# documentación de Django - 1.10.7

# Código fuente para django.contrib.gis.forms.fields

    from __future__ import unicode_literals

    from django import forms
    from django.contrib.gis.geos import GEOSException, GEOSGeometry
    from django.utils.translation import ugettext_lazy as _

    from .widgets import OpenLayersWidget

    class GeometryField(forms.Field):
        """
        This is the basic form field for a Geometry.  Any textual input that is
        accepted by GEOSGeometry is accepted by this form.  By default,
        this includes WKT, HEXEWKB, WKB (in a buffer), and GeoJSON.
        """
        widget = OpenLayersWidget
        geom_type = 'GEOMETRY'

        default_error_messages = {
            'required': _('No geometry value provided.'),
            'invalid_geom': _('Invalid geometry value.'),
            'invalid_geom_type': _('Invalid geometry type.'),
            'transform_error': _('An error occurred when transforming the geometry '
                                 'to the SRID of the geometry form field.'),
        }

        def __init__(self, **kwargs):
            # Pop out attributes from the database field, or use sensible
            # defaults (e.g., allow None).
            self.srid = kwargs.pop('srid', None)
            self.geom_type = kwargs.pop('geom_type', self.geom_type)
            super(GeometryField, self).__init__(**kwargs)
            self.widget.attrs['geom_type'] = self.geom_type

        def to_python(self, value):
            """
            Transforms the value to a Geometry object.
            """
            if value in self.empty_values:
                return None

            if not isinstance(value, GEOSGeometry):
                try:
                    value = GEOSGeometry(value)
                except (GEOSException, ValueError, TypeError):
                    raise forms.ValidationError(self.error_messages['invalid_geom'], code='invalid_geom')

            # Try to set the srid
            if not value.srid:
                try:
                    value.srid = self.widget.map_srid
                except AttributeError:
                    if self.srid:
                        value.srid = self.srid
            return value

        def clean(self, value):
            """
            Validates that the input value can be converted to a Geometry
            object (which is returned).  A ValidationError is raised if
            the value cannot be instantiated as a Geometry.
            """
            geom = super(GeometryField, self).clean(value)
            if geom is None:
                return geom

            # Ensuring that the geometry is of the correct type (indicated
            # using the OGC string label).
            if str(geom.geom_type).upper() != self.geom_type and not self.geom_type == 'GEOMETRY':
                raise forms.ValidationError(self.error_messages['invalid_geom_type'], code='invalid_geom_type')

            # Transforming the geometry if the SRID was set.
            if self.srid and self.srid != -1 and self.srid != geom.srid:
                try:
                    geom.transform(self.srid)
                except GEOSException:
                    raise forms.ValidationError(
                        self.error_messages['transform_error'], code='transform_error')

            return geom

        def has_changed(self, initial, data):
            """ Compare geographic value of data with its initial value. """

            try:
                data = self.to_python(data)
                initial = self.to_python(initial)
            except forms.ValidationError:
                return True

            # Only do a geographic comparison if both values are available
            if initial and data:
                data.transform(initial.srid)
                # If the initial value was not added by the browser, the geometry
                # provided may be slightly different, the first time it is saved.
                # The comparison is done with a very low tolerance.
                return not initial.equals_exact(data, tolerance=0.000001)
            else:
                # Check for change of state of existence
                return bool(initial) != bool(data)

    class GeometryCollectionField(GeometryField):
        geom_type = 'GEOMETRYCOLLECTION'

    class PointField(GeometryField):
        geom_type = 'POINT'

    class MultiPointField(GeometryField):
        geom_type = 'MULTIPOINT'

    class LineStringField(GeometryField):
        geom_type = 'LINESTRING'

    class MultiLineStringField(GeometryField):
        geom_type = 'MULTILINESTRING'

    class PolygonField(GeometryField):
        geom_type = 'POLYGON'

    class MultiPolygonField(GeometryField):
        geom_type = 'MULTIPOLYGON'