Sometimes you need to load an existing project from a plugin or (more often) when developing a stand-alone QGIS Python application (see: Python アプリケーション).
To load a project into the current QGIS aplication you need a QgsProject instance() object and call its read() method passing to it a QFileInfo object that contains the path from where the project will be loaded:
# If you are not inside a QGIS console you first need to import
# qgis and PyQt4 classes you will use in this script as shown below:
from qgis.core import QgsProject
from PyQt4.QtCore import QFileInfo
# 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
u'/home/user/projects/my_qgis_project.qgs'
# Load another project
project.read(QFileInfo('/home/user/projects/my_other_qgis_project.qgs'))
print project.fileName
u'/home/user/projects/my_other_qgis_project.qgs'
In case you need to make some modifications to the project (for example add or remove some layers) and save your changes, you can call the write() method of your project instance. The write() method also accepts an optional QFileInfo that allows you to specify a path where the project will be saved:
# Save the project to the same
project.write()
# ... or to a new file
project.write(QFileInfo('/home/user/projects/my_new_qgis_project.qgs'))
Both read() and write() funtions return a boolean value that you can use to check if the operation was successful.