Currently the best way to add server-side JS functions via PyMongo is:

db.system.js.save({ '_id': 'add', 'value': Code('function(a,b)

{ return a+b; }')})

And to call that function via PyMongo:

db.eval("function(a,b) {return add(a,b);}", var1, var2)

A simpler API could be provided like:

# Function creation
db.js.save('add', 'function(a,b) { return a+b; }

')

1. Function execution
   db.js.add(var1, var2)

Sample code here:
http://github.com/schmichael/mongo-python-driver/commit/57e6df4ca14314eb8c0a3a9e4b8c97c0186a0121