Worthwhile to reverse a dictionary

[rudysanford at hotmail.com]

What is the difference between

 " d1 = {'A' : '1', 'B' : '2', 'C' : '3'} "

and

" d1 = dict(A = 1, B = 2, C = 3) "  ?

All of the dictionary examples I saw (python.org, aspn.activestate.com,
Learning Python by Lutz, among others) use d={'x' : 'y'}.