The astropy.io.votable subpackage converts VOTable XML files to and from Numpy record arrays.
To read in 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.
VOTable files are made up of nested RESOURCE elements, each of which may contain one or more TABLE elements. The TABLE elements contain the arrays of data.
To get at the TABLE elements, one can write a loop over the resources in the VOTABLE file:
for resource in votable.resources:
for table in resource.tables:
# ... do something with the table ...
pass
However, if the nested structure of the resources is not important, one can use iter_tables to return a flat list of all tables:
for table in votable.iter_tables():
# ... do something with the table ...
pass
Finally, if there is expected to be only one table in the file, it might be simplest to just use get_first_table:
table = votable.get_first_table()
Even easier, there is a convenience method to parse a VOTable file and return the first table all in one step:
from astropy.io.votable import parse_single_table
table = parse_single_table("votable.xml")
From a Table object, one can get the data itself in the array member variable:
data = table.array
This data is a Numpy record array.
The columns get their names from both the ID and name attributes of the FIELD elements in the VOTABLE file. For example, suppose we had a FIELD specified as follows:
<FIELD ID="Dec" name="dec_targ" datatype="char" ucd="POS_EQ_DEC_MAIN"
unit="deg">
<DESCRIPTION>
representing the ICRS declination of the center of the image.
</DESCRIPTION>
</FIELD>
Note
The mapping from VOTable name and ID attributes to Numpy dtype names and titles is highly confusing.
In VOTable, ID is guaranteed to be unique, but is not required. name is not guaranteed to be unique, but is required.
In Numpy record dtypes, names are required to be unique and are required. titles are not required, and are not required to be unique.
Therefore, VOTable’s ID most closely maps to Numpy’s names, and VOTable’s name most closely maps to Numpy’s titles. However, in some cases where a VOTable ID is not provided, a Numpy name will be generated based on the VOTable name. Unfortunately, VOTable fields do not have an attribute that is both unique and required, which would be the most convenient mechanism to uniquely identify a column.
This column of data can be extracted from the record array using:
>>> table.array['dec_targ']
array([17.15153360566, 17.15153360566, 17.15153360566, 17.1516686826,
17.1516686826, 17.1516686826, 17.1536197136, 17.1536197136,
17.1536197136, 17.15375479055, 17.15375479055, 17.15375479055,
17.1553884541, 17.15539736932, 17.15539752176,
17.25736014763,
# ...
17.2765703], dtype=object)
or equivalently:
>>> table.array['Dec']
array([17.15153360566, 17.15153360566, 17.15153360566, 17.1516686826,
17.1516686826, 17.1516686826, 17.1536197136, 17.1536197136,
17.1536197136, 17.15375479055, 17.15375479055, 17.15375479055,
17.1553884541, 17.15539736932, 17.15539752176,
17.25736014763,
# ...
17.2765703], dtype=object)
It is also possible to build a new table, define some field datatypes and populate it with data:
from astropy.io.votable.tree import VOTableFile, Resource, Table, Field
# Create a new VOTable file...
votable = VOTableFile()
# ...with one resource...
resource = Resource()
votable.resources.append(resource)
# ... with one table
table = Table(votable)
resource.tables.append(table)
# Define some fields
table.fields.extend([
Field(votable, name="filename", datatype="char", arraysize="*"),
Field(votable, name="matrix", datatype="double", arraysize="2x2")])
# Now, use those field definitions to create the numpy record arrays, with
# the given number of rows
table.create_arrays(2)
# Now table.array can be filled with data
table.array[0] = ('test1.xml', [[1, 0], [0, 1]])
table.array[1] = ('test2.xml', [[0.5, 0.3], [0.2, 0.1]])
# Now write the whole thing to a file.
# Note, we have to use the top-level votable file object
votable.to_xml("new_votable.xml")
To save a VOTable file, simply 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 can not 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')
astropy.io.votable.tree.Table supports the VOTable Format Definition Version 1.1, Version 1.2, and the Version 1.3 proposed recommendation. Some flexibility is provided to support the 1.0 draft version and other non-standard usage in the wild. To support these cases, set the keyword argument pedantic to False when parsing.
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 or 1.3 spec, depending on the input.
Many VOTABLE files in the wild do not conform to the VOTABLE specification. If reading one of these files causes exceptions, you may turn off pedantic mode in astropy.io.votable by passing pedantic=False to the parse or parse_single_table functions:
from astropy.io.votable import parse
votable = parse("votable.xml", pedantic=False)
Note, however, that it is good practice to report these errors to the author of the application that generated the VOTABLE file to bring the file into compliance with the specification.
Even with pedantic turned off, many warnings may still be omitted. These warnings are all of the type VOTableSpecWarning and can be turned off using the standard Python warnings module.
Any value in the table may be “missing”. astropy.io.votable stores a Numpy masked array in each Table instance. This behaves like an ordinary Numpy masked array, except for variable-length fields. For those fields, the datatype of the column is “object” and another Numpy masked array is stored there. Therefore, operations on variable length columns will not work – this is simply because variable length columns are not directly supported by Numpy masked arrays.
