Today I am going to talk of one of those things that can get you bordered many hours Googling the web before find out the right way to do it.
I just land in the GWT island with Duke, on his Google App Engine, coming from the old Java EE world. Working on Fastcall I had to export - import data from the Google's App Engine Datastore. At first glance, I expected  to do it in two or three clicks, but that was not the case.
Very quickly I get stuck because my project was developed on the Java runtime environment where the native support for export – import  data is missing.
So if you are concerned with that, take a closer look at this post.
1- Due to the use of python, we’ll install on our local machine:
“Google App Engine SDK for Python” (http://code.google.com/appengine/downloads.html)  and 
the python interpreter from http://www.python.org/download/.
2- In our application’s web.xml deployment descriptor file we will add the following servlet com.google.apphosting.utils.remorteapi.RemorteApiServlet in order to interact with our web application using python commands.
------------------------------------------------------------
<servlet>
      <servlet-name>remoteapi</servlet-name>
      <servlet-class>com.google.apphosting.utils.remoteapi.RemoteApiServlet</servlet-class>
</servlet>
<servlet-mapping>
      <servlet-name>remoteapi</servlet-name>
      <url-pattern>/remote_api</url-pattern>
</servlet-mapping>
<security-constraint>
      <web-resource-collection>
            <web-resource-name>remoteapi</web-resource-name>
            <url-pattern>/remote_api</url-pattern>
       </web-resource-collection>
       <auth-constraint>
              <role-name>admin</role-name>
       </auth-constraint>
</security-constraint>
------------------------------------------------------------
3- Deploy your web application on the App Engine.
So for so good, the first part of the work is done!!!!
Next we will see two kind of import - exports:
 A- Raw data import - export
We can make use of the python commands to do some import - export tasks:
Note: In the commands below you can ommit the –kind=<…> argument. In that case all our entities will be backup.
Dump:  (be sur that the our_dump.txt doesn’t exist)
bulkloader.py --dump --app_id=<our_app_id> --kind=<our_entity> --url=http://<our_app_name>.appspot.com/remote_api --filename=<our_dump.txt>
Restore:
bulkloader.py --restore --app_id=<our_app_id> --kind=<our_ entity> --filename=<our_dump.txt> --url=http://<our_app_name>.appspot.com/remote_api 
When running each of those commands, you will be prompted for authentication. Just put your Google email account and you password.
 B- Formatted data import – export
 We'll use the bulkloader.yaml configuration file to describe how your data should be transformed when uploaded or downloaded. But where is this file??? Well, we are going to create it with this command:
appcfg.py create_bulkloader_config --filename=bulkloader.yaml --url http://<our_app_id>.appspot.com/remote_api
 Then let’s customize it.
Go to the transformers section and do your own customization: This configuration below will probably be not suitable for you. Use it just like a sample.
------------------------------------------------------------
transformers:
- kind: Contact
  connector: CSV
  property_map:
    - property: __key__
      external_name: email
      export_transform: transform.key_id_or_name_as_string
    - property: name
      external_name: name
    - property: surname
      external_name: prenom
    - property: numTelF
      external_name: numTelF
    - property: numTelM
      external_name: numTelM
    - property: Fax
      external_name: Fax
    - property: adresse
      external_name: adresse
------------------------------------------------------------
This file has 2 main parts:
The connector configuration: lets you choose the type of the backup file. Available connectors are csv, xml or simpletext. The last connector can only be used for export not import (keep that in mind).
The property mapping: this is where you define the properties you want to export, the mappings between the entity properties and the exported names and finally some import – export transformation functions that will occur during the process.
The kind keyword means our persisted entity class: Contact in our case.
Run the following commands:
Note: the –kind=<…> is mandatory this time.
Dump:
appcfg.py download_data --config_file=bulkloader.yaml --filename=<filename> --kind=<our_ entity> --url http://<our_app_id>.appspot.com/remote_api 
Restore:
appcfg.py upload_data --config_file=bulkloader.yaml --filename=<filename> --kind=<our_ entity> --url http://<our_app_id>.appspot.com/remote_api 
And that's it.
For any further information have a quick look at http://code.google.com/intl/fr-FR/appengine/docs/python/tools/uploadingdata.html