field names are not guaranteed to be unique in the VOTable specification, but column names are required to be unique in numpy structured arrays (and thus astropy.table.Table objects), the names may be renamed by appending numbers to the end in some cases.

Performance Considerations#

File reads will be moderately faster if the TABLE element includes an nrows attribute. If the number of rows is not specified, the record array must be resized repeatedly during load.

Data Origin#

Introduction#

Extract basic provenance information from VOTable header. The information is described in DataOrigin IVOA note: https://www.ivoa.net/documents/DataOrigin/.

DataOrigin includes both the query information (such as publisher, contact, versions, etc.) and the Dataset origin (such as Creator, bibliographic links, URL, etc.)

This API retrieves Metadata from INFO in VOTable.

Getting Started#

To extract DataOrigin from VOTable

Example: VizieR catalogue J/AJ/167/18

>>> from astropy.io.votable import parse
>>> from astropy.io.votable.dataorigin import extract_data_origin
>>> votable = parse("https://vizier.cds.unistra.fr/viz-bin/conesearch/J/AJ/167/18/table4?RA=265.51&DEC=-22.71&SR=0.1")
>>> data_origin = extract_data_origin(votable)
>>> print(data_origin)
publisher: CDS
server_software: 7.4.5
service_protocol: ivo://ivoa.net/std/ConeSearch/v1.03
request_date: 2025-03-05T14:18:05
contact: cds-question@unistra.fr
publisher: CDS

ivoid: ivo://cds.vizier/j/aj/167/18
citation: doi:10.26093/cds/vizier.51670018
reference_url: https://cdsarc.cds.unistra.fr/viz-bin/cat/J/AJ/167/18
rights_uri: https://cds.unistra.fr/vizier-org/licences_vizier.html
creator: Hong K.
...

Contents and metadata#

astropy.io.votable.dataorigin.extract_data_origin returns a astropy.io.votable.dataorigin.DataOrigin (class) container which is made of:

  • a astropy.io.votable.dataorigin.QueryOrigin (class) container describing the request. QueryOrigin is considered to be unique for the whole VOTable. It includes metadata like the publisher, the contact, date of execution, query, etc.

  • a list of astropy.io.votable.dataorigin.DatasetOrigin (class) container for each Element having DataOrigin information. DataSetOrigin is a basic provenance of the datasets queried. Each attribute is a list. It includes metadata like authors, ivoid, landing pages, ….

Examples#

Get the (Data Center) publisher and the Creator of the dataset

>>> print(data_origin.query.publisher)
CDS
>>> print(data_origin.origin[0].creator)
['Hong K.']

Other capabilities#

DataOrigin container includes VO Elements:

  • Extract list of astropy.io.votable.tree.Info

    >>> # get DataOrigin with the description of each INFO
>>> for dataset_origin in data_origin.origin:
...    for info in dataset_origin.infos:
...        print(f"{info.name}: {info.value} ({info.content})")
ivoid: ivo://cds.vizier/j/aj/167/18 (IVOID of underlying data collection)
creator: Hong K. (First author or institution)
cites: bibcode:2024AJ....167...18H (Article or Data origin sources)
editor: Astronomical Journal (AAS) (Editor name (article))
original_date: 2024 (Year of the article publication)
...

  • Extract tree node astropy.io.votable.tree.Element

The following example extracts the citation from the header (in APA style).

>>> # get the Title retrieved in Element
>>> origin = data_origin.origin[0]
>>> vo_elt = origin.get_votable_element()
>>> title = vo_elt.description if vo_elt else ""
>>> print(f"APA: {','.join(origin.creator)} ({origin.publication_date[0]}). {title} [Dataset]. {data_origin.query.publisher}. {origin.citation[0]}")
APA: Hong K. (2024-11-06). Period variations of 32 contact binaries (Hong+, 2024) [Dataset]. CDS. doi:10.26093/cds/vizier.51670018

  • Add Data Origin INFO into VOTable:

>>> votable = parse("votable.xml")
>>> dataorigin.add_data_origin_info(votable, "query", "Data center name")
>>> dataorigin.add_data_origin_info(votable.resources[0], "creator", "Author name")

See Also#

Reference/API#