# documentación de Django - 1.10.7

# Código fuente para django.contrib.gis.db.models.functions

    from decimal import Decimal

    from django.contrib.gis.db.models.fields import GeometryField
    from django.contrib.gis.db.models.sql import AreaField
    from django.contrib.gis.measure import (
        Area as AreaMeasure, Distance as DistanceMeasure,
    )
    from django.core.exceptions import FieldError
    from django.db.models import BooleanField, FloatField, IntegerField, TextField
    from django.db.models.expressions import Func, Value
    from django.utils import six

    NUMERIC_TYPES = six.integer_types + (float, Decimal)

    class GeoFunc(Func):
        function = None
        output_field_class = None
        geom_param_pos = 0

        def __init__(self, *expressions, **extra):
            if 'output_field' not in extra and self.output_field_class:
                extra['output_field'] = self.output_field_class()
            super(GeoFunc, self).__init__(*expressions, **extra)

        @property
        def name(self):
            return self.__class__.__name__

        @property
        def srid(self):
            expr = self.source_expressions[self.geom_param_pos]
            if hasattr(expr, 'srid'):
                return expr.srid
            try:
                return expr.field.srid
            except (AttributeError, FieldError):
                return None

        def as_sql(self, compiler, connection):
            if self.function is None:
                self.function = connection.ops.spatial_function_name(self.name)
            return super(GeoFunc, self).as_sql(compiler, connection)

        def resolve_expression(self, *args, **kwargs):
            res = super(GeoFunc, self).resolve_expression(*args, **kwargs)
            base_srid = res.srid
            if not base_srid:
                raise TypeError("Geometry functions can only operate on geometric content.")

            for pos, expr in enumerate(res.source_expressions[1:], start=1):
                if isinstance(expr, GeomValue) and expr.srid != base_srid:
                    # Automatic SRID conversion so objects are comparable
                    res.source_expressions[pos] = Transform(expr, base_srid).resolve_expression(*args, **kwargs)
            return res

        def _handle_param(self, value, param_name='', check_types=None):
            if not hasattr(value, 'resolve_expression'):
                if check_types and not isinstance(value, check_types):
                    raise TypeError(
                        "The %s parameter has the wrong type: should be %s." % (
                            param_name, str(check_types))
                    )
            return value

    class GeomValue(Value):
        geography = False

        @property
        def srid(self):
            return self.value.srid

        def as_sql(self, compiler, connection):
            return '%s(%%s, %s)' % (connection.ops.from_text, self.srid), [connection.ops.Adapter(self.value)]

        def as_mysql(self, compiler, connection):
            return '%s(%%s)' % (connection.ops.from_text), [connection.ops.Adapter(self.value)]

        def as_postgresql(self, compiler, connection):
            if self.geography:
                self.value = connection.ops.Adapter(self.value, geography=self.geography)
            else:
                self.value = connection.ops.Adapter(self.value)
            return super(GeomValue, self).as_sql(compiler, connection)

    class GeoFuncWithGeoParam(GeoFunc):
        def __init__(self, expression, geom, *expressions, **extra):
            if not hasattr(geom, 'srid') or not geom.srid:
                raise ValueError("Please provide a geometry attribute with a defined SRID.")
            super(GeoFuncWithGeoParam, self).__init__(expression, GeomValue(geom), *expressions, **extra)

    class SQLiteDecimalToFloatMixin(object):
        """
        By default, Decimal values are converted to str by the SQLite backend, which
        is not acceptable by the GIS functions expecting numeric values.
        """
        def as_sqlite(self, compiler, connection):
            for expr in self.get_source_expressions():
                if hasattr(expr, 'value') and isinstance(expr.value, Decimal):
                    expr.value = float(expr.value)
            return super(SQLiteDecimalToFloatMixin, self).as_sql(compiler, connection)

