瞭解程序

Question

def cross(A, B): 
    "Cross product of elements in A and elements in B." 
    return [a+b for a in A for b in B] 

digits = '123456789' 
rows  = 'ABCDEFGHI' 
cols  = digits 
squares = cross(rows, cols) 
unitlist = ([cross(rows, c) for c in cols] + 
      [cross(r, cols) for r in rows] + 
      [cross(rs, cs) for rs in ('ABC','DEF','GHI') for cs in ('123','456','789')]) 
units = dict((s, [u for u in unitlist if s in u]) 
      for s in squares) 
peers = dict((s, set(sum(units[s],[]))-set([s])) 
      for s in squares) 

有人請幫我理解（簡單來說）最後兩行是什麼意思？

Answer 1

units = dict((s, [u for u in unitlist if s in u]) 
      for s in squares) 

相同

units = dict() 
for s in squares: 
    units[s] = [u for u in unitlist if s in u] 

甚至類似以下低效版本

units = dict() 
for s in squares: 
    units[s] = [] 
    for u in unitlist: 
     if s in u: 
      units[s].append(u)