Most people who learn Python will say that the language itself is much more productive than Java, but if you are skilled in Java, and you like getting code-assist in your IDE and lots of compiler checking for correct code, you might dispute this. Another reason you might find Python web application programming significantly more productive is how easy it is to do unit testing locally using just your Python program and a web browser. Consider this application written in the file WSGI/application.py.
from wsgiref.simple_server import make_server
def application(environ, start_response):
status = '200 OK' # HTTP Status
response_body = 'Hello World'
headers = [('Content-type', 'text/plain'), ('Content-length', str(len(response_body)))]
if __name__ == '__main__': #only true when local unit testing
httpd = make_server('localhost', 8000, application)
print "Serving on port 8000..."
If I run this file locally as a simple Python program, I can test it in a local browser window using any url that begins with http://localhost:8000, but I can also copy it unaltered into a WSGI-based production environment. This is extremely convenient and allows me to trivially use local debuggers, IDEs and so on. IDEs like Rational Applicaton Developer (previously known as WeSphere Studio) provide this sort of easy local debug environment for WebSphere/Java aplications, but it takes a lot of engineering in RAD to do that, and the user is often ultimately exposed to the presence of WebSphere and the need to configure it, and therefore to match that configuration to the production one. In Python, there is no external application server to configure - almost all configuration is configuration of the application itself - so this issue doesn't arise.
Databases like MongoDB fit very nicely with this experience. Installation of Mongodb is a simple unzip - it is completely stand-alone, and there is nothing to configure. You do not create databases, or create schema via configuration - you just use it. Even index creation is done from your program, not externally configured. When you move your app up to a production environment, only the URL of mongodb changes, and you can get that from a simple environment variable to avoid changing the code.