Getting path of project, or layer file in PyQGIS?

I am generating new raster files from vector layers on a project. I would like to save them in the same directory as the project or layer files.

How do I find this path using PyQGIS?

Answer

For python console:

QgsProject.instance().readPath("./") # also try "../"

Or with

dir(QgsProject.instance())

you find something like

fileName()

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For python plugin (not tested!):

from PyQt4.QtGui import QMessageBox
from qgis.core import QgsProject
path_absolute = QgsProject.instance().readPath("./")
QMessageBox.information(None, "Title", "AP: " + unicode(path_absolute))