Error logs in debian using python

There are two kind of errors when for example a USB device disconnected:

1. System Errors
2. Application Errors

System errors is logging in built-in log files (usually in /var/log/). In this case you could use rsyslog to categorize errors in files and watching them with another application.

Here you could find some useful rsyslog configuration: http://wiki.rsyslog.com/index.php/Configuration_Samples

I believe it's clear that in case of application error, you should log errors with application and watch them.

OK, I re-wrote the answer based on your comments: Install PyUSB

Checking if the USB device is working:

def getUSBDeviceStatus(vendor, product):

    #idVendor 0xfffe, idProduct=0x0001
    dev = usb.core.find(idVendor=vendor, idProduct=product)
    if dev is None:
        raise ValueError("Device failed or is not connected!")
    else:
        return 1

Retrieving the logs based on their type:

def retrieveLogs(type):

    with open('/home/pi/log/log.log') as file:
        lines = sum(1 for _ in file)

    readerPosition = 0
    readingLine = file.readline() 

    while readerPosition >= lines:

        if readingLine.find(type):
            print readingLine
        readerPosition += 1

PS: I had the assumption you are logging errors with their respective criticality.