The datatype specified by a FIELD element is mapped to a Numpy type according to the following table:
VOTABLE type Numpy type boolean b1 bit b1 unsignedByte u1 char (variable length) O - In Python 2.x, a str object; in 3.x, a bytes() object. char (fixed length) S unicodeChar (variable length) O - In Python 2.x, a unicode object, in utf-16; in 3.x a str object unicodeChar (fixed length) U short i2 int i4 long i8 float f4 double f8 floatComplex c8 doubleComplex c16
If the field is a fixed size array, the data is stored as a Numpy fixed-size array.
If the field is a variable size array (that is arraysize contains a ‘*’), the cell will contain a Python list of Numpy values. Each value may be either an array or scalar depending on the arraysize specifier.
To look up more information about a field in a table, one can use the get_field_by_id method, which returns the Field object with the given ID. For example:
>>> field = table.get_field_by_id('Dec')
>>> field.datatype
'char'
>>> field.unit
'deg'
Note
Field descriptors should not be mutated. To change the set of columns, convert the Table to an astropy.table.Table, make the changes, and then convert it back.
VOTable supports a number of different serialization formats.
The serialization format can be selected in two ways:
1) By setting the format attribute of a astropy.io.votable.tree.Table 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")
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.
This package reads and writes data formats used by the Virtual Observatory (VO) initiative, particularly the VOTable XML format.
| parse(source[, columns, invalid, pedantic, ...]) | Parses a VOTABLE xml file (or file-like object), and returns a VOTableFile object. |
| parse_single_table(source, **kwargs) | Parses a VOTABLE xml file (or file-like object), reading and returning only the first Table instance. |
| validate(source[, output, xmllint, filename]) | Prints a validation report for the given file. |
| from_table(table[, table_id]) | Given an Table object, return a VOTableFile file structure containing just that single table. |
| is_votable(source) | Reads the header of a file to determine if it is a VOTable file. |
| writeto(table, file[, tabledata_format]) | Writes a VOTableFile to a VOTABLE xml file. |
| Conf | Configuration parameters for astropy.io.votable. |
| Link([ID, title, value, href, action, id, ...]) | LINK elements: used to reference external documents and servers through a URI. |
| Info([ID, name, value, id, xtype, ref, ...]) | INFO elements: arbitrary key-value pairs for extensions to the standard. |
| Values(votable, field[, ID, null, ref, ...]) | VALUES element: used within FIELD and PARAM elements to define the domain of values. |
| Field(votable[, ID, name, datatype, ...]) | FIELD element: describes the datatype of a particular column of data. |
| Param(votable[, ID, name, value, datatype, ...]) | PARAM element: constant-valued columns in the data. |
| CooSys([ID, equinox, epoch, system, id, ...]) | COOSYS element: defines a coordinate system. |
| FieldRef(table, ref[, ucd, utype, config, pos]) | FIELDref element: used inside of GROUP elements to refer to remote FIELD elements. |
| ParamRef(table, ref[, ucd, utype, config, pos]) | PARAMref element: used inside of GROUP elements to refer to remote PARAM elements. |
| Group(table[, ID, name, ref, ucd, utype, ...]) | GROUP element: groups FIELD and PARAM elements. |
| Table(votable[, ID, name, ref, ucd, utype, ...]) | TABLE element: optionally contains data. |
| Resource([name, ID, utype, type, id, ...]) | RESOURCE element: Groups TABLE and RESOURCE elements. |
| VOTableFile([ID, id, config, pos, version]) | VOTABLE element: represents an entire file. |
This module handles the conversion of various VOTABLE datatypes to/from TABLEDATA and BINARY formats.
| get_converter(field[, config, pos]) | Get an appropriate converter instance for a given field. |
| table_column_to_votable_datatype(column) | Given a astropy.table.Column instance, returns the attributes necessary to create a VOTable FIELD element that corresponds to the type of the column. |
This file contains routines to verify the correctness of UCD strings.
| parse_ucd(ucd[, ...]) | Parse the UCD into its component parts. |
| check_ucd(ucd[, ...]) | Returns False if ucd is not a valid unified content descriptor. |
Various utilities and cookbook-like things.
| convert_to_writable_filelike(*args, **kwds) | Returns a writable file-like object suitable for streaming output. |
| coerce_range_list_param(p[, frames, numeric]) | Coerces and/or verifies the object p into a valid range-list-format parameter. |
Validates a large collection of web-accessible VOTable files, and generates a report as a directory tree of HTML files.
| make_validation_report([urls, destdir, ...]) | Validates a large collection of web-accessible VOTable files. |
Various XML-related utilities
| check_id(ID[, name, config, pos]) | Raises a VOTableSpecError if ID is not a valid XML ID. |
| fix_id(ID[, config, pos]) | Given an arbitrary string, create one that can be used as an xml id. |
| check_token(token, attr_name[, config, pos]) | Raises a ValueError if token is not a valid XML token. |
| check_mime_content_type(content_type[, ...]) | Raises a VOTableSpecError if content_type is not a valid MIME content type. |
| check_anyuri(uri[, config, pos]) | Raises a VOTableSpecError if uri is not a valid URI. |
| validate_schema(filename[, version]) | Validates the given file against the appropriate VOTable schema. |