The code snippets on this page need the following imports if you’re outside the pyqgis console:
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from qgis.core import ( QgsProject, QgsPathResolver ) from qgis.gui import ( QgsLayerTreeMapCanvasBridge, )
2. Loading Projects¶
Sometimes you need to load an existing project from a plugin or (more often) when developing a standalone QGIS Python application (see: Python Applications).
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:
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# If you are not inside a QGIS console you first need to import # qgis and PyQt classes you will use in this script as shown below: from qgis.core import QgsProject # Get the project instance project = QgsProject.instance() # Print the current project file name (might be empty in case no projects have been loaded) # print(project.fileName()) # Load another project project.read('testdata/01_project.qgs') print(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')
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 with the
allows setting a custom path pre-processor function, which allows for
manipulation of 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.
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:
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def my_processor(path): path= path.replace("user='gis_team'", "user='team_awesome'") path = path.replace("password='cats'", "password='g7as!m*'") return path QgsPathResolver.setPathPreprocessor(my_processor)