Get started with PyQGIS 03 – Manipulate vector layers with QGIS python console

Get started with PyQGIS (3 Part Series)

1 Get started with PyQGIS 01 – Some basic commands with QGIS python console
2 Get started with PyQGIS 02 – Manage layers with QGIS python console
3 Get started with PyQGIS 03 – Manipulate vector layers with QGIS python console

On the previous chapter, we get started with vector layer classes called QgsVectorLayer.

It could be set as follow:

file_path = "your/path/data/target_file.shp"
layer_name = "my fantastic layer"

# declare a new layer and specify file_path and layer name layer = QgsVectorLayer(file_path, layer_name, "ogr")
# import declared layer QgsProject.instance().addMapLayer(layer)

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and layer name can be retrieved as follow :

# each layer is a QgsVectorLayer object for layer in QgsProject.instance().mapLayers().values():
     print(layer.name())

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Regarding documentation, QgsVectorLayer class is full of functions. Let check some basic functions with QGIS python console!

1. Open script editor of Python Console

In Python console we can launch commands one by one, but we need several commands to manage layers.
To run those at once, we need to open the script editor.

1.1. Open Python console in QGIS

As for the first chapter :
Open QGIS, menu -> Plugins -> Python Console

Or use shortcut Ctrl+Alt+P (Windows) command+option+P (mac)

Python console open usually at the bottom of QGIS frame

1.2. Open Script Editor

Click on the Show Editor button to open script editor as below:

2. Vector Layer related commands

Import the vector layer you want, here is Japan Prefectures

2.1. Retrieve target layer by name

As last chapter, we will retrieve layer by name (here japan_pref ), by

layer = QgsProject.instance().mapLayersByName("japan_pref")[0]

# check print(layer)

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Result :

<QgsVectorLayer: 'japan_pref' (ogr)>

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2.2. Retrieve layer extent

Simply add

layer_extent = layer.extent()
print("extent: ", layer_extent)

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The result should be a QgsRectangle object.

extent: <QgsRectangle: 122.93372516199046629 24.04561582899132333, 148.89439264601270452 45.55724341400912891>

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Coordinates looks to be in geographic system, let’s check.

2.3. Retrieve layer extent

Simply write

layer_crs = layer.sourceCrs()
print("CRS: ", layer_crs)

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The result should be the WGS 84 (EPSG:4326) reference system.

CRS: <QgsCoordinateReferenceSystem: EPSG:4326>

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2.4. Retrieve the number of features of the target layer

featureCount() method can do it.
you can combine with name() method to write a clean message

msg = f"{layer.name()} layer has {layer.featureCount()} features."
print(msg)

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And here is the number of Japan prefectures:

japan_pref layer has 47 features.

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2.5. Retrieves vector layer fields

You can try

layer_fields = layer.fields()
print (layer_fields)

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Result is an unreadable code, BUT!
class name QgsFields makes us to find a way to solve it.

<qgis._core.QgsFields object at 0x0000024950231160>

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Let’s check documentation.
You can check by searching QgsFields with your favourite search engine.
Regarding, Qgs Fields Documentation names method may be fine.

Let’s try!

layer_fields = layer.fields()
print (layer_fields.names())

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I got it !

3. Check output PyQgis class and its methods, a good pratice to make you grow up in PyQGIS

In this chapter, QgsVectorLayer basic methods has been used, and regarding documentation, many other usages can be done.

The last method to retrieve field is a combination of QgsVectorLayer and QgsFields.
However, we saw that when we hit to an unknown class, we can still moving on by checking documentation of this class, and find the good method easily.

This is a good way to understand easily a huge library such as PyQGIS.

Get started with PyQGIS (3 Part Series)

1 Get started with PyQGIS 01 – Some basic commands with QGIS python console
2 Get started with PyQGIS 02 – Manage layers with QGIS python console
3 Get started with PyQGIS 03 – Manipulate vector layers with QGIS python console

原文链接：Get started with PyQGIS 03 – Manipulate vector layers with QGIS python console