Using expression-based labels

Expressions are a kind of mini-programming language or SQL-like language found throughout different QGIS functions to selected features. One important use of expressions is controlling labels. Maps easily become cluttered if you label every single feature. Expressions make it easy to limit labels to important features. You can filter labels using expressions from within Python, as we will do in this recipe.

Getting ready

You will need to download the following zipped shapefile and decompress it to a directory named ms in your qgis_data directory:

https://github.com/GeospatialPython/Learn/raw/master/MS_UrbanAnC10.zip.

How to do it...

We'll use the QGIS PAL labeling engine to filter labels based on a field name. After loading the layer, we'll create our PAL settings and write it to the layer. Finally, we'll add the layer to the map.

1. First, we'll set up the path to our shapefile:

           pth = "/Users/joellawhead/qgis_data/ms/MS_UrbanAnC10.shp...