Getting Started with Astropy

Importing Astropy

In order to encourage consistency amongst users in importing and using Astropy functionality, we have put together the following guidelines.

Since most of the functionality in Astropy resides in sub-packages, importing astropy as:

>>> import astropy

is not very useful. Instead, it is best to import the desired sub-package with the syntax:

>>> from astropy import subpackage  

For example, to access the FITS-related functionality, you can import astropy.io.fits with:

>>> from astropy.io import fits
>>> hdulist = fits.open('data.fits')  

In specific cases, we have recommended shortcuts in the documentation for specific sub-packages, for example:

>>> from astropy import units as u
>>> from astropy import coordinates as coord
>>> coord.SkyCoord(ra=10.68458*u.deg, dec=41.26917*u.deg, frame='icrs')  
<SkyCoord (ICRS): (ra, dec) in deg
    ( 10.68458,  41.26917)>

Finally, in some cases, most of the required functionality is contained in a single class (or a few classes). In those cases, the class can be directly imported:

>>> from astropy.cosmology import WMAP7
>>> from astropy.table import Table
>>> from astropy.wcs import WCS

Note that for clarity, and to avoid any issues, we recommend to never import any Astropy functionality using *, for example:

>>> from astropy.io.fits import *  # NOT recommended

Some components of Astropy started off as standalone packages (e.g. PyFITS, PyWCS), so in cases where Astropy needs to be used as a drop-in replacement, the following syntax is also acceptable:

>>> from astropy.io import fits as pyfits

Getting started with subpackages

Because different subpackages have very different functionality, further suggestions for getting started are in the documentation for the subpackages, which you can reach by browsing the sections listed in the User Documentation.

Or, if you want to dive right in, you can either look at docstrings for particular a package or object, or access their documentation using the find_api_page function. For example, doing this:

>>> from astropy import find_api_page
>>> from astropy.units import Quantity
>>> find_api_page(Quantity)  

Will bring up the documentation for the Quantity class in your browser.

Command-line utilities

For convenience, several of Astropy’s subpackages install utility programs on your system which allow common tasks to be performed without having to open a Python interpreter. These utilities include:

  • fitsheader: prints the headers of a FITS file.
  • fitscheck: verifies and optionally re-writes the CHECKSUM and DATASUM keywords of a FITS file.
  • fitsdiff: compares two FITS files and reports the differences.
  • Scripts: converts FITS images to bitmaps, including scaling and stretching.
  • samp_hub: starts a SAMP hub.
  • volint: checks a VOTable file for compliance against the standards.
  • wcslint: checks the WCS keywords in a FITS file for compliance against the standards.