Question

Being new to Python, I’m having a heck of a time figuring out what these two lines do:

for i in [j for j in xrange(0, N) if [k for k in xrange(j) if now[k] == now[j]] == []]:
   for j in [k for k in xrange(1, K + 1) if [l for l in xrange(i) if now[l] == k] == []]:

Is there a way to rewrite this so noobs like myself could understand it? Thanks.

Answer 1

I’m almost positive that this code is equivalent to:

for i in range(0, N):
  prefix = set(now[:i])
  if now[i] in prefix: continue

  for j in range(1, K + 1):
    if j in prefix: continue

    # do something

or (which would be a bit less efficient, but nicely reflects the idea behind the code):

for i, j in itertools.product(range(0, N), range(1, K + 1)):
  prefix = now[:i]
  if now[i] not in prefix and j not in prefix:
    # do something with i and j

BUT it’s written in a convulted, inefficient way (especially the [...] == [] is nasty and pointless). Maybe by someone who feels pretty clever using complex structures where simple ones would be much better suited.