2. Loading Projects
The code snippets on this page need the following imports if you’re outside the pyqgis console:
1from qgis.core import ( 2 Qgis, 3 QgsProject, 4 QgsPathResolver 5) 6 7from qgis.gui import ( 8 QgsLayerTreeMapCanvasBridge, 9)
Sometimes you need to load an existing project from a plugin or (more often) when developing a standalone QGIS Python application (see: Приложения на Python).
To load a project into the current QGIS application you need to create
an instance of the
This is a singleton class, so you must use its
instance() method to do it.
You can call its
read() method, passing the path of the project to be loaded:
1# If you are not inside a QGIS console you first need to import 2# qgis and PyQt classes you will use in this script as shown below: 3from qgis.core import QgsProject 4# Get the project instance 5project = QgsProject.instance() 6# Print the current project file name (might be empty in case no projects have been loaded) 7# print(project.fileName()) 8 9# Load another project 10project.read('testdata/01_project.qgs') 11print(project.fileName())
If you need to make modifications to the project (for example to add or remove some layers)
and save your changes, call the
write() method of your project instance.
write() method also accepts an optional
path for saving the project to a new location:
# Save the project to the same project.write() # ... or to a new file project.write('testdata/my_new_qgis_project.qgs')
return a boolean value that you can use to check if the operation was successful.
If you are writing a QGIS standalone application, in order to synchronise the loaded project with
the canvas you need to instantiate a
QgsLayerTreeMapCanvasBridge as in the example below:
bridge = QgsLayerTreeMapCanvasBridge( \ QgsProject.instance().layerTreeRoot(), canvas) # Now you can safely load your project and see it in the canvas project.read('testdata/my_new_qgis_project.qgs')
2.1. Resolving bad paths
It can happen that layers loaded in the project are moved to another location.
When the project is loaded again all the layer paths are broken.
QgsPathResolver class helps you rewrite
layers path within the project.
method allows setting a custom path pre-processor function to
manipulate paths and data sources prior to resolving them to file references
or layer sources.
The processor function must accept a single string argument (representing the original file path or data source) and return a processed version of this path. The path pre-processor function is called before any bad layer handler. If multiple preprocessors are set, they will be called in sequence based on the order in which they were originally set.
Some use cases:
replace an outdated path:
def my_processor(path): return path.replace('c:/Users/ClintBarton/Documents/Projects', 'x:/Projects/') QgsPathResolver.setPathPreprocessor(my_processor)
replace a database host address with a new one:
def my_processor(path): return path.replace('host=10.1.1.115', 'host=10.1.1.116') QgsPathResolver.setPathPreprocessor(my_processor)
replace stored database credentials with new ones:
1def my_processor(path): 2 path= path.replace("user='gis_team'", "user='team_awesome'") 3 path = path.replace("password='cats'", "password='g7as!m*'") 4 return path 5 6QgsPathResolver.setPathPreprocessor(my_processor)
method is available for a path writer function.
An example to replace the path with a variable:
def my_processor(path): return path.replace('c:/Users/ClintBarton/Documents/Projects', '$projectdir$') QgsPathResolver.setPathWriter(my_processor)
Both methods return an
id that can be used to remove the pre-processor
or writer they added.
2.2. Using flags to speed up things
In some instances where you may not need to use a fully functional project, but only
want to access it for a specific reason, flags may be helpful. A full list of flags is available under
ProjectReadFlag. Multiple flags can be added together.
As an example, if we do not care about actual layers and data and simply want to
access a project (e.g. for layout or 3D view settings), we can use
to bypass the data validation step and prevent the bad layer dialog from appearing.
The following can be done:
readflags = Qgis.ProjectReadFlags() readflags |= Qgis.ProjectReadFlag.DontResolveLayers project = QgsProject() project.read('C:/Users/ClintBarton/Documents/Projects/mysweetproject.qgs', readflags)
To add more flags the python Bitwise OR operator (
|) must be used.