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The Python console

API Note: Any QGIS APIs referenced will link out to the Python API Docs.

Before Starting

Open the project.qgz

Exploring the console

The Python Console and Python script editor is a great way to explore the QGIS api in order to try out the logic of your code before adding it to you scripts.

We won't spend long in here, just enough to try out some basic QGIS APIs and get used to the UI.

Exercise: Using the console

Open the console using Plugins -> Python Console

Running Commands

Inside the console we can run any commands we want to try. This is a full interactive Python console so any python code will work here.

Exercise: First Python commands

Try something basic like the following and hit enter

print("foss4g rocks")

The output is:

foss4g rocks

Exercise: First PyQGIS commands

Lets try some QGIS commands. Try each one and check the results

print(Qgis.QGIS_RELEASE_NAME)

print(iface.activeLayer().name())

print(iface.activeLayer().featureCount())

Intro the iface object

QGIS exposes a object called iface (QgisInterface). This object exposes methods that you can use to make some more complex operations easier. It's a wrapper around some of the lower level object. One example is iface.newProject().

Exercise: New project from PyQGIS

Run the following command to create a new project

iface.newProject()

Exercise: Access the main canvas

We can access the main canvas and it's scale function from here.

iface.mapCavnas().scale()

Lets move on the the script editor

The script editor

Before Starting

Open the project.qgz

From inside the console we can open the script editor. This editor can be used to write more complex scripts where the console is normally only used for smaller one off logic.

Exercise: Opening the editor

Open the editor from inside the Python Console using the following button and the script editor will open on the left.

Function Editor

For a quick test we can write the a simple one line command and run the script

Exercise: Running the first commands

print(iface.activeLayer().name())

Function Editor

Tip: Run selected text

You can run just the selected text as well with Ctrl+E if you have a more complex script and need to check just parts of it.

Lower case all layer names

For this example we will create a new file using the plus button and write a script to lower case all the layer names in the project.

Exercise: Import QGIS modules

Although QGIS has auto imported all the QGIS APIs into the console it's good practice to import them in your scripts to make things clear

In the new script add the following at the top

from qgis.core import QgsProject

Now we are ready to loop over all the layers and rename them to lower case. Copy the following into the script editor and use the green edit button to run the script

Exercise: Upper case all layer names

for layer in QgsProject.instance().mapLayers().values():
     print(layer.name())
     layer.setName(layer.name().upper())
     print("New name", layer.name())

The full script should look like this:

from qgis.core import QgsProject
for layer in QgsProject.instance().mapLayers().values():
     print(layer.name())
     layer.setName(layer.name().upper())
     print("New name", layer.name())

The output in the console should be

Function Editor

and the layers panel with the new names

Function Editor