What’s New in Astropy 0.4?

Overview

Astropy 0.4 is a major release that adds new functionality since the 0.3.x series of releases. A new sub-package is included (see SAMP), a major overhaul of the Coordinates sub-package has been completed (see Coordinates), and many new features and improvements have been implemented for the existing sub-packages. In addition to usability improvements, we have made a number of changes in the infrastructure for setting up/installing the package (see astropy-helpers package), as well as reworking the configuration system (see Configuration).

In addition to these major changes, a large number of smaller improvements have occurred. Since v0.3, by the numbers:

  • 819 issues have been closed

  • 511 pull requests have been merged

  • 57 distinct people have contributed code

Coordinates

The Astronomical Coordinate Systems (astropy.coordinates) sub-package has been largely re-designed based on broad community discussion and experience with v0.2 and v0.3. The key motivation was to implement coordinates within an extensible framework that cleanly separates the distinct aspects of data representation, coordinate frame representation and transformation, and user interface. This is described in the APE5 document. Details of the new usage are given in the Astronomical Coordinate Systems (astropy.coordinates) section of the documentation.

An important point is that this sub-package is now considered stable and we do not expect any further major interface changes.

For most users the major change is that the recommended user interface to coordinate functionality is the SkyCoord class instead of classes like ICRS or Galactic (which are now called “frame” classes). For example:

>>> from astropy import units as u
>>> from astropy.coordinates import SkyCoord
>>> coordinate = SkyCoord(123.4*u.deg, 56.7*u.deg, frame='icrs')

The frame classes can still be used to create coordinate objects as before, but they are now more powerful because they can represent abstract coordinate frames without underlying data. The more typical use for frame classes is now:

>>> from astropy.coordinates import FK4  # Or ICRS, Galactic, or similar
>>> fk4_frame = FK4(equinox='J1980.0', obstime='2011-06-12T01:12:34')
>>> coordinate.transform_to(fk4_frame)
<SkyCoord (FK4): equinox=J1980.000, obstime=2011-06-12T01:12:34.000, ra=123.001698182 deg, dec=56.7609162301 deg>

At the lowest level of the framework are the representation classes which describe how to represent a point in a frame as a tuple of quantities, for instance as spherical, cylindrical, or cartesian coordinates. Any coordinate object can now be created using values in a number of common representations and be displayed using those representations. For example:

>>> coordinate = SkyCoord(1*u.pc, 2*u.pc, 3*u.pc, representation='cartesian')
>>> coordinate
<SkyCoord (ICRS): x=1.0 pc, y=2.0 pc, z=3.0 pc>

>>> coordinate.representation = 'physicsspherical'
>>> coordinate
<SkyCoord (ICRS): phi=63.4349488229 deg, theta=36.6992252005 deg, r=3.74165738677 pc>

SAMP

The SAMP (Simple Application Messaging Protocol (astropy.samp) sub-package is a new sub-package (adapted from the SAMPy package) that contains an implementation of the Simple Application Messaging Protocol (SAMP) standard that allows communication with any SAMP-enabled application (such as TOPCAT, SAO Ds9, and Aladin). This sub-package includes both classes for a hub and a client, as well as an integrated client which automatically connects to any running SAMP hub and acts as a client:

>>> from astropy.vo.samp import SAMPIntegratedClient
>>> client = SAMPIntegratedClient()
>>> client.connect()

We can then use the client to communicate with other clients:

>>> client.get_registered_clients()
['hub', 'c1', 'c2']
>>> client.get_metadata('c1')
{'author.affiliation': 'Astrophysics Group, Bristol University',
 'author.email': 'm.b.taylor@bristol.ac.uk',
 'author.name': 'Mark Taylor',
 'home.page': 'http://www.starlink.ac.uk/topcat/',
 'samp.description.text': 'Tool for OPerations on Catalogues And Tables',
 'samp.documentation.url': 'http://127.0.0.1:2525/doc/sun253/index.html',
 'samp.icon.url': 'http://127.0.0.1:2525/doc/images/tc_sok.gif',
 'samp.name': 'topcat',
 'topcat.version': '4.0-1'}

and we can then send for example tables and images over SAMP to other applications (see SAMP (Simple Application Messaging Protocol (astropy.samp) for examples of how to do this).

Quantity

The Quantity class has seen a series of optimizations and is now substantially faster. Additionally, the time, coordinates, and table subpackages integrate better with Quantity, with further improvements on the way for table. See Quantity and the other subpackage documentation sections for more details.

Inspecting FITS headers from the command line

The FITS File handling (astropy.io.fits) sub-package now provides a command line script for inspecting the header(s) of a FITS file. With Astropy 0.4 installed, run fitsheader file.fits in your terminal to print the header information to the screen in a human-readable format. Run fitsheader --help to see the full usage documentation.

Reading and writing HTML tables

The ASCII Tables (astropy.io.ascii) sub-package now provides the capability to read a table within an HTML file or web URL into an astropy Table object. This requires the BeautifulSoup4 package to be installed. Conversely a Table object can now be written out as an HTML table.

Documentation URL changes

Starting in v0.4, the astropy documentation (and any package that uses astropy-helpers) will show the full name of functions and classes prefixed by the intended user-facing location. This is in contrast to previous versions, which pointed to the actual implementation module, rather than the intended public API location.

This will affect URLs pointing to specific documentation pages. For example, this URL points to the v0.3 location of the astropy.cosmology.luminosity_distance function:

while the appropriate URL for v0.4 and later is:

astropy-helpers package

We have now extracted our set-up and documentation utilities into a separate package, astropy-helpers. In practice, this does not change anything from a user point of view, but it is a big internal change that will allow any other packages to benefit from the set-up utilities developed for the core package without having to first install astropy.

Configuration

The configuration framework has been re-factored based on the design described in APE3. If you have previously edited the astropy configuration file (typically located at ~/.astropy/config/astropy.cfg) then you should read over Configuration Transition in order to understand how to update it to the new mechanism.

Deprecation and backward-incompatible changes

  • Quantity comparisons with == or != now always return True or False, even if units do not match (for which case a UnitsError used to be raised). [#2328]

  • The functional interface for astropy.cosmology (e.g. cosmology.H(z=0.5) is now deprecated in favor of the objected-oriented approach (WMAP9.H(z=0.5)). [#2343]

  • The astropy.coordinates sub-package has undergone major changes for implementing the APE5 plan for the package. A compatibility layer has been added that will allow common use cases of pre-v0.4 coordinates to work, but this layer will be removed in the next major version. Hence, any use of the coordinates package should be adapted to the new framework. Additionally, the compatibility layer cannot be used for convenience functions (like the match_catalog_*() functions), as these have been moved to SkyCoord. From this point on, major changes to the coordinates classes are not expected. [#2422]

  • The configuration framework has been re-designed to the scheme of APE3. The previous framework based on ConfigurationItem is deprecated, and will be removed in a future release. Affiliated packages should update to the new configuration system, and any users who have customized their configuration file should migrate to the new configuration approach. Until they do, warnings will appear prompting them to do so.

Full change log

To see a detailed list of all changes in version 0.4 and prior, please see the Full Changelog.

Note on future versions

While the current release supports Python 2.6, 2.7, and 3.1 to 3.4, the next release (1.0) will drop support for Python 3.1 and 3.2.