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?

Answer 1

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)
...