Using generators for layer queries

Python generators provide an efficient way to process large datasets. A QGIS developer named Nathan Woodrow has created a simple Python QGIS query engine that uses generators to easily fetch features from QGIS layers. 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 that includes a Python setup file:

easy_install 

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

You can also download the ZIP file from https://github.com/NathanW2/qquery/archive/master.zip and copy the contents to your working directory or the site-packages directory of your QGIS Python installation.

You will also need to download the zipped shapefile and decompress it to a directory named ms in your qgis_data directory from...