Can I have two init functions in a python class?

Question

I'm porting some geolocation java code from http://janmatuschek.de/LatitudeLongitudeBoundingCoordinates#Java (shown below) to python. It can be initialized using two functions (fromDegrees or fromRadians). I thought I could do something like

class geoLocation:

    _radLat = 0
    _radLong = 0
    _degLat = 0
    _degLong = 0


    def fromDegrees(lat, long):
        #set _radLat, _radLong, _degLat, _degLong

    def fromRadians(lat, long):
        #set _radLat, _radLong, _degLat, _degLong

    ...

But that does not seem optimal since I set the values for _radLat, _radLong, _degLat and _degLong twice. Can I define two init functions? What's the best way to do that?

Thanks

/**
 * <p>Represents a point on the surface of a sphere. (The Earth is almost
 * spherical.)</p>
 *
 * <p>To create an instance, call one of the static methods fromDegrees() or
 * fromRadians().</p>
 *
 * <p>This code was originally published at
 * <a href="http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates#Java">
 * http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates#Java</a>.</p>
 *
 * @author Jan Philip Matuschek
 * @version 27 May 2010
 */
public class GeoLocation {

    private double radLat;  // latitude in radians
    private double radLon;  // longitude in radians

    private double degLat;  // latitude in degrees
    private double degLon;  // longitude in degrees

    private static final double MIN_LAT = Math.toRadians(-90d);  // -PI/2
    private static final double MAX_LAT = Math.toRadians(90d);   //  PI/2
    private static final double MIN_LON = Math.toRadians(-180d); // -PI*2
    private static final double MAX_LON = Math.toRadians(180d);  //  PI*2

    private GeoLocation () {
    }

    /**
     * @param latitude the latitude, in degrees.
     * @param longitude the longitude, in degrees.
     */
    public static GeoLocation fromDegrees(double latitude, double longitude) {
        GeoLocation result = new GeoLocation();
        result.radLat = Math.toRadians(latitude);
        result.radLon = Math.toRadians(longitude);
        result.degLat = latitude;
        result.degLon = longitude;
        result.checkBounds();
        return result;
    }

    /**
     * @param latitude the latitude, in radians.
     * @param longitude the longitude, in radians.
     */
    public static GeoLocation fromRadians(double latitude, double longitude) {
        GeoLocation result = new GeoLocation();
        result.radLat = latitude;
        result.radLon = longitude;
        result.degLat = Math.toDegrees(latitude);
        result.degLon = Math.toDegrees(longitude);
        result.checkBounds();
        return result;
    }

    private void checkBounds() {
        if (radLat < MIN_LAT || radLat > MAX_LAT ||
                radLon < MIN_LON || radLon > MAX_LON)
            throw new IllegalArgumentException();
    }

    /**
     * @return the latitude, in degrees.
     */
    public double getLatitudeInDegrees() {
        return degLat;
    }

    /**
     * @return the longitude, in degrees.
     */
    public double getLongitudeInDegrees() {
        return degLon;
    }

    /**
     * @return the latitude, in radians.
     */
    public double getLatitudeInRadians() {
        return radLat;
    }

    /**
     * @return the longitude, in radians.
     */
    public double getLongitudeInRadians() {
        return radLon;
    }

    @Override
    public String toString() {
        return "(" + degLat + "\u00B0, " + degLon + "\u00B0) = (" +
                radLat + " rad, " + radLon + " rad)";
    }

    /**
     * Computes the great circle distance between this GeoLocation instance
     * and the location argument.
     * @param radius the radius of the sphere, e.g. the average radius for a
     * spherical approximation of the figure of the Earth is approximately
     * 6371.01 kilometers.
     * @return the distance, measured in the same unit as the radius
     * argument.
     */
    public double distanceTo(GeoLocation location, double radius) {
        return Math.acos(Math.sin(radLat) * Math.sin(location.radLat) +
                Math.cos(radLat) * Math.cos(location.radLat) *
                Math.cos(radLon - location.radLon)) * radius;
    }

