XML Default namespaces for unqualified attribute names

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I'm trying to understand the correct interpretation of the "Namespaces in XML 1.0 (Third Edition)" definition for unqualified attribute namespaces.

"The namespace name for an unprefixed attribute name always has no value."

And later in the same section:

"The attribute value in a default namespace declaration MAY be empty. This has the same effect, within the scope of the declaration, of there being no default namespace."

So if I want to declare a default namespace for an element (and its children), do I also have to declare a prefix-namespace mapping for any attributes which reside within that namespace?

For example, in this example

<parent xmlns="http://example.com/foo">
    <child attrib="value">text</child>
<parent>

I would interpret the above definition to say that the namespace of attrib is empty.

So if I needed attrib to have the same namespace as parent, then I would be forced to do this?

<foo:parent xmlns:foo="http://example.com/foo">
    <foo:child foo:attrib="value">text</foo:child>
<foo:parent>

or this?

<parent xmlns="http://example.com/foo" xmlns:foo="http://example.com/foo">
    <child foo:attrib="value">text</child>
<parent>

This seems silly to me as it appears to defeat the purpose of default namespaces. I'm hoping that I'm just misunderstanding the spec.

Best Answer

You're correct. The idea behind attributes not being part of the default namespace is that they are considered to exist in an "element namespace" — so in this case, <foo:child/> is considered to be the 'namespace' for @attrib. Note that this is just conceptual; there's no API or anything that refers to attribute namespaces this way.

This was chosen because multiple elements may have attributes with the same names, but different meanings — unlike a traditional namespace, which is a set of names (so no duplicates). In a way, it gives more structure to the namespace, instead of having a flat set.

You can read about this in a very old version of the Namespaces recommendation.

This convention means that whenever you see a prefixed attribute, it represents some 'additional' information which isn't related to the main schema in the document.

Related Solutions

XML namespaces and attributes

Most of the time, attributes will not be in any namespace. The namespace spec says (emphasis mine):

A default namespace declaration applies to all unprefixed element names within its scope. Default namespace declarations do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear.

There's a reason that most XML vocabularies use non-namespaced attributes:
When your elements have a namespace and those elements have attributes, then there can be no confusion: the attributes belong to your element, which belongs to your namespace. Adding a namespace prefix to the attributes would just make everything more verbose.

So why do namespaced attributes exist?
Because some vocabularies do useful work with mostly attributes, and can do this when mixed in with other vocabularies. The best known example is XLink.

Lastly, W3C XML Schema has an all too easy way (<schema attributeFormDefault="qualified">) of declaring your attributes as being in a namespace, forcing you to prefix them in your documents, even when you use a default namespace.

Python – How to parse XML and count instances of a particular node attribute

I suggest ElementTree. There are other compatible implementations of the same API, such as lxml, and cElementTree in the Python standard library itself; but, in this context, what they chiefly add is even more speed -- the ease of programming part depends on the API, which ElementTree defines.

First build an Element instance root from the XML, e.g. with the XML function, or by parsing a file with something like:

import xml.etree.ElementTree as ET
root = ET.parse('thefile.xml').getroot()

Or any of the many other ways shown at ElementTree. Then do something like:

for type_tag in root.findall('bar/type'):
    value = type_tag.get('foobar')
    print(value)

And similar, usually pretty simple, code patterns.

Best Answer

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XML namespaces and attributes

Python – How to parse XML and count instances of a particular node attribute

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