Dynamic variable creation from string

[Massi]

Thank you all for your replies, first of all my Sum function was an
example simplifying what I have to do in my real funciton. In general
the D dictionary is complex, with a lot of keys, so I was searching
for a quick method to access all the variables in it without doing the
explicit creation:

a, b, c = D['a'], D['b'], D['c']

and without using directly the D dictionary (boring...).
When I talked about nested function I meant both cases Chris, but this
is not a really tighten bound.
I tried to follow the hints of Chris together with some help by google
and used the following code:

for k in D : exec "%s = D[k]" %k

That seems to do the trick, but someone speaks about "dirty code", can
anyone point me out which problems this can generate?
Again, thank you for your help!