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Date: Mon, 23 Dec 2013 00:33:24 -0800
From: Gary Herron <gary.herron@islandtraining.com>
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To: python-list@python.org
Subject: Re: 2nd Try: Trouble writing lines to file that include line feeds - Newbie
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On 12/22/2013 04:55 PM, Dan Healy wrote:
> Overview: I'm attempting to read strings from a serial port. Each string ends with a carriage return and line feed. I want to write those strings to a file, like a log file. So, if I send P1 and the P2 on a new line, I would expect to open this file and find (line 1) P1 (line 2) P2.
>
> Problem: The file only contains P2. It always overwrites the first line. I can send 20 strings and the file will always contain the last string received.
>
> Code:
>
> #Import the serial module
> import serial
>
> #Open the serial port w/ settings
> ser=serial.Serial(
>          port="/dev/ttyUSB0",
>          baudrate=9600,
>          timeout=None)
>
> #Print data received on the serial port after removing the CR and LF characters
> while True:
>          rawcode=ser.readline()
>          codelog=open('/home/pi/avdms/codes.log','w')
>          codelog.write(rawcode)
>          codelog.close()

First, that code is quite foolish:  If you want to write multiple lines 
to a file, open the file once before the loop, looped through all your 
writes, and then closed it after the loop has exited.:

codelog=open('/home/pi/avdms/codes.log','w')
while True:
         rawcode=ser.readline()
         codelog.write(rawcode)
         
codelog.close()


However, on the chance that you are writing this code for testing 
purposes, and you really do have a need to open a file, write something 
onto the end and then close the file, and that you are going to do this 
many times, I'll continue with this response:

    The default behavior of the "open" call is to *truncate* (i.e.,
    clear the  contents) and start writing at the beginning. The
    behavior you want is called *append*, and you get it with a 'wa' as
    the second parameter of the open call.


See http://docs.python.org/3/library/functions.html#open for a list of 
other modes available for the open call.

Gary Herron


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    <div class="moz-cite-prefix">On 12/22/2013 04:55 PM, Dan Healy
      wrote:<br>
    </div>
    <blockquote
      cite="mid:f4f962d9-0e23-4b33-ba88-ce61afbb3418@googlegroups.com"
      type="cite">
      <pre wrap="">Overview: I'm attempting to read strings from a serial port. Each string ends with a carriage return and line feed. I want to write those strings to a file, like a log file. So, if I send P1 and the P2 on a new line, I would expect to open this file and find (line 1) P1 (line 2) P2. 

Problem: The file only contains P2. It always overwrites the first line. I can send 20 strings and the file will always contain the last string received. 

Code: 

#Import the serial module 
import serial 

#Open the serial port w/ settings 
ser=serial.Serial( 
        port="/dev/ttyUSB0", 
        baudrate=9600, 
        timeout=None) 

#Print data received on the serial port after removing the CR and LF characters 
while True: 
        rawcode=ser.readline() 
        codelog=open('/home/pi/avdms/codes.log','w') 
        codelog.write(rawcode) 
        codelog.close() 
</pre>
    </blockquote>
    <br>
    First, that code is quite foolish:&nbsp; If you want to write multiple
    lines to a file, open the file once before the loop, looped through
    all your writes, and then closed it after the loop has exited.:<br>
    <pre wrap="">codelog=open('/home/pi/avdms/codes.log','w')
while True: 
        rawcode=ser.readline()  
        codelog.write(rawcode) 
        
codelog.close() </pre>
    <br>
    However, on the chance that you are writing this code for testing
    purposes, and you really do have a need to open a file, write
    something onto the end and then close the file, and that you are
    going to do this many times, I'll continue with this response:<br>
    <blockquote>
      The default behavior of the "open" call is to <b>truncate</b>
      (i.e., clear the&nbsp; contents) and start writing at the beginning.&nbsp;
      The behavior you want is called <b>append</b>, and you get it
      with a 'wa' as the second parameter of the open call.<br>
    </blockquote>
    <br>
    See <a class="moz-txt-link-freetext" href="http://docs.python.org/3/library/functions.html#open">http://docs.python.org/3/library/functions.html#open</a> for a list
    of other modes available for the open call.<br>
    <br>
    Gary Herron<br>
    <br>
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