2. Loading Projects

Hint

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

from qgis.core import (
    QgsProject,
    QgsPathResolver
)

from qgis.gui import (
    QgsLayerTreeMapCanvasBridge,
)

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 QgsProject class. 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:

# 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())

testdata/01_project.qgs

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. The 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')

Both read() and write() functions return a boolean value that you can use to check if the operation was successful.

Note

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. The QgsPathResolver class helps you rewrite layers path within the project.

Its setPathPreprocessor() 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:

1. replace an outdated path:

   def my_processor(path):
       return path.replace('c:/Users/ClintBarton/Documents/Projects', 'x:/Projects/')
   
   QgsPathResolver.setPathPreprocessor(my_processor)
   

2. 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)
   

3. replace stored database credentials with new ones:

   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)
   

Likewise, a setPathWriter() 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. See removePathPreprocessor() and removePathWriter().