How to check the type of a document element (sub document vs list)? "ReqlServerCompileError: Variable name not found in: var_1"

I extend the RethinkDb API by providing some extra functions.

For example I simplify the expression

site_ids = r.table('periods')\
            ['regions']\
            .concat_map(lambda row: row['sites'])\
            ['id']

to

site_ids = f['periods']\
            .unwind('regions.sites.id')

using a custom unwind method that is able to resolve a path of nested document elements. If an item in the given path is a list, its entries are concatenated with concat_map. Otherwise the item is accessed with bracket notation:

def unwind(self, path):
    items = path.split('.')
    cursor = self._cursor
    for item in items:
        is_list = isinstance(cursor[item].run().next(), list)
        if is_list:
            cursor = cursor.concat_map(lambda row: row[item])
        else:
            cursor = cursor[item]

    return self.wrap(self._f, cursor)

=> How can I improve the type check to find out if an element is a list? The check should not require an extra .run() and it should work in main queries as well as in sub queries.

My current implementation with the expression

is_list = isinstance(cursor[item].run().next(), list)

works fine in "main queries" like

result = f['periods'] \
 .unwind('regions.sites.plants.product.process.technologies')\
 .populate_with('periods', 'technologies')\
 .sum('specific_cost_per_year') \
 .run()

It does not work in sub queries, e.g. inside a mapping function:

def period_mapper(period):
    return {
         'year': period['start'],
         'site_ids': f.wrap(period).unwind('regions.sites.id')
    }


f.table('periods')\
 .map(period_mapper)\
 .run()

I get the error

rethinkdb.errors.ReqlServerCompileError: Variable name not found in:
var_1['regions']
^^^^^  

because I am not able to .run() a query on the passed variable argument "period".

I tried to replace the if-then-else condition with r.branch but that did not help.

=> How can I choose an operator based on the type of the current cursor content in a better way?

Code of my selection class that wraps a RethinkDb cursor:

from rethinkdb.ast import RqlQuery


# needs to inherit from RqlQuery for the json serialization to work
class AbstractSelection(RqlQuery):

    def __init__(self, f, cursor):
        self._f = f
        self._cursor = cursor

    def __getitem__(self, identifier):
        cursor = self._cursor[identifier]
        return self.wrap(self._f, cursor)

    def __repr__(self):
        return self._cursor.__repr__()

    def __str__(self):
        return self._cursor.__str__()

    def build(self):
        return self._cursor.build()

    @property
    def _args(self):  # required for json serialization
        return self._cursor._args

    @property
    def optargs(self):  # required for json serialization
        return self._cursor.optargs

    def wrap(self, r, cursor):
        raise NotImplemented('Needs to be implemented by inheriting class')

    def unwind(self, path):
        items = path.split('.')
        cursor = self._cursor
        for item in items:
            is_list = isinstance(cursor[item].run().next(), list)
            if is_list:
                cursor = cursor.concat_map(lambda row: row[item])
            else:
                cursor = cursor[item]

        return self.wrap(self._f, cursor)

    def pick(self, path, query):
        return self.unwind(path).get(query)

    def populate(self, collection_name, path):
        return self.map(lambda identifier:
                        self._f[collection_name]
                        .pick(path, {'id': identifier})
                        )

    def get(self, query):
        cursor = self._cursor.filter(query)[0]
        return self.wrap(self._f, cursor)

    def to_array(self):
        return [item for item in self._cursor]

I managed to use type_of in combination with branch. Accessing the item with bracket notation returns a STREAM and I had to get the first item with [0] before using type_of to check for the 'ARRAY' type. This also works if the property is not an array:

def unwind(self, path):
    items = path.split('.')
    cursor = self._cursor
    r = self._f._r
    for item in items:
        cursor = r.branch(
            cursor[item][0].type_of() == 'ARRAY',
            cursor.concat_map(lambda row: row[item]),
            cursor[item]
        )

    return self.wrap(self._f, cursor)