有效的XPath表达式

Question

Just two questions:

1. How can I check if the string assigned to a variable corresponds to a valid XPath expression?
2. How can I return a customized error message in case the requested resource does not exist?

Answer 1

1. If the XPath is invalid, you'll get an exception.
2. If the requested node does not exist, you'll get an empty result set.

For example:

from lxml import etree
from StringIO import StringIO
tree = etree.parse(StringIO('<foo><bar></bar></foo>'))
try:
  tree.xpath('\BAD XPATH')
  print '1. Valid XPath'
except etree.XPathEvalError, e:
  print '1. Invalid XPath: ', e
if not tree.xpath('/foo/xxx'):
  print '2. Requested node does not exist.'

Runs as follows:

1. Invalid XPath:  Invalid expression
2. Requested node does not exist.