Make me beautiful (code needs help)

[Mark]

Ok, I have the following data:

data["A"] = [1,2,4,10,50]

and I want to get:

data["new"] = [?, 1, 2, 6, 40]

That is, I want to get the successive differences into their proper place.
I don't care what is in the question mark spot.  I'll figure that out later.

Here's my current code:

for i in range(len(data["A"])):
     try:
          data["new"].append(data["A"][i] - data["A"][i-1])
     except OutOfBounds:
          data["new"].append("?")

Now, the other problem, is that in general, I want to fill data["new"] with 
an arbitary function of the values in data["A"].  As a dumb example, I 
might want to fill data["new"] with copies of the mean (average) of 
data["A"].  So, in some cases, the new values are relative to the current 
location (ie i and i-1) and in other they are absolute (ie all of them).

I can certainly hack through this, but I'm looking for something pretty.  I 
know python is up to it.

Regards,
Mark