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****************************
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 # doctest: +SKIP
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') # doctest: +SKIP
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') # doctest: +FLOAT_CMP
<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 :ref:`user-docs`.
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
`~astropy.utils.misc.find_api_page` function. For example, doing this::
>>> from astropy import find_api_page
>>> from astropy.units import Quantity
>>> find_api_page(Quantity) # doctest: +SKIP
Will bring up the documentation for the `~astropy.units.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:
- `~astropy.io.fits.scripts.fitsheader`: prints the headers of a FITS file.
- `~astropy.io.fits.scripts.fitscheck`: verifies and optionally re-writes
the CHECKSUM and DATASUM keywords of a FITS file.
- :ref:`fitsdiff`: compares two FITS files and reports the differences.
- :ref:`fits2bitmap`: converts FITS images to bitmaps, including scaling and
stretching.
- :ref:`samp_hub <vo-samp-example_hub>`: starts a :ref:`SAMP <vo-samp>` hub.
- ``volint``: checks a :ref:`VOTable <astropy-io-votable>`
file for compliance against the standards.
- :ref:`wcslint <wcslint>`: checks the :ref:`WCS <astropy-wcs>` keywords in a
FITS file for compliance against the standards.
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