Getting exactly one element or raising exception with lxml xpath

Question

The xpath() function in lxml normally returns a list of elements.

If I have an XPath which I expect to return exactly one element, what's the nicest way to:

• Check that one element is returned or else raise an exception, and:
• Get that element (as opposed to a one-element list)?

I'm really looking for the analogue of SQLAlchemy's one() function.

Answer 1

try:
    (element,) = tree.xpath('//xpath/selector')
except ValueError:
    raise InvalidSelector()
    # happened because the list was either empty or contained multiple elements