Michael Wayne Goodman

Computational Linguist

Python one-liner: Getting only the first match in a list comprehension

2015 November 04

Python's ElementTree is a great interface for XML documents, but it has terrible support for namespaces; you basically have to do it yourself. ElementTree's XPath support (a subset called ElementPath, although that name is not used publicly) will take a namespaces argument with mapping of prefixes to URIs, but it does not handle default namespaces.

I had to do some digging, but apparently the XPath designers didn't consider default namespaces when creating the API. It is referenced in the specification, however. From the section on node tests (emphasis mine):

A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. This is the same way expansion is done for element type names in start and end-tags except that the default namespace declared with xmlns is not used: if the QName does not have a prefix, then the namespace URI is null (this is the same way attribute names are expanded). It is an error if the QName has a prefix for which there is no namespace declaration in the expression context.

It is again referenced (contradictorily?) in the section on Element Nodes (again, emphasis mine):

There is an element node for every element in the document. An element node has an expanded-name computed by expanding the QName of the element specified in the tag in accordance with the XML Namespaces Recommendation [XML Names]. The namespace URI of the element's expanded-name will be null if the QName has no prefix and there is no applicable default namespace.

Now back to Python, the design of ElementPath chooses to follow the first quote from the spec, even though the qualified names of tags and attributes are strings with the URIs embedded. Recent versions of Python allow functions like find() to take a namespaces argument, and a given mapping will find the qualified names when a prefix is given, but if a default namespace is in effect, you'd need to create the qualified name manually, or do something destructive like remove the default namespaces from all elements. That is, for an XML document like this:

<root xmlns="http://example.com" xmlns:pre="http://example.com/prefix">

You could not find <a> (or even <root>) with a simple path like this:

As far as I know, there is not a good solution to this problem. Even lxml is aware of the problem but suggests modifying the document with a bogus prefix. The design of ElementPath seems to selectively choose which parts of the XPath spec to implement (when considered in conjunction with the model that ElementTree provides). In its defense, the XPath spec itself is lacking in some ways. Nevertheless, a more useful (if perhaps technically incorrect) implementation in the Python standard library would replace the xpath_tokenizer() function shown here:

with this: