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I'm trying to over write a file in python so it only keeps the most up to date information read from a serial port. I've tried several different methods and read quite a few different posts but the file keeps writing the information over and over with out overwriting the previous entry. 

 import serial

 ser=serial.Serial('/dev/ttyUSB0',57600)

 target=open( 'wxdata' , 'w+' )

 with ser as port, target as outf:
      while 1:
           target.truncate()
           outf.write(ser.read))
           outf.flush()

I have a weather station sending data wirelessly to a raspberry pi, I just want the file to keep one line of current data received. right now it just keeps looping and adding over and over. Any help would be greatly appreciated..

Joshua Foster
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3 Answers3

3

I would change your code to look like:

from serial import Serial

with Serial('/dev/ttyUSB0',57600) as port:
    while True:
        with open('wxdata', 'w') as file:
            file.write(port.read())

This will make sure it gets truncated, flushed, etc. Why do work you don't have to? :)

Travis Griggs
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1

By default, truncate() only truncates the file to the current position. Which, with your loop, is only at 0 the first time through. Change your loop to:

while 1:
    outf.seek(0)
    outf.truncate()
    outf.write(ser.read())
    outf.flush()

Note that truncate() does accept an optional size argument, which you could pass 0 for, but you'd still need to seek back to the beginning before writing the next part anyway.

Cameron
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0

Before you start writing the file, add the following line:

outf.seek(0)
outf.truncate()

This will make it so that whatever you write next will overwrite the file

Vedaad Shakib
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  • Careful with this in general -- if what you write next is shorter than what was there before, only the start of the file will be overwritten! The call to `truncate()` is necessary too. – Cameron Nov 21 '14 at 23:00