Automate QGIS: How to Use the Python Console

Introduction

This article explains the Python console feature in QGIS and shows how to run Python code with the interactive console and the code editor. The same content is also covered in a video, so watch it for detailed instructions.

This article is a revised version of the “QGIS Blog” content that was published on the MIERUNE Inc. website.

What is the Python console?

The Python console is a feature for running Python from an interactive console and a code editor, as used in QGIS plugin development. It is useful for automation too small to need a plugin, and for checking how PyQGIS, the QGIS API for Python, behaves.

How to use the interactive console

First, open the Python console. In the menu bar, click Plugins → Python Console.

The Python Console panel opens at the bottom of the QGIS window.

Enter a Python command in the input area (area 7) and the result appears in the output area (area 8), so you can run commands interactively, as in a command-line interface. When the output grows long and becomes hard to read, click the Clear Console button (button 1) to clear the output area. You can also change the console's look by changing the theme in Options.

The table below describes each function.

How to use the editor

In the editor, you can write, run, and save Python scripts. It is also useful when you want to try out a process you plan to implement, including in QGIS plugin development. In particular, because the editor and the console share the same scope, you can change the arguments of a function defined in the editor and test it interactively.

The table below describes each function.

Python sample code

Now use Python to perform some simple layer operations. The data is the Lake Data from National Land Numerical Information. Download it and add it to QGIS.

On the lake layer you added, get the number of features in the layer and zoom to a feature selected with an expression. Enter the code below in the editor and click Run Script.

# Get the lake layer by specifying the layer name
layer = QgsProject.instance().mapLayersByName('W09-05-g_Lake')[0]
# Print the number of features
print(layer.featureCount())
# Select the feature whose W09_001 field value is Lake Sapporo
layer.selectByExpression('"W09_001"=\'さっぽろ湖\'')
# Store the first selected feature in a variable
selected_feature = layer.selectedFeatures()[0]
# Store the map canvas in a variable
canvas = iface.mapCanvas()
# Zoom to the selected feature
canvas.setExtent(selected_feature.geometry().boundingBox())
# Refresh the canvas
canvas.refresh()

The map canvas zooms to Lake Sapporo (さっぽろ湖), and the output area shows 556, the number of features in the lake layer.

Conclusion

This article gave a brief overview of how to use the Python console. The console runs code easily and lets you work with layers, the map canvas, and more. It is a very useful feature that is easy to recommend. Give it a try.