    /**
     * <p>Computes the bounding coordinates of all points on the surface
     * of a sphere that have a great circle distance to the point represented
     * by this GeoLocation instance that is less or equal to the distance
     * argument.</p>
     * <p>For more information about the formulae used in this method visit
     * <a href="http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates">
     * http://JanMatuschek.de/LatitudeLongitudeBoundingCoordinates</a>.</p>
     * @param distance the distance from the point represented by this
     * GeoLocation instance. Must me measured in the same unit as the radius
     * argument.
     * @param radius the radius of the sphere, e.g. the average radius for a
     * spherical approximation of the figure of the Earth is approximately
     * 6371.01 kilometers.
     * @return an array of two GeoLocation objects such that:<ul>
     * <li>The latitude of any point within the specified distance is greater
     * or equal to the latitude of the first array element and smaller or
     * equal to the latitude of the second array element.</li>
     * <li>If the longitude of the first array element is smaller or equal to
     * the longitude of the second element, then
     * the longitude of any point within the specified distance is greater
     * or equal to the longitude of the first array element and smaller or
     * equal to the longitude of the second array element.</li>
     * <li>If the longitude of the first array element is greater than the
     * longitude of the second element (this is the case if the 180th
     * meridian is within the distance), then
     * the longitude of any point within the specified distance is greater
     * or equal to the longitude of the first array element
     * <strong>or</strong> smaller or equal to the longitude of the second
     * array element.</li>
     * </ul>
     */
    public GeoLocation[] boundingCoordinates(double distance, double radius) {

        if (radius < 0d || distance < 0d)
            throw new IllegalArgumentException();

        // angular distance in radians on a great circle
        double radDist = distance / radius;

        double minLat = radLat - radDist;
        double maxLat = radLat + radDist;

        double minLon, maxLon;
        if (minLat > MIN_LAT && maxLat < MAX_LAT) {
            double deltaLon = Math.asin(Math.sin(radDist) /
                Math.cos(radLat));
            minLon = radLon - deltaLon;
            if (minLon < MIN_LON) minLon += 2d * Math.PI;
            maxLon = radLon + deltaLon;
            if (maxLon > MAX_LON) maxLon -= 2d * Math.PI;
        } else {
            // a pole is within the distance
            minLat = Math.max(minLat, MIN_LAT);
            maxLat = Math.min(maxLat, MAX_LAT);
            minLon = MIN_LON;
            maxLon = MAX_LON;
        }

        return new GeoLocation[]{fromRadians(minLat, minLon),
                fromRadians(maxLat, maxLon)};
    }

}

Answer 1

Chose one default ( radians or degrees ) and stick with it. You can write a classmethod to automatically convert to the other:

class geoLocation:
    def __init__(self, lat, long):
        """init class from lat,long as radians"""

    @classmethod
    def fromDegrees(cls, dlat, dlong):
        """creat `cls` from lat,long in degrees """
        return cls( to_radians(dlat), to_radians(dlong))

    @classmethod
    def fromRadians(cls, lat, long): # just in case
        return cls(lat, long)

obj = geoLocation.fromDegrees(10,20) # returns a new geoLocation object

Answer 2

I would just include a boolean in your init method. Instead of having two __init__ methods, do the following:

class geoLocation:
    def __init__(self, lat, long, degrees=True):
        if degrees:
            # process as fromDegrees
            (self._radLat, self._radLong, self._degLat, self._degLong) = self.fromDegrees(lat, long)
        else:
            (self._radLat, self._radLong, self._degLat, self._degLong) = self.fromRadians(lat, long)

    def fromDegrees(self, lat, long):
        # some function returning radLat and long and degLat and long in a tuple
    def fromRadians(self, lat, long):
        # same idea but different calculations

Answer 3

An option is to use factory class methods:

class geoLocation(object):
    @classmethod
    def fromDegrees(cls, lat, long):
        return cls(lat, long, True)

    @classmethod
    def fromRadians(cls, lat, long):
        return cls(lat, long, False)

    def __init__(self, lat, long, degrees=True):
        if degrees:
            #blah
        else:
            #blah

Answer 4

Another option is to have to subclasses of GeoLocation, say DegreesGeoLocation and RadiansGeoLocation. Now you can give each their own init function.

You are now storing the location twice in your class, once using radians and once using degrees. This can cause problems if you accidentally modify one representation but forget the other. I think you could best use one representation, and provide getters and setters which eventually do the conversion to the other representation.