Fix issue with assignment in case root element is a list.

[rolling-robot]

This is a fix for https://github.com/h2non/jsonpath-ng/issues/108

[michaelmior]

@rolling-robot Would you be able to add a test for this new case?

[rolling-robot]

Here you go. An error message with older clean algorithm

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ jsonpath_ng/jsonpath.py:295: in update_or_create
    return _clean_list_keys(data)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
dict_ = [{'foo': 1}, {'bar': 2, 'foo': 42}]

    def _clean_list_keys(dict_):
        """
        Replace {LIST_KEY: ['foo', 'bar']} with ['foo', 'bar'].

        >>> _clean_list_keys({LIST_KEY: ['foo', 'bar']})
        ['foo', 'bar']

        """
>       for key, value in dict_.items():
E       AttributeError: 'list' object has no attribute 'items'

jsonpath_ng/jsonpath.py:809: AttributeError

[rolling-robot]

Older version implicitly suggests that root element is a dict, but it may not.

[rolling-robot]

BTW, upon looking at original code now, a year later, I started to doubt: although everything works, this may be a wrong fix for this bug. It may be intended by original author of this code that library-internal representation for list is dictionary with special structure {LIST_KEY: [.......]}, so zero-th level in the tree MUST be a dict type, but somehow _create_list_key() is not run for 0-th level in the first place, so bug manifests as an exception in _clean_list_keys . I did not dig into implementation and philosophy of structure representation of jsonpath-ng that deep.

[rolling-robot]

No, it seems like the bugfix is safe and does not break conventions. _create_list_key() is used twice in the whole repo, and both times its argument is an empty dictionary. So IMHO LIST_KEY hack looks like a generic way to allow assignment or access to uninitialized or empty element be it either a list, or a dict in more or less uniform way.

[michaelmior]

@rolling-robot Thanks for the fix!