[Yaml-core] Partial representations with PyYAML

[Geert J. <ge...@gm...>]

Hi,

i'm using version 3.09 of PyYAML. When parsing a document with
unresolvable tags, the default behavior of PyYAML seems to be to raise
an exception. However i believe the YAML spec allows for a "partial"
representation to be returned instead, according to section 3.3.2 of
the YAML 1.1 spec:

"""If a document contains unresolved tags, the YAML processor is
unable to compose a complete representation graph. In such a case, the
YAML processor may compose an partial representation, based on each
node’s kind and allowing for non-specific tags."""

These partial representations are useful with documents that you get
from other sources and for which do you not have constructors for all
tags.

Example:

  s = "!tag test"
  print yaml.load(s)

raises an exception.

It's possible to parse the document by using this hack:

  from yaml import Loader
  del Loader.yaml_constructors[None]

Two questions...

 - Would it be possible to return partial representations by default
in case of unresolvable tags?
 - If not, would it be possible to have an API to enable it (so that i
don't need to mess around with PyYAML internals).

Thanks,
Geert