A large component of most web applications, including those built on Google App Engine, is a backing persistent data store. Bulk data import and export to and from the data store critical for purposes of backup, testing, and migrating between cloud service providers.
You can find detailed information on how to download all data from one of your App Engine applications here.
You can find detailed information on how to upload all data from one of your App Engine applications here.
The Google App Engine Python SDK includes bulkloader.py, a tool for importing and exporting data from your App Engine data store. The upload and download functionality are also integrated into appcfg.py. Full instructions for import and export can be found at here.