    class OracleToleranceMixin(object):
        tolerance = 0.05

        def as_oracle(self, compiler, connection):
            tol = self.extra.get('tolerance', self.tolerance)
            self.template = "%%(function)s(%%(expressions)s, %s)" % tol
            return super(OracleToleranceMixin, self).as_sql(compiler, connection)

    class Area(OracleToleranceMixin, GeoFunc):
        output_field_class = AreaField
        arity = 1

        def as_sql(self, compiler, connection):
            if connection.ops.geography:
                self.output_field.area_att = 'sq_m'
            else:
                # Getting the area units of the geographic field.
                source_fields = self.get_source_fields()
                if len(source_fields):
                    source_field = source_fields[0]
                    if source_field.geodetic(connection):
                        # TODO: Do we want to support raw number areas for geodetic fields?
                        raise NotImplementedError('Area on geodetic coordinate systems not supported.')
                    units_name = source_field.units_name(connection)
                    if units_name:
                        self.output_field.area_att = AreaMeasure.unit_attname(units_name)
            return super(Area, self).as_sql(compiler, connection)

        def as_oracle(self, compiler, connection):
            self.output_field = AreaField('sq_m')  # Oracle returns area in units of meters.
            return super(Area, self).as_oracle(compiler, connection)

    class AsGeoJSON(GeoFunc):
        output_field_class = TextField

        def __init__(self, expression, bbox=False, crs=False, precision=8, **extra):
            expressions = [expression]
            if precision is not None:
                expressions.append(self._handle_param(precision, 'precision', six.integer_types))
            options = 0
            if crs and bbox:
                options = 3
            elif bbox:
                options = 1
            elif crs:
                options = 2
            if options:
                expressions.append(options)
            super(AsGeoJSON, self).__init__(*expressions, **extra)

    class AsGML(GeoFunc):
        geom_param_pos = 1
        output_field_class = TextField

        def __init__(self, expression, version=2, precision=8, **extra):
            expressions = [version, expression]
            if precision is not None:
                expressions.append(self._handle_param(precision, 'precision', six.integer_types))
            super(AsGML, self).__init__(*expressions, **extra)

    class AsKML(AsGML):
        def as_sqlite(self, compiler, connection):
            # No version parameter
            self.source_expressions.pop(0)
            return super(AsKML, self).as_sql(compiler, connection)

    class AsSVG(GeoFunc):
        output_field_class = TextField

        def __init__(self, expression, relative=False, precision=8, **extra):
            relative = relative if hasattr(relative, 'resolve_expression') else int(relative)
            expressions = [
                expression,
                relative,
                self._handle_param(precision, 'precision', six.integer_types),
            ]
            super(AsSVG, self).__init__(*expressions, **extra)

    class BoundingCircle(GeoFunc):
        def __init__(self, expression, num_seg=48, **extra):
            super(BoundingCircle, self).__init__(*[expression, num_seg], **extra)

    class Centroid(OracleToleranceMixin, GeoFunc):
        arity = 1

    class Difference(OracleToleranceMixin, GeoFuncWithGeoParam):
        arity = 2

    class DistanceResultMixin(object):
        def source_is_geography(self):
            return self.get_source_fields()[0].geography and self.srid == 4326

        def convert_value(self, value, expression, connection, context):
            if value is None:
                return None
            geo_field = GeometryField(srid=self.srid)  # Fake field to get SRID info
            if geo_field.geodetic(connection):
                dist_att = 'm'
            else:
                units = geo_field.units_name(connection)
                if units:
                    dist_att = DistanceMeasure.unit_attname(units)
                else:
                    dist_att = None
            if dist_att:
                return DistanceMeasure(**{dist_att: value})
            return value

    class Distance(DistanceResultMixin, OracleToleranceMixin, GeoFuncWithGeoParam):
        output_field_class = FloatField
        spheroid = None

