old_contents: #!/usr/bin/env python
  # -*- encoding: utf-8 -*-
  #
  # Copyright (c) 2015 ASMlover. All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without
  # modification, are permitted provided that the following conditions
  # are met:
  #
  #  * Redistributions of source code must retain the above copyright
  #    notice, this list ofconditions and the following disclaimer.
  #
  #  * Redistributions in binary form must reproduce the above copyright
  #    notice, this list of conditions and the following disclaimer in
  #    the documentation and/or other materialsprovided with the
  #    distribution.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  # FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
  # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
  # BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
  # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  # POSSIBILITY OF SUCH DAMAGE.
  
  def file_reading():
      import sys
      file_obj = open('ch01.py')
      line = file_obj.readline()
      while line:
          sys.stdout.write(line)
          line = file_obj.readline()
      file_obj.close()
  
  def echoing():
      while True:
          read_info = input('please input >>')
          if read_info == 'exit':
              break
          print (read_info)
  
  def listing():
      import sys
      file_obj = open('ch01.py')
      [sys.stdout.write(line) for line in file_obj.readlines()]
      file_obj.close()
  
  if __name__ == '__main__':
      # file_reading()
      # echoing()
      listing()
  
new_contents: #!/usr/bin/env python
  # -*- encoding: utf-8 -*-
  #
  # Copyright (c) 2015 ASMlover. All rights reserved.
  #
  # Redistribution and use in source and binary forms, with or without
  # modification, are permitted provided that the following conditions
  # are met:
  #
  #  * Redistributions of source code must retain the above copyright
  #    notice, this list ofconditions and the following disclaimer.
  #
  #  * Redistributions in binary form must reproduce the above copyright
  #    notice, this list of conditions and the following disclaimer in
  #    the documentation and/or other materialsprovided with the
  #    distribution.
  #
  # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  # FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
  # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
  # BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
  # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  # POSSIBILITY OF SUCH DAMAGE.
  
  def file_reading():
      import sys
      file_obj = open('ch01.py')
      line = file_obj.readline()
      while line:
          sys.stdout.write(line)
          line = file_obj.readline()
      file_obj.close()
  
  def echoing():
      while True:
          read_info = input('please input >>')
          if read_info == 'exit':
              break
          print (read_info)
  
  def listing():
      import sys
      file_obj = open('ch01.py')
      [sys.stdout.write(line) for line in file_obj.readlines()]
      file_obj.close()
  
  def yield_func():
      def counting(n):
          while n > 0:
              yield n
              n -= 1
      for c in counting(5):
          print (c)
  
  if __name__ == '__main__':
      # file_reading()
      # echoing()
      # listing()
      yield_func()
  
subject: add demo about using yield