I have been playing with google app engine. And I found that google app engine does have anything that allows developer to download the code. Thanks to Voodoo programming blog ( http://www.cneophytou.com/2008/04/17/downloading-your-code-from-google-app-engine/ ) It shows a way to do it in mac, I take a similar approach in windows.

1. create an Archive folder in your project folder.

2. create a batch file:
for %%* in (.) do set SUBDIR=%%~n*
cd ..
del "%SUBDIR%\Archive\Archive.zip"
zip -r -9 "%SUBDIR%\Archive\Archive.zip" %SUBDIR%
cd %SUBDIR%
"C:\Program Files\Google\google_appengine\appcfg.py" update ./

3. Add these line in your app.yaml file
- url: /Archive
static_dir: Archive
login: admin

Use the update.bat to upload your application. And you can download your source code from http://yourapp.appspot.com/archive/Archive.zip