Python Lxml (objectify): Checking whether a tag exists

问题

I need to check whether a certain tag exists in an xml file.

For example, I want to see if the tag exists in this snippet:

 <main>
       <elem1/>
       <elem2>Hi</elem2>
       <elem3/>
       ...
 </main>

Currently, I am using an ugly hack with error checking, like this:

try:
   if root.elem1.tag:
      foo = elem1
except AttributeError:
   foo = "error finding elem1"

I also want to customize the string if it is unable to find the node (i.e. "unable to find -tagname-").

I have to check a long list of variables, and I don't want to repeat the code 100 times.

Any suggestions?

Edit:

Here is a snip of the actual xml file:

<main>
 <asset name="Virtual Dvaered Unpresence">
  <virtual/>
  <presence>
   <faction>Dvaered</faction>
   <value>-1000.000000</value>
   <range>0</range>
  </presence>
 </asset>
 <asset name="Virtual Empire Small">
  <virtual/>
  <presence>
   <faction>Empire</faction>
   <value>100.000000</value>
   <range>2</range>
  </presence>
 </asset>
</main>

I want to check whether the tag exists, and, if so, to get the contents.

Edit edit: Ok, I am going to combine two of the answers, but I can only vote for one. Sorry.

Edit 3: Related question about XPath here: Python lxml (objectify): Xpath troubles

回答1:

Assume you want to get elem2's value, you can use xpath to find it.

tree = etree.parse(StringIO(htmlString), etree.HTMLParser()).getroot()
youWantValue = tree.xpath('/main/elem2')[0].text

回答2:

hasattr() works for this:

if hasattr(root, 'elem1'):
    foo = root.elem1

回答3:

Edit: updated answer for sample file.

I'm assuming you want to search each asset for certain tags. If so, the following worked for me:

import lxml.objectify

# Parse the file.
tree = lxml.objectify.parse('sample.xml')
root = tree.getroot()

# Which elements to find.
to_find = set(['presence/faction', 'presence/value', 'fake'])

# Go through each asset in the document.
for asset in root.findall('asset'):
    # Check for each element. 
    for name in to_find:
        node = asset.find(name)
        if node is not None:
            print 'Found %s, its value is %s' % (name, node)
        else:
            print 'Unable to find %s' % name

The output was:

Found presence/value, its value is -1000.0
Found presence/faction, its value is Dvaered
Unable to find fake
Found presence/value, its value is 100.0
Found presence/faction, its value is Empire
Unable to find fake

回答4:

If your document tends to be relatively short you can iterate over all children of <main> looking for tags matching your set of variable names:

tree = lxml.etree.fromstring(DATA)
NAMES = set(['elem1', 'elem3'])
for node in tree.iterchildren():
    if node.tag in NAMES:
        print 'found', node.tag

Or you can search for each variable name one at a time:

for tag in ('elem1', 'elem3'):
    if tree.find(tag) is not None:
        print 'found', tag

来源：https://stackoverflow.com/questions/5385821/python-lxml-objectify-checking-whether-a-tag-exists

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