"""

The GeometryProxy object, allows for lazy-geometries.  The proxy uses

Python descriptors for instantiating and setting Geometry objects

corresponding to geographic model fields.

Thanks to Robert Coup for providing this functionality (see #4322).

"""

class GeometryProxy(object):

    def __init__(self, klass, field):

        """

        Proxy initializes on the given Geometry class (not an instance) and

        the GeometryField.

        """

        self._field = field

        self._klass = klass

    def __get__(self, obj, type=None):

        """

        This accessor retrieves the geometry, initializing it using the geometry

        class specified during initialization and the HEXEWKB value of the field.

        Currently, only GEOS or OGR geometries are supported.

        """

        if obj is None:

            # Accessed on a class, not an instance

            return self

        # Getting the value of the field.

        geom_value = obj.__dict__[self._field.attname]

        if isinstance(geom_value, self._klass):

            geom = geom_value

        elif (geom_value is None) or (geom_value==''):

            geom = None

        else:

            # Otherwise, a Geometry object is built using the field's contents,

            # and the model's corresponding attribute is set.

            geom = self._klass(geom_value)

            setattr(obj, self._field.attname, geom)

        return geom

    def __set__(self, obj, value):

        """

        This accessor sets the proxied geometry with the geometry class

        specified during initialization.  Values of None, HEXEWKB, or WKT may

        be used to set the geometry as well.

        """

        # The OGC Geometry type of the field.

        gtype = self._field.geom_type

        # The geometry type must match that of the field -- unless the

        # general GeometryField is used.

        if isinstance(value, self._klass) and (str(value.geom_type).upper() == gtype or gtype == 'GEOMETRY'):

            # Assigning the SRID to the geometry.

            if value.srid is None: value.srid = self._field.srid

        elif value is None or isinstance(value, (basestring, buffer)):

            # Set with None, WKT, HEX, or WKB

            pass

        else:

            raise TypeError('cannot set %s GeometryProxy with value of type: %s' % (obj.__class__.__name__, type(value)))

        # Setting the objects dictionary with the value, and returning.

        obj.__dict__[self._field.attname] = value

        return value