        def __init__(self, expr1, expr2, spheroid=None, **extra):
            expressions = [expr1, expr2]
            if spheroid is not None:
                self.spheroid = spheroid
                expressions += (self._handle_param(spheroid, 'spheroid', bool),)
            super(Distance, self).__init__(*expressions, **extra)

        def as_postgresql(self, compiler, connection):
            geo_field = GeometryField(srid=self.srid)  # Fake field to get SRID info
            if self.source_is_geography():
                # Set parameters as geography if base field is geography
                for pos, expr in enumerate(
                        self.source_expressions[self.geom_param_pos + 1:], start=self.geom_param_pos + 1):
                    if isinstance(expr, GeomValue):
                        expr.geography = True
            elif geo_field.geodetic(connection):
                # Geometry fields with geodetic (lon/lat) coordinates need special distance functions
                if self.spheroid:
                    self.function = 'ST_Distance_Spheroid'  # More accurate, resource intensive
                    # Replace boolean param by the real spheroid of the base field
                    self.source_expressions[2] = Value(geo_field._spheroid)
                else:
                    self.function = 'ST_Distance_Sphere'
            return super(Distance, self).as_sql(compiler, connection)

        def as_oracle(self, compiler, connection):
            if self.spheroid:
                self.source_expressions.pop(2)
            return super(Distance, self).as_oracle(compiler, connection)

    class Envelope(GeoFunc):
        arity = 1

    class ForceRHR(GeoFunc):
        arity = 1

    class GeoHash(GeoFunc):
        output_field_class = TextField

        def __init__(self, expression, precision=None, **extra):
            expressions = [expression]
            if precision is not None:
                expressions.append(self._handle_param(precision, 'precision', six.integer_types))
            super(GeoHash, self).__init__(*expressions, **extra)

    class Intersection(OracleToleranceMixin, GeoFuncWithGeoParam):
        arity = 2

    class IsValid(GeoFunc):
        output_field_class = BooleanField

    class Length(DistanceResultMixin, OracleToleranceMixin, GeoFunc):
        output_field_class = FloatField

        def __init__(self, expr1, spheroid=True, **extra):
            self.spheroid = spheroid
            super(Length, self).__init__(expr1, **extra)

        def as_sql(self, compiler, connection):
            geo_field = GeometryField(srid=self.srid)  # Fake field to get SRID info
            if geo_field.geodetic(connection) and not connection.features.supports_length_geodetic:
                raise NotImplementedError("This backend doesn't support Length on geodetic fields")
            return super(Length, self).as_sql(compiler, connection)

        def as_postgresql(self, compiler, connection):
            geo_field = GeometryField(srid=self.srid)  # Fake field to get SRID info
            if self.source_is_geography():
                self.source_expressions.append(Value(self.spheroid))
            elif geo_field.geodetic(connection):
                # Geometry fields with geodetic (lon/lat) coordinates need length_spheroid
                self.function = 'ST_Length_Spheroid'
                self.source_expressions.append(Value(geo_field._spheroid))
            else:
                dim = min(f.dim for f in self.get_source_fields() if f)
                if dim > 2:
                    self.function = connection.ops.length3d
            return super(Length, self).as_sql(compiler, connection)

        def as_sqlite(self, compiler, connection):
            geo_field = GeometryField(srid=self.srid)
            if geo_field.geodetic(connection):
                if self.spheroid:
                    self.function = 'GeodesicLength'
                else:
                    self.function = 'GreatCircleLength'
            return super(Length, self).as_sql(compiler, connection)

    class MakeValid(GeoFunc):
        pass

    class MemSize(GeoFunc):
        output_field_class = IntegerField
        arity = 1

    class NumGeometries(GeoFunc):
        output_field_class = IntegerField
        arity = 1

    class NumPoints(GeoFunc):
        output_field_class = IntegerField
        arity = 1

        def as_sqlite(self, compiler, connection):
            if self.source_expressions[self.geom_param_pos].output_field.geom_type != 'LINESTRING':
                raise TypeError("SpatiaLite NumPoints can only operate on LineString content")
            return super(NumPoints, self).as_sql(compiler, connection)

