... """Decode a JSON document from ``s`` (a ``str`` beginning with
    345 a JSON document) and return a 2-tuple of the Python
    346 representation and the index in ``s`` where the document ended.
   (...)    350 
    351 """
    352 try:
--> 353     obj, end = self.scan_once(s, idx)
    354 except StopIteration as err:
    355     raise JSONDecodeError("Expecting value", s, err.value) from None

JSON...