Implement custom function for parsing embedded JSON (nested JSON docs)

[lowell80]

Come up with a mechanism that allows a nested JSON document to be parsed as a custom function within the JMSEPath language.

Essentially this would just call json.loads().

If this fails, then just assume the object wasn't a JSON string and return the original value unmodified. That seems like a nice safety mechanism in case the there's a mixture of JSON strings and other strings that have to be processed at the same time.

[lowell80]

Proof-of-concept implementation:

from jmespath import functions

class JmesPathSplunkExtraFunctions(functions.Functions):
    @functions.signature({'types': ['string', 'array']})
    def _func_jsonstr(self, s):
        if s is None:
            return None
        if isinstance(s, (list,tuple)):
            return [ json.loads(i) for i in s ]
        try:
            return json.loads(s)
        except:
            return s

I tried, unsuccessfully to make this handle null types as well, but couldn't get it right. This resulted in runtime errors.