Stack trace shows different exception type than print

[micseydel]

Python's JSON module supports deserializing Javascript infinities (although the official JSON spec doesn't). Specifically, it supports "Infinity" and "-Infinity" in particular. I need to be able to deserialize "+Infinity" though. Here is my attempt at achieving this

import json

class PositiveInfinityJSONDecoder(json.JSONDecoder):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.original_scan_once = self.scan_once
        self.scan_once = self._scan_once

    def _scan_once(self, string, idx):
        print("Custom _scan_once entered; trying default first...")
        try:
            return self.original_scan_once(string, idx)
        except json.decoder.JSONDecodeError as e:
            print("Default failed, trying custom logic...")
            try:
                nextchar = string[idx]
            except