gistfile1.py

import abc

class DataAccessor(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractproperty
    def wcs(self):
        pass

    @abc.abstractproperty
    def data(self):
        pass

    @abc.abstractproperty
    def url(self):
        pass

    def extract_metadata(self):
        return {
            'url': self.url
        }

class FitsImage(DataAccessor):
    def __init__(self, url, plane, beam, hdu=0):
        self._plane = plane
        self._hdu = hdu
        self._url = url
        self._beam = beam

        # We must provide all the abstract properties, or we can't instantiate
        # the class. We may also provide whatever extra properties we like:
        if 'TELESCOP' in self.header:
            self.telescope = self.header['TELESCOP']
        # But Trap code is explicitly not allowed to depend on their existence
        # (ie, if the attribute doesn't exist, Trap code must gracefully
        # degrade rather than throw an exception).

    # Optional properties, to match current FitsImage class
    @property
    def hdu(self):
        return pyfits.open(self.url)[self._hdu]

    @property
    def header(self):
        return self.hdu.header

    # Properties required to comply with interface
    @property
    def wcs(self):
        if not hasattr(self, "_wcs"):
            self._wcs = parse_coordinates(self.header)
        return self._wcs

    @property
    def data(self):
        return read_data(self.hdu, self._plane)

    @property
    def url(self):
        return self._url

f = FitsImage('url', 'plane', 'beam')