Python - Continuing after exception at the point of exception

Question

I'm trying to extract data from an xml file. A sample of my code is as follows:

    from xml.dom import minidom
    dom = minidom.parse("algorithms.xml")
    ...
       parameter = dom.getElementsByTagName("Parameters")[0]
       # loop over parameters
       try:
            while True:
                parameter_id = parameter.getElementsByTagName("Parameter")[m].getAttribute("Id")
                parameter_name = parameter.getElementsByTagName("Name")[m].lastChild.data
                ...
                parameter_default = parameter.getElementsByTagName("Default")[m].lastChild.data
                print parameter_id
                print parameter_default
                m = m+1
        except IndexError:
            #reached end of available parameters
            pass
        #except AttributeError:
            #parameter doesn't exist
            #?

If all elements for each parameter exist, the code runs correctly. Unfortunately the data I am supplied often has missing entries in it, raising an AttributeError exception. If I simply pass on that error, then any elements that do exist but are retrieved later in the loop than when the exception occurred are skipped, which I don't want. I need some way to continue where the code left off and skip to the next line of code if this specific exception is raised.

The only way to work around this that I can think of would be to override the minidom's class methods and catch the exception there, but that seems far too messy and too much work to handle what should be a very simple and common problem. Is there some easier way to handle this that I am missing?

AttributeError is not a minidom exception... it refers to the attempt to access a nonexistent Python object attribute. Minidom's getAttribute() method returns an empty string if the attribute doesn't exist. Please provide a sample stack trace and identify the statement in your code that threw the exception. — Jim Garrison
– Jim Garrison, Commented Feb 8, 2012 at 19:08

ben w · Accepted Answer · 2012-02-08 19:08:41Z

3

Instead of "an individual try-except block for every statement", why not abstract out that part?

def getParam(p, tagName, index, post=None):
    post = post or lambda i: i
    try:
        return post(p.getElementsByTagName(tagname)[index])
    except AttributeError:
        print "informative message"
    return None # will happen anyway, but why not be explicit?

then in the loop you could have things like:

parameter_id = getParam(parameter, "Parameter", m, lambda x: x.getAttribute("Id"))
parameter_name = getParam(parameter, "Name", m, lambda x: x.lastChild.data)
...

answered Feb 8, 2012 at 19:08

ben w

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3 Comments

ben w Over a year ago

Empty strings don't have the attribute getAttribute or lastChild, so AttributeError can still be raised here and in the original question.

user1197931 Over a year ago

Doesn't catching AttributeError at that level prevent the IndexError from correctly firing and exiting the loop?

Johan Lundberg Over a year ago

No AttributeError is distinct from IndexError

Johan Lundberg · Accepted Answer · 2012-02-08 19:27:52Z

1

I think there are two parts to your question. First, you want the loop to continue after the first AttributeError. This you do by moving the try and except into the loop.

Something like this:

try:
    while True:
        try:
            parameter_id = parameter.getElementsByTagName("Parameter")[m].getAttribute("Id")
            parameter_name = parameter.getElementsByTagName("Name")[m].lastChild.data
            ...
            parameter_default = parameter.getElementsByTagName("Default")[m].lastChild.data
            print parameter_id
            print parameter_default
            m = m+1
        except AttributeError:
            print "parameter doesn't exist"
        #?
except IndexError:
    #reached end of available parameters
    pass

The second part is more tricky. But it is nicely solved by the other answer.

edited Feb 8, 2012 at 19:27

answered Feb 8, 2012 at 18:55

Johan Lundberg

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5 Comments

user1197931 Over a year ago

Simply moving it into the loop wouldn't change the problem at all (and not handling IndexError outside the loop would cause an infinite loop). I could have an individual try-except block for every single element to be retrieved, but to me that just seems a horrible way to code and requires way too much effort.

thavan Over a year ago

where is the break for your while/

Johan Lundberg Over a year ago

@thavan, I had an indentation error. I'm not familiar with his codebase but the escape from the loop is via IndexError as in his original code.

ben w Over a year ago

This doesn't seem to do the right thing; if parameter_name couldn't be read but parameter_default could have been, we don't have either of them after the exception is handled, because we skip the call where parameter_default would have been read. The questioner wants to be able to read everything that can be read, regardless of whether previous reads fail. Storing things in a dict just lets you know what things were read prior to failure.

Johan Lundberg Over a year ago

@thavan, who are you asking about infinite loop? I don't understand what you mean.

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