Many times it is useful for a plugin to save some variables so that the user does not have to enter or select them again next time the plugin is run.
These variables can be saved and retrieved with help of Qt and QGIS API. For each variable, you should pick a key that will be used to access the variable — for user’s favourite color you could use key “favourite_color” or any other meaningful string. It is recommended to give some structure to naming of keys.
We can make difference between several types of settings:
global settings — they are bound to the user at particular machine. QGIS itself stores a lot of global settings, for example, main window size or default snapping tolerance. This functionality is provided directly by Qt framework by the means of QSettings class. By default, this class stores settings in system’s “native” way of storing settings, that is — registry (on Windows), .plist file (on macOS) or .ini file (on Unix). The QSettings documentation is comprehensive, so we will provide just a simple example
def store():
s = QSettings()
s.setValue("myplugin/mytext", "hello world")
s.setValue("myplugin/myint", 10)
s.setValue("myplugin/myreal", 3.14)
def read():
s = QSettings()
mytext = s.value("myplugin/mytext", "default text")
myint = s.value("myplugin/myint", 123)
myreal = s.value("myplugin/myreal", 2.71)
The second parameter of the value() method is optional and specifies the default value if there is no previous value set for the passed setting name.
project settings — vary between different projects and therefore they are connected with a project file. Map canvas background color or destination coordinate reference system (CRS) are examples — white background and WGS84 might be suitable for one project, while yellow background and UTM projection are better for another one. An example of usage follows
proj = QgsProject.instance()
# store values
proj.writeEntry("myplugin", "mytext", "hello world")
proj.writeEntry("myplugin", "myint", 10)
proj.writeEntry("myplugin", "mydouble", 0.01)
proj.writeEntry("myplugin", "mybool", True)
# read values
mytext = proj.readEntry("myplugin", "mytext", "default text")[0]
myint = proj.readNumEntry("myplugin", "myint", 123)[0]
As you can see, the writeEntry() method is used for all data types, but several methods exist for reading the setting value back, and the corresponding one has to be selected for each data type.
map layer settings — these settings are related to a particular instance of a map layer with a project. They are not connected with underlying data source of a layer, so if you create two map layer instances of one shapefile, they will not share the settings. The settings are stored in project file, so if the user opens the project again, the layer-related settings will be there again. This functionality has been added in QGIS v1.4. The API is similar to QSettings — it takes and returns QVariant instances
# save a value
layer.setCustomProperty("mytext", "hello world")
# read the value again
mytext = layer.customProperty("mytext", "default text")