The code snippets on this page needs the following imports if you’re outside the pyqgis console:

from qgis.core import (
  QgsProject,
  QgsSettings,
  QgsVectorLayer
)

Leggere E Memorizzare Impostazioni¶

Avvertimento

Despite our constant efforts, information beyond this line may not be updated for QGIS 3. Refer to https://qgis.org/pyqgis/master for the python API documentation or, give a hand to update the chapters you know about. Thanks.

Molte volte è utile per un plugin salvare alcune variabili in modo tale che l’utente non debba inserirle o selezionarle di nuovo la volta successiva che il plugin è eseguito.

Questa variabili possono essere salvate e recuperate con l’aiuto delle API di Qt e QGIS. Per ogni variabile, dovresti scegliere una chiave che sarà usata per accedere alla variabile — per il colore preferito dell’utente potresti usare la chiave «favourite_color» o una qualunque altra stringa significativa. È raccomandabile dare una qualche struttura alla denominazione delle chiavi.

We can differentiate between several types of settings:

global settings — they are bound to the user at a particular machine. QGIS itself stores a lot of global settings, for example, main window size or default snapping tolerance. Settings are handled using the QgsSettings class, through for example the setValue() and value() methods.

Here you can see an example of how these methods are used.

def store():
  s = QgsSettings()
  s.setValue("myplugin/mytext", "hello world")
  s.setValue("myplugin/myint",  10)
  s.setValue("myplugin/myreal", 3.14)

def read():
  s = QgsSettings()
  mytext = s.value("myplugin/mytext", "default text")
  myint  = s.value("myplugin/myint", 123)
  myreal = s.value("myplugin/myreal", 2.71)
  nonexistent = s.value("myplugin/nonexistent", None)
  print(mytext)
  print(myint)
  print(myreal)
  print(nonexistent)

The second parameter of the value() method is optional and specifies the default value that is returned 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 (returns a tuple with the value, and a status boolean
# which communicates whether the value retrieved could be converted to
# its type, in these cases a string, an integer, a double and a boolean
# respectively)

mytext, type_conversion_ok = proj.readEntry("myplugin",
                                            "mytext",
                                            "default text")
myint, type_conversion_ok = proj.readNumEntry("myplugin",
                                              "myint",
                                              123)
mydouble, type_conversion_ok = proj.readDoubleEntry("myplugin",
                                                    "mydouble",
                                                    123)
mybool, type_conversion_ok = proj.readBoolEntry("myplugin",
                                                "mybool",
                                                123)

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 inside the project file, so if the user opens the project again, the layer-related settings will be there again. The value for a given setting is retrieved using the customProperty() method, and can be set using the setCustomProperty() one.
```
vlayer = QgsVectorLayer()
# save a value
vlayer.setCustomProperty("mytext", "hello world")

# read the value again (returning "default text" if not found)
mytext = vlayer.customProperty("mytext", "default text")
```