Using expression-based labels

Expressions are a kind of mini-programming language or SQL-like language found throughout different QGIS functions to select features. One important use of expressions is to control 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 zipped shapefile and decompress it to a directory named ms in your qgis_data directory from the following:

https://geospatialpython.googlecode.com/files/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 them to the layer. Finally, we'll add the layer to the map. To do this, we need to perform the following steps:

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

   pth = "/Users...