gistfile1.txt

vl = QgsVectorLayer("Point?crs=epsg:4326&field=id:integer&field=name:string(20)&index=yes", "temporary_points", "memory")
pr = vl.dataProvider()
pr.addAttributes([QgsField("name", QVariant.String), QgsField("age",  QVariant.Int), QgsField("size", QVariant.Double)])
vl.updateFields()

fet = QgsFeature()
fet.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
fet.setAttributes(["Johny", 2, 0.3])
pr.addFeatures([fet])

vl.updateExtents()

("fields:", len(pr.fields()))
("features:", pr.featureCount())
e = vl.extent()
print("extent:", e.xMinimum(), e.yMinimum(), e.xMaximum(), e.yMaximum())

features = vl.getFeatures()
for fet in features:
    print("F:", fet.id(), fet.attributes(), fet.geometry().asPoint())