Create Memory layer in pyQGIS with projected coordinates

Question

I am trying to make a memory layer in QGIS using python and then generate a grid base on calculations in meters. When I run the script, a banner comes and says the CRS is not defined. Then when I look at the derived information I see that the area of the Geometry and its position is wrong.

vector_layer = QgsVectorLayer('Polygon?crs:epsg=3879&field=ID:integer(3)&field=value:double','test',"memory")
poly = QgsGeometry.fromPolygonXY([[
    QgsPointXY(34.18639744, 95.15668787),
    QgsPointXY(12.0161471 , 82.35668787),
    QgsPointXY(24.8161471 , 60.18643753),
    QgsPointXY(46.98639744, 72.98643753)])
f = QgsFeature(vector_layer.fields())
f.setGeometry(poly)
vector_layer.dataProvider().addFeatures([f])
QgsProject.instance().addMapLayer(vector_layer)

This code will generate a square, but the location is wrong. I tried to define CRS afterward in the code:

crs = vector_layer.crs()
crs.createFromId(3879)
vector_layer.setCrs(crs)
QgsMapLayerRegistry.instance().addMapLayer(vector_layer)

But it did not work either. Can you please tell where I am making mistake?

Answer 1

As @Fran Raga said, you probably have a typo in your layer definition.

The reference system is set using ?crs=epsg:3879 and not ?crs:epsg=3879.

Check the QGIS Python API | Class: QgsVectorLayer for more details:

crs=definition Defines the coordinate reference system to use for the layer. definition is any string accepted by QgsCoordinateReferenceSystem.createFromString()

Change the syntax and try again, it should solve your problem.