Question

I am hoping this is a simple stupid noob mistake that can be fixed with the addition of a single line of code somewhere.

I’m using pySerial to read in serial data from a USB port and print it out to standard output. I’m running Mac OSX 10.6. I open terminal and type “python”, then the following:

>>> import serial;
>>> ser = serial.Serial('/dev/tty.usbserial-XXX', 9600, timeout=1);
>>> while True:
>>>      if ser.inWaiting() > 0:
>>>            ser.readline();
>>> [done, I hit enter...]

This works beautifully. It starts outputting my serial data nicely, exactly as I’d expect it to. Great, I think, let me put this into its own script with command line arguments and then I can call it any time I want to:

import sys;
import serial;

serialPort = sys.argv[1]
baudRate = sys.argv[2]

ser = serial.Serial(serialPort, baudRate, timeout=1)

while True:
    if ser.inWaiting() > 0:
        ser.readline()

On the command line, I type “python myScript.py [my serial port] 9600” and sit back and wait for a beautiful flow of serial data – but nothing comes out. It just kinda hangs until I kill the process.

I thought, well maybe for some reason it’s not working – so I put some debugging prints into the code. I update my while loop to look like this:

 while True:
    print("looping...")
    print(ser.inWaiting());
    if ser.inWaiting() > 0:
        ser.readline()

I run it again, and I get a repeating output stream of “Looping…” and “0”. I think, well maybe there’s something wrong with my command line arguments – so I hard-coded the port and the baud rate into the script, same thing.

So, why would this be the case? Is my use of while True: somehow blocking the script from accepting serial data? Is there a better way to do this?

I’m a complete Python noob. I am writing this script in order to create a faster way to communicate between Adobe AIR and an Arduino board. I’m hoping there’s a magic bullet I can drop in here to make it work – is there?

Answer 1

Is it printing anything? I’d assume it would print:

looping...
<ser.inWaiting() val>

And then nothing. Add a print statement so it looks like

print ser.readline()

and see if that works. I’m guessing that the interpreter is printing the returned strings when you do it as a command, but in a script the returned strings from readline() are getting discarded.