QGIS has a powerful programming interface that allows you to extend the core functionality of the software as well as write scripts to automate your tasks. QGIS supports the popular Python scripting language. Even if you are a beginner, learning a little bit of Python and QGIS programming interface will allow you to be much more productive in your work. This tutorial assumes no prior programming knowledge and is intended to give an introduction to python scripting in QGIS (PyQGIS).
We will load a vector point layer representing all major airports and use python scripting to create a text file with the airport name, airport code, latitude and longitude for each of the airport in the layer.
ne_10m_airports.zip
file in the QGIS Browser and expand it. Select the ne_10m_airports.shp
file and drag it to the canvas.ne_10m_airports
layer loaded in QGIS.name
and iata_code
respectively. You can close the Identify window.>>>
at the bottom where you can type commands. For interacting with the QGIS environment, we must use the iface
variable. To access the currently active layer in QGIS, you can type the following and press Enter
. This command fetches the reference to the currently loaded layer and stores it in the layer
variable.layer = iface.activeLayer()
dir()
in python that shows you all available methods for any object. This is useful when you are not sure what functions are available for the object. Run the following command to see what operations we can do on the layer
variable.dir(layer)
getFeatures()
which will gets you the reference to all features of a layer. In our case, each feature will be a point representing an airport. You can type the following command to iterate through each of the features in the current layer.Note
Indentation (or number of spaces before each statement) is very important in Python. If you get error in this step, make sure you have added 2 spaces before typing the second line.
As the print(f) statement is inside a for-loop, you will need to press Enter
twice after that statement - once to exit the loop - and another to execute the command.
for f in layer.getFeatures():
print(f)
f
variable. We can use the f
variable to access the attributes of each feature. Type the following to print the name
and iata_code
for each airport feature.for f in layer.getFeatures():
print(f['name'], f['iata_code'])
geometry()
function. This function returns a geometry object that we can store in the variable geom
. You can run asPoint()
function on the geometry object to get the x and y coordinates of the point. If your feature is a line or a polygon, you can use asPolyline()
or asPolygon()
functions. Type the following code at the prompt and press Enter
to see the x and y coordinates of each feature.for f in layer.getFeatures():
geom = f.geometry()
print(geom.asPoint())
x
cordinate of the feature? You can call the x()
function on the point object and get its x coordinate.for f in layer.getFeatures():
geom = f.geometry()
print(geom.asPoint().x())
.format()
method which gives more control on printing multiple variables. The .2f
notation is to limit the coordinates to 2 decimals.for f in layer.getFeatures():
geom = f.geometry()
print('{},{},{:.2f},{:.2f}'.format(f['name'], f['iata_code'], geom.asPoint().y(), geom.asPoint().x()))
\n
at the end of our line formatting. This is to add a newline after we add the data for each feature.Note
There are 2 levels of code blocks below. Do make sure to add 4 spaces to the code starting line 3.
with open('/Users/ujaval/Desktop/airports.txt', 'w') as file:
for f in layer.getFeatures():
geom = f.geometry()
line = '{},{},{:.2f},{:.2f}\n'.format(f['name'], f['iata_code'], geom.asPoint().y(), geom.asPoint().x())
file.write(line)
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