do/while structure needed

[John Salerno]

1 random.shuffle(letters)
2 trans_letters = ''.join(letters)[:len(original_set)]
3 trans_table = string.maketrans(original_set, trans_letters)

So what I'd like to do is have lines 1 and 2 run once, then I want to do 
some comparison between original_set and trans_letters before running 
line 3. If the comparison passes, line 3 runs; otherwise, lines 1 and 2 
run again.

A do/while would be good for this, but perhaps I'm looking at it in the 
wrong way? Or is there some kind of do/while type of idiom that I could use?

Thanks.