#### Overview of this book

QGIS is a desktop geographic information system that facilitates data viewing, editing, and analysis. Paired with the most efficient scripting language—Python, we can write effective scripts that extend the core functionality of QGIS. Based on version QGIS 2.18, this book will teach you how to write Python code that works with spatial data to automate geoprocessing tasks in QGIS. It will cover topics such as querying and editing vector data and using raster data. You will also learn to create, edit, and optimize a vector layer for faster queries, reproject a vector layer, reduce the number of vertices in a vector layer without losing critical data, and convert a raster to a vector. Following this, you will work through recipes that will help you compose static maps, create heavily customized maps, and add specialized labels and annotations. As well as this, we’ll also share a few tips and tricks based on different aspects of QGIS.

## Calculating length for all selected lines

If you need to calculate the total of a given dataset property such as length, the easiest thing to do is just use Python. In this recipe, we'll total the length of all of the railways in a dataset.

You will need to download a zipped shapefile from the following URL:

Unzip it and place it in directory named `ms` in your `qgis_data` directory.

### How to do it...

We will load the layer, loop through the features while keeping a running total of line lengths, and finally, convert the result to kilometers:

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

```        pth = "/Users/joellawhead/qgis_data/ms/ms_rails_mstm.shp"
```
2. Then, we'll load the layer:

```        lyr = QgsVectorLayer(pth, "Railroads", "ogr")
```
3. Next, we need a variable to `total` the line lengths:

```        total = 0
```
4. Now, we loop through the layer getting the length of each line:

`        for f in lyr.getFeatures():...`