Separating a shapefile with specific features into a new shapefile using PyQGIS

Question

I have a shapefile. I would like to know if it is possible to classify the different layers in that shapefile (like highways, local roads, rail lines, etc.) with MTFCC and extract a separate shapefile from the existing one.

So for example, if I had a shapefile of our existing county with all vectors present could I make another, separate shapefile, of that same county containing only highways and local roads?

This is all using PyQGIS v2.14.0

Answer 1

There are existing algorithms from the Processing Toolbox which you can call. We could use two such algorithms:

1. Select by expression - Selects all features based on an expression.
2. Save selected features - Saves the selected features in a shapefile.

---

import processing

layer = qgis.utils.iface.activeLayer()
expression = """ "fieldName" = 'value_1' OR "fieldName" = 'value_2' """
result = "C:/Users/You/Desktop/result.shp"

processing.runalg("qgis:selectbyexpression", layer, expression, 0)
processing.runalg("qgis:saveselectedfeatures", layer, result)