title: how to put quote in write function in python
body: <p>I am totaly newbe.
  I try to use a gsm card (Sim900) with a Raspberry and Python 3.</p>
  <p>I find some exemples, that need to be adapted.
  For exemple, I find that I need to sennd a request like :
  port.write(('AT\r\n')
  That I need to transform to:
  port.write(('AT'+'\r\n').encode('utf-8'))</p>
  <p>I am able to send SMS.</p>
  <p>Now I try to read my SMS.
  If I send:
  port.write(('AT+CMGR=1'+'\r\n').encode('utf-8'))
  I can read the first SMS in the List.</p>
  <p>If I want to read all the SMS, I need to change 1 by 'ALL', with the quote.
  But I am not able to see how to do it.
  I try a lot of things, but don't find the right way to do it.
  To be clear, I need to send: AT+CMGDA='DEL ALL'</p>
  <p>Thanks for your help</p>