Why does strip() fix this pexpect script?

Question

I have an object.

This object has a connect() method which spawns a pexpect process.

The process that's spawned is a custom serial interface. On launch, this tool prints a menu of serial devices to connect to, like so:

libftdi device (0): A6005jpt
libftdi device (1): acFX9DQf
Serial device (a) : /dev/cu.Bluetooth-PDA-Sync
Select a device by its letter (^D to abort): 

My connect() determines which number to pass based on a given devicename (e.g. 'acFX9DQf'): (self.connection is the pexpect spawn)

USBSERIAL_DEVICE_NAME = "acFX9DQf"    

try:
    while self.connection.expect(['libftdi device \(([0-9])\): (.*)','Serial device']) == 0:
        if self.connection.match.group(2).strip() == USBSERIAL_DEVICE_NAME:
            # do stuff
except:
    # do stuff

Now, my problem is that I connect()/kill() the process multiple times in my main logic and sometimes, one of those times, connect() decides to throw a pexpect.TIMEOUT exception, unexpectedly.

For example, when I add the following debug statements to my logic, like so:

USBSERIAL_DEVICE_NAME = "acFX9DQf"

try:
    while self.connection.expect(['libftdi device \(([0-9])\): (.*)','Serial device'], timeout=10) == 0:
        print "MATCHED A DEVICE LINE!"
        if self.connection.match.group(2).strip() == USBSERIAL_DEVICE_NAME:
            print "MATCHED THE DESIRED USBSERIAL..."

...I get this output for many calls of connect():

libftdi device (0): A6005jpt
MATCHED A DEVICE LINE!
libftdi device (1): acFX9DQf
MATCHED A DEVICE LINE!
MATCHED THE DESIRED USBSERIAL...
Serial device (a) : /dev/cu.Bluetooth-PDA-Sync
Select a device by its letter (^D to abort): 1

...then, one of my connect() calls will unexpectedly do....

libftdi device (0): A6005jpt
MATCHED A DEVICE LINE!
libftdi device (1): acFX9DQf
Serial device (a) : /dev/cu.Bluetooth-PDA-Sync
Select a device by its letter (^D to abort): MATCHED A DEVICE LINE!

<<EXCEPTION>>

But, if I revise my code to this:

try:
    while self.connection.expect(['libftdi device \(([0-9])\): (.*)','Serial device'], timeout=10) == 0:
        devicename = self.connection.match.group(2).strip()
        if devicename == USBSERIAL_DEVICE_NAME:
            # do stuff

My problems go away! I can run it repeatedly and no problems will occur - no exceptions, no nothin'.

So wot in the heck is going on here? I'm not even sure where to begin with this problem.

Answer 1

I'm guessing you're getting a TIMEOUT exception. Based on where the "MATCHED A DEVICE LINE!" message is in the output, my hypothesis is that the spawn instance has multiple lines when it tests for a match. When pexpect compiles the regexes, it sets re.DOTALL, so the .* includes the newline and the additional lines. This causes the test against USBSERIAL_DEVICE_NAME to fail and the loop continues its next iteration. The next call to expect blocks because there is no additional input and you get a timeout.

To fix this, you can pass in your own compiled regexes (which will lack the re.DOTALL flag) or change the .* so it explicitly doesn't match the newline (e.g. \S*).

As for why storing the matched group in a variable seems to fix things, I can only guess this introduces a subtle timing change that prevents the call to expect from receiving multiple lines of input at once.