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# Importing `astropy` and Sub-packages

In order to encourage consistency among 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’s 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 never importing 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 Sub-packages

Because different sub-packages have very different functionalities, each sub-package has its own getting started guide. These can be found by browsing the sections listed in the User Guide.

You can also look at docstrings for a particular package or object, or access their documentation using the `find_api_page` function. For example,

    >>> 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.