Re: [Yaml-core] External entities in yaml

[Steve H. <sh...@zi...>]

From: "Tom Harris" <To...@op...>
> We use XML for serialization/configuration and its a pain, yaml looks much
> more suitable. However, we typically use a few main files that include other
> XML files as external entities, this allows the configuration for a
> particular library to be kept apart from anything else, so we can update the
> library and its configuration in the field with only a few files. There
> appears to be no support for external-entity-like things in yaml (and I can
> see why this is a good thing), how would I go about using yaml with files
> including other yaml files?
>

There is a way to do it with the Python implementation, although YAML purists
might consider it a little kludgy.  You would mark nodes in your YAML that
should act like "include" statements with a type indicator of "!include", and
then when you go to parse the YAML, you would give the YAML library a
type-resolving object that reads in the included YAML files.  The Python library
is reentrant, so the code below works as expected:

====
import yaml

def writeFile(fn, data):
    f = open(fn, 'w')
    f.write(data)

writeFile('toplevel.yml', """
    - doc1: !!include doc1.yml
    - doc2: !!include doc2.yml
    """)

writeFile('doc1.yml', "foo: bar")
writeFile('doc2.yml', "more: stuff")

class Includer:
    def resolveType(self, value, url):
        return yaml.loadFile(value).next()

print yaml.loadFile('toplevel.yml', Includer()).next()
=====

Here is the output:

[{'doc1': {'foo': 'bar'}}, {'doc2': {'more': 'stuff'}}]

If you're interested in the Python implementation specifically, let me know.  I
made a tiny patch to the library to make this code work, which will be in future
tarballs, but which is not yet up on the website.

There would be a solution for automatically emitting the YAML with the
"!!include"s as well, but I haven't gotten there quite yet.  For configuration
files, you might be creating the files manually anyway.  For pure serialization
applications, there's probably no point in breaking out the files, although I
haven't thought it through.

BTW the notion of including files may eventually become part of core YAML, but
we have been waiting for a use case.  Sounds like you have one. :)

-- Steve