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

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

Configuración de lectura y almacenamiento¶

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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.

Muchas veces es útil para un complemento guardar algunas variables para que el usuario no tenga que introducir o seleccionar de nuevo la próxima vez que el complemento se ejecute.

Estas variables se pueden guardar y recuperar con ayuda de Qt y QGIS API. Para cada variable, se debe escoger una clave que será utilizada para acceder a la variable — para el color favorito del usuario podría utilizarse la clave «favourite_color» o cualquier otra cadena que tenga sentido. Es recomendable dar un poco de estructura al nombrar las claves.

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")
```