Le fragment de code sur cette page nécessite les importations suivantes:

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

Lecture et sauvegarde de configurations¶

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Il est souvent utile pour une extension de sauvegarder des variables pour éviter à l’utilisateur de saisir à nouveau leur valeur ou de faire une nouvelle sélection à chaque lancement de l’extension.

Ces variables peuvent être sauvegardées et récupérées grâce à Qt et à l’API QGIS. Pour chaque variable, vous devez fournir une clé qui sera utilisée pour y accéder — pour la couleur préférée de l’utilisateur, vous pourriez utiliser la clé « couleur_favorite » ou toute autre chaîne de caractères explicite. Nous vous recommandons d’utiliser une convention pour nommer les clés.

Nous pouvons identifier différents types de paramètres :

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.

Ci-après un exemple d’utilisation de ces méthodes.

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.

Ci-après un exemple d’utilisation.

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")
```