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

source_file = open('source.txt', 'r')
target_file = open('target.txt', 'w')

def read_line(source_file, target_file):
  source_line = source_file.readline()
  if source_line == '':
    return
  else:
    target_file.write(source_line)
    read_line(source_file, target_file)

read_line(source_file, target_file)

source_file.close()
target_file.close()