Projections in GIS are commonly referred to by their “EPSG” codes, these are identifiers managed by the European Petroleum Survey Group.

One common identifier is “EPSG:4326”, which describes maps where latitude and longitude are treated as X/Y values.

Spherical Mercator has an official designation of EPSG:3857. However, before this was established, a large amount of software used the identifier EPSG:900913. This is an unofficial code, but is still the commonly usedin many GIS systems.

Any time you see the string “EPSG:4326”, you can assume it describes latitude/longitude coordinates.

Any time you see the string “EPSG:900913”, it will be describing coordinates in meters in x/y.

In python its quite easy to transform coordinates using OGR and OSR tools with their python wrapper.

    # Spatial Reference System  
    inputEPSG = 4326  
    outputEPSG = 3857  
    # create a geometry from coordinates  
    point = ogr.Geometry(ogr.wkbPoint)  
    point.AddPoint(pointX, pointY)  
    # create coordinate transformation  
    inSpatialRef = osr.SpatialReference()  
    outSpatialRef = osr.SpatialReference()  
    coordTransform = osr.CoordinateTransformation(inSpatialRef, outSpatialRef)  
    # transform point  
    # return point in EPSG 3857  
    return point.GetX(), point.GetY()  

Basically you can use coordTransform to transform any geometry (point,polygon,line e.t.c) between any osr supported projections.

Hope this helps