Creating QgsGeometry from Extended-WKT or Extended-WKB

Question

Creating a QgsGeometry using a WKT/WKB string is a fairly standard procedure using pyqgis:

wkt = 'MULTIPOLYGON (((0 0, 1 1, 1 0, 0 0)))'
wkb = '0106000000010000000103000000010000000400000000000000000000000000000000000000000000000000f03f000000000000f03f000000000000f03f000000000000000000000000000000000000000000000000'

geometry_wkt = QgsGeometry.fromWkt(wkt)
print(geometry_wkt) # <QgsGeometry: MultiPolygon (((0 0, 1 1, 1 0, 0 0)))>

geometry_wkb = QgsGeometry()
geometry_wkb.fromWkb(bytes.fromhex(wkb))
print(geometry_wkb) # <QgsGeometry: MultiPolygon (((0 0, 1 1, 1 0, 0 0)))>

But now I have data in the extended WKT/WKB format which also embeds a spatial reference identifier. I could't find a way to create a QgsGeometry from such a string. If I just use the same as above I always get <QgsGeometry: null> as result.

ewkt = 'SRID=4326;POLYGON((0 0,1 0,1,0 1,0 0))'
ewkb = '0103000020E6100000010000000500000000000000000000000000000000000000000000000000F03F0000000000000000000000000000F03F000000000000F03F0000000000000000000000000000F03F00000000000000000000000000000000'

geometry_ewkt = QgsGeometry.fromWkt(ewkt)
print(geometry_ewkt) # <QgsGeometry: null>

geometry_ewkb = QgsGeometry()
geometry_ewkb.fromWkb(bytes.fromhex(ewkb))
print(geometry_ewkb) # <QgsGeometry: null>

Is there any way to create those geometries using pyqgis API?

If not, I probably could extract the relevant wkt/wkb parts from the extended representation. I guess for EWKT it would be just dropping everything until the ;. But what do I do to convert EWKB string to WKB string?

Answer 1

I had the same problem and found a solution, though it uses an additional PostGis function on the EWKB before working with python:

i added ST_AsBinary to my query converting the EWKB to a WKB.
The result of psycopg2 cursor.fetchone()[0] is then of type memoryview, i convert it using the tobytes method then feeds it to the fromWkb method of QgsGeometry.

Example:

def function(connection):
    try:
        cursor = connection.cursor()
        req = "SELECT ST_AsBinary( myPostGisRequest(%s) );"
        parameters = (myParameter,)

        cursor.execute(req, parameters)
        memoryview_wkb = cursor.fetchone()[0]
        connection.commit()
    finally:
        cursor.close()

    if memoryview_wkb is None:
            return

    qgs_geometry = QgsGeometry()
    qgs_geometry.fromWkb(memoryview_wkb.tobytes())
    return qgs_geometry