    class Perimeter(DistanceResultMixin, OracleToleranceMixin, GeoFunc):
        output_field_class = FloatField
        arity = 1

        def as_postgresql(self, compiler, connection):
            geo_field = GeometryField(srid=self.srid)  # Fake field to get SRID info
            if geo_field.geodetic(connection) and not self.source_is_geography():
                raise NotImplementedError("ST_Perimeter cannot use a non-projected non-geography field.")
            dim = min(f.dim for f in self.get_source_fields())
            if dim > 2:
                self.function = connection.ops.perimeter3d
            return super(Perimeter, self).as_sql(compiler, connection)

        def as_sqlite(self, compiler, connection):
            geo_field = GeometryField(srid=self.srid)  # Fake field to get SRID info
            if geo_field.geodetic(connection):
                raise NotImplementedError("Perimeter cannot use a non-projected field.")
            return super(Perimeter, self).as_sql(compiler, connection)

    class PointOnSurface(OracleToleranceMixin, GeoFunc):
        arity = 1

    class Reverse(GeoFunc):
        arity = 1

    class Scale(SQLiteDecimalToFloatMixin, GeoFunc):
        def __init__(self, expression, x, y, z=0.0, **extra):
            expressions = [
                expression,
                self._handle_param(x, 'x', NUMERIC_TYPES),
                self._handle_param(y, 'y', NUMERIC_TYPES),
            ]
            if z != 0.0:
                expressions.append(self._handle_param(z, 'z', NUMERIC_TYPES))
            super(Scale, self).__init__(*expressions, **extra)

    class SnapToGrid(SQLiteDecimalToFloatMixin, GeoFunc):
        def __init__(self, expression, *args, **extra):
            nargs = len(args)
            expressions = [expression]
            if nargs in (1, 2):
                expressions.extend(
                    [self._handle_param(arg, '', NUMERIC_TYPES) for arg in args]
                )
            elif nargs == 4:
                # Reverse origin and size param ordering
                expressions.extend(
                    [self._handle_param(arg, '', NUMERIC_TYPES) for arg in args[2:]]
                )
                expressions.extend(
                    [self._handle_param(arg, '', NUMERIC_TYPES) for arg in args[0:2]]
                )
            else:
                raise ValueError('Must provide 1, 2, or 4 arguments to `SnapToGrid`.')
            super(SnapToGrid, self).__init__(*expressions, **extra)

    class SymDifference(OracleToleranceMixin, GeoFuncWithGeoParam):
        arity = 2

    class Transform(GeoFunc):
        def __init__(self, expression, srid, **extra):
            expressions = [
                expression,
                self._handle_param(srid, 'srid', six.integer_types),
            ]
            super(Transform, self).__init__(*expressions, **extra)

        @property
        def srid(self):
            # Make srid the resulting srid of the transformation
            return self.source_expressions[self.geom_param_pos + 1].value

        def convert_value(self, value, expression, connection, context):
            value = super(Transform, self).convert_value(value, expression, connection, context)
            if not connection.ops.postgis and not value.srid:
                # Some backends do not set the srid on the returning geometry
                value.srid = self.srid
            return value

    class Translate(Scale):
        def as_sqlite(self, compiler, connection):
            func_name = connection.ops.spatial_function_name(self.name)
            if func_name == 'ST_Translate' and len(self.source_expressions) < 4:
                # Always provide the z parameter for ST_Translate (Spatialite >= 3.1)
                self.source_expressions.append(Value(0))
            elif func_name == 'ShiftCoords' and len(self.source_expressions) > 3:
                raise ValueError("This version of Spatialite doesn't support 3D")
            return super(Translate, self).as_sqlite(compiler, connection)

    class Union(OracleToleranceMixin, GeoFuncWithGeoParam):
        arity = 2