Drawing some points in QGIS 2.4 using Python

Question

I'm new to PyQGIS so here goes: Give a new QGIS project, how can we draw some Points on the map?

I tried the following but nothing showed up. Should they be placed in a Vector layer before they can be rendered?

line_start = QgsPoint(50,50)
line_end = QgsPoint(100,150)
line = QgsGeometry.fromPolyline([line_start,line_end])

---

Update

from PyQt4.QtCore import QVariant  

lineLayer = QgsVectorLayer("LineString", 'test layer', "memory")  

lineLayer.startEditing()  
layerData = lineLayer.dataProvider() 
layerData.addAttributes([ QgsField("ID", QVariant.String), QgsField("latStart", QVariant.String), QgsField("lonStart", QVariant.String), QgsField("latEnd", QVariant.String), QgsField("lonEnd", QVariant.String) ])
lineLayer.commitChanges()

QgsMapLayerRegistry.instance().addMapLayer(lineLayer)  

This code created a layer but nothing appears on the map even after using the Zoom to layer function on that layer.

Answer 1

You have forgotten to add the feature:

lineLayer = QgsVectorLayer("LineString", 'test layer', "memory")  
pr = lineLayer.dataProvider()
# define the fields of the layer
pr.addAttributes([ QgsField("ID", QVariant.String), QgsField("latStart", QVariant.String), QgsField("lonStart", QVariant.String), QgsField("latEnd", QVariant.String), QgsField("lonEnd", QVariant.String) ])
# create a feature
feat = QgsFeature()
# add the geometry to the feature
line_start = QgsPoint(50,50)
line_end = QgsPoint(100,150)
feat.setGeometry(QgsGeometry.fromPolyline([line_start,line_end]))
# add the attributes (5 fields, string format) to the feature
feat.setAttributes(["a", "b", "c","d","e"])
# add the feature to the layer
pr.addFeatures([feat])
# update the extent of the layer
lineLayer.updateExtents()
# update the fields
lineLayer.updateFields()
# show the layer
QgsMapLayerRegistry.instance().addMapLayer(lineLayer)  

Answer 2

Hi Athena and welcome to PyQGIS programming! For having a good start you could take a look at PyQGIS Cookbook! Anyway, the snippets below is an example what you are trying to do of:

from PyQt4.QtCore import QVariant

## create an empty memory layer
vl = QgsVectorLayer("Point", "myLayer", "memory")
## define and add a field ID to memory layer "myLayer"
provider = vl.dataProvider()
provider.addAttributes([QgsField("ID", QVariant.Int)])
## create a new feature for the layer "myLayer"
ft = QgsFeature()
## set the value 1 to the new field "ID"
ft.setAttributes([1])
## set the geometry defined from the point X: 50, Y: 100
ft.setGeometry(QgsGeometry.fromPoint(QgsPoint(100,200)))
## finally insert the feature
pr.addFeatures([ft])
## add layer to the registry and over the map canvas
QgsMapLayerRegistry.instance().addMapLayer(vl)

Happy QGISsing!

Answer 3

I am no expert but I would assume when you draw any points, lines or polygons, they will have to be placed in a vector layer beforehand.

The following code is from here which describes how to add a line layer and attributes:

from qgis.core import QgsVectorLayer, QgsField, QgsMapLayerRegistry  
from PyQt4.QtCore import QVariant  

#create Layer
self.lineLayer = QgsVectorLayer("LineString", name, "memory")

Hope this helps.