read from source file one line at a time and write to target file one line at time.

# write a function to read from source file one line at a time and write to target file one line at time.

def copy(source, target):
  source_file = open(source, 'r')
  target_file = open(target, 'w')
  for line in source_file.readlines():
    target_file.write(line)
    
  source_file.close()
  target_file.close()
  
copy('a.txt', 'b.txt')