URI Resolution

rnginline uses URI Resolution to map relative paths like common/name.rng to absolute URIs, such as file:/a/b/proj/common/name.rng which can be handled by a URL Handler.

Resolution 101

A quick crash course in URI resolution. Check out RFC 3986 if you want all the details.

URI Types

There are two types of URIs we need to know about. Absolute URIs and relative URIs. In simple terms, Absolute URIs have a scheme, which is the part of a URI before the first colon, e.g. in http://example.org http is the scheme. Relative URIs don’t have a scheme. e.g. file.txt, //example.org/foo and /tmp/file.txt are all relative URIs.


URI Resolution is the process of merging two URIs. A base URI which is always absolute, and a reference URI which can be absolute or relative.

There are two aspects of URI resolution we need to know about. Resolving schemes and resolving paths.


If we resolve a relative reference URI against an absolute base URI, the resulting URI has the scheme of the base URI, with other parts overridden by the reference URI:

>>> from rnginline import uri
>>> uri.resolve('file:', 'somefile.txt')

Resolving an absolute reference URI results in the base being replaced by the reference:

>>> uri.resolve('file:somefile.txt', 'other:blah')


The path component of a relative reference URI is resolved against the path component of the base URI:

>>> uri.resolve('file:/some/dir/', 'other/dir/file.txt')

If the reference URI’s path is absolute (starts with a /) then it replaces the base URIs path:

>>> uri.resolve('file:/some/dir/', '/tmp/foo.txt')

If the reference URI is absolute, its path replaces the base URIs path, regardless of whether or not the reference URIs path is absolute or not:

>>> uri.resolve('file:/some/dir/', 'file:file.txt')

Also, note that trailing slashes are significant for path resolution. Without a trailing slash, the base’s final path segment is replaced when resolving paths:

>>> uri.resolve('file:/some/dir', 'other/dir/file.txt')

URI Resolution in rnginline

When the Inliner sees a URI like common/name.rng, it needs to resolve it to to absolute URIL, such as file:/a/b/proj/common/name.rng in order to determine the location it points to, and which URL Handler can fetch the URL.

To do this, rnginline uses a heirachy of URIs:

  1. The default base URI — The catch-all, top-most URI. This has to be

    absolute. By default it’s file:<current-dir> but can be set to anything.

  2. The current document’s base URI — The location the current document was

    loaded from.

  3. Any xml:base attributes on, or on ancestors of the an

    <inline>/<externalRef> XML element.

  4. The URI value of the href attribute of the <inline>/<externalRef>


The absolute URI of an href attribute is resolved by resolving 2 against 1, then 3 against the result of that, then 4 against the result of that.

Because the default base URI is file:<current-dir>, relative paths/URIs passed to inline() get resolved to file: URLs, which are handled by the filesystem handler without having to specify the file: scheme on the input to inline().

Similarly, following what we’ve learnt above, if an input’s base URI is pydata://my.pkg/some/dir/a.rng and an href attribute in a.rng contains the value b.rng, it will be resolved to the absolute URL pydata://my.pkg/some/dir/b.rng, and therefore handled by the pydata handler, not the filesystem handler.