Multiple Try-Excepts followed by an Else in python

Question

Is there some way to have several consecutive Try-Except clauses that trigger a single Else only if all of them are successful?

As an example:

 try:
    private.anodization_voltage_meter = Voltmeter(voltage_meter_address.value) #assign voltmeter location
except(visa.VisaIOError): #channel time out
    private.logger.warning('Volt Meter is not on or not on this channel')
try:
    private.anodization_current_meter = Voltmeter(current_meter_address.value) #assign voltmeter as current meter location
except(visa.VisaIOError): #channel time out
    private.logger.warning('Ammeter is not on or not on this channel')
try:
    private.sample_thermometer = Voltmeter(sample_thermometer_address.value)#assign voltmeter as thermomter location for sample.
except(visa.VisaIOError): #channel time out
    private.logger.warning('Sample Thermometer is not on or not on this channel')
try:
    private.heater_thermometer = Voltmeter(heater_thermometer_address.value)#assign voltmeter as thermomter location for heater.
except(visa.VisaIOError): #channel time out
    private.logger.warning('Heater Thermometer is not on or not on this channel')
else:
    private.logger.info('Meters initialized')

As you can see, you only want to print meters initialized if all of them went off, however as currently written it only depends on the heater thermometer. is there some way to stack these?

Answer 1

You can keep a boolean, initialized at the beginning to: everythingOK=True Then set it to false in all the except blocks and log the final line only if true.

Answer 2

Consider breaking the try/except structure into a function that returns True if the call worked and False if it failed, then use e.g. all() to see that they all succeded:

def initfunc(structure, attrname, address, desc):
  try:
    var = Voltmeter(address.value)
    setattr(structure, attrname, var)
    return True
  except(visa.VisaIOError):
    structure.logger.warning('%s is not on or not on this channel' % (desc,))

if all([initfunc(*x) for x in [(private, 'anodization_voltage_meter', voltage_meter_address, 'Volt Meter'), ...]]):
  private.logger.info('Meters initialized')

Answer 3

Personally I'd just have a trip variable init_ok or somesuch.

Set it as True, and have all the except clauses set it False, then test at the end?

Answer 4

Try something like this. keeping the original behaviour of not stopping after the first exception

success = True
for meter, address, message in (
        ('anodization_voltage_meter',voltage_meter_address,'Volt Meter is not on or not on this channel'),
        ('anodization_current_meter',current_meter_address,'Ammeter is not on or not on this channel'),
        ('sample_thermometer',sample_thermometer_address,'Sample Thermometer is not on or not on this channel'),
        ('heater_thermometer',heater_thermometer_address,'Heater Thermometer is not on or not on this channel')):
   try:
       setattr(private,meter, Voltmeter(address.value):
   except (visa.VisaIOError,):
       success = False
       private.logger.warning(message)

if success: # everything is ok
    private.logger.info('Meters initialized')

Answer 5

allDone = 0
try: #1
    ...
    allDone += 1
except:
    ...
try: #2
    ...
    allDone += 1
except:
    ...
try: #3
    ...
    allDone += 1
except:
    ...

if allDone == 3:
    # execute this block only when all the try blocks and none of the except blocks were executed

Answer 6

You've got a number of quite similar objects, and in cases like that it's often better to treat them uniformly and use a data-driven approach.

So, I'd start with your ..._meter_address objects. To me they sound like configuration for the meters, so I'd have a class that looks something like this:

class MeterConfiguration(object):
    def __init__(self, name, address):
        self.name = name
        self.address = address

    def english_name(self):
        """A readable form of the name for this meter."""
        return ' '.join(x.title() for x in self.name.split('_')) + ' Meter'

Perhaps you have some more configuration (currently stored in variables) that could go in here too.

Then, I'd have a summary of all the meters that your program deals with. I'm creating these statically, but it may well be right for you to read all or part of this information from a config file. I've no idea what your addresses look like, so I made something up :)

ALL_METERS = [
    MeterConfiguration('anodization_voltage', 'PORT0001'),
    MeterConfiguration('anodization_current', 'PORT0002'),
    MeterConfiguration('sample_thermometer', 'PORT0003'),
    MeterConfiguration('heater_thermometer', 'PORT0004')
]

Cool, now all configuration is in one place, and we can use this to make things uniform and simpler.

private.meters = {}
any_errors = False
for meter in ALL_METERS:
    try:
        private.meters[meter.name] = Voltmeter(meter.address)
    except VisaError:
        logging.error('%s not found at %s', meter.english_name(), meter.address)
        any_errors = True
if not any_errors:
    logging.info('All meters initialized.')

You can use, for example private.meters['anodization_voltage'] to refer to a particular meter, or iterate over the meters dict if you need to do something to them all.