def characters(c):
print ' '.join(map(chr, range(ord(c), ord('a'), -1) + range(ord('a'), ord(c)+1)))
>>> characters('d')
d c b a b c d
или
def characters(c):
for n in xrange(ord(c), ord('a'), -1):
print chr(n),
for n in xrange(ord('a'), ord(c)+1):
print chr(n),
print