PyQGIS: Select Features from a Vector Layer

Objectives:

• Select features from a vector layer
• Iterate over selected features

Open the QGIS Python Console

From the menu, select Plugins -> Python Console, or Ctrl + Alt + P (Windows)

You can type directly into the console, or select the pad and paper icon to write code in the editor. I prefer to write code in the editor because it allows me to save my work and run a compilation of commands.

Load a vector layer

Define the path to a vector

In the code editor, or console, create a variable containing the path to your vector file.

fn = 'c:/path/to/vector/file.shp'

Load vector into QGIS interface

Now we’ll simultaneously assign the vector layer to the layer variable and load it into the QGIS interface.

layer = iface.addVectorLayer(fn, '', 'ogr')

This adds the layer into the QGIS interface. In the legend, the vecotr is labeled with the file name.

Select features

There are multiple ways to select features from a shapefile (or other vector file). In this tutorial, we will cover selecting all features, selection by ID and selection by attribute values.

Select all features

It is very simple to select all features in a layer. We simply use the code layer.selectAll().

Select by ID

Selecting by ID selects a feature based on the feature id, of FID. The FID is a unique identifier for each feature in a layer. Generally, the FID corresponds to the order in which features were created. For example, the first feature created for a layer would have a FID of 1.

To select features, we simply pass a list of FIDs to layer.select().

The code below selects the features in layer that correspond to an FID in selectid. When you run this code in the QGIS python console you will see that these features become highlighted (see video below for demonstration).

selectid = [1, 3, 6, 8, 11]
layer.select(selectid)

Select by attribute values

Another common way to select features is by referencing attribute values for specific features. For example, we may want to select all features that have a value of ‘1’ for stream order.

Features can be selected based on attribute values with layer.selectByExpression(). We must pass a SQL expression defining the attributes we want to select.

The code below selects features in layer where the field ‘GRID_CODE’ is equal to ‘4’. The ‘GRID_CODE’ field represents stream order for this particular layer. See the video below for a visual demonstration.

layer.selectByExpression('"GRID_CODE"=4')

Iterate over a selection

Once we have selected features, we may want to perform operations for only the selected features. This can be accomplished by iterating through only the layers that are selected.

We can return the selected features with layer.selectedFeatures(), then iterate through those features. The code below shows how to iterate over a selection and print out an attribute.

# retrieve selected features
selection = layer.selectedFeatures()
# iterate over selected features
for feat in selection:
    # print an attribute
    print(feat['GRID_CODE'])

Video tutorial

See this tutorial described in a video.