Using generators for layer queries

Python generators provide an efficient way to process large datasets. A QGIS developer named Nathan Woodrow created a simple Python QGIS query engine, which uses generators to make fetching features from QGIS layers easier. We'll use this engine in this recipe to query a layer.

Getting ready

You need to install the query engine using easy_install or by downloading it and adding it to your QGIS Python installation. To use easy_install, run the following command from a console, which downloads a clone of the original code but includes a Python setup file:

easy_install https://github.com/GeospatialPython/qquery/archive
             /master.zip

You can also download the ZIP file from the following URL and copy the contents to your working directory or the site-packages directory of your QGIS Python installation:

https://github.com/NathanW2/qquery/archive/master.zip

You will also need to download the following zipped shapefile and decompress it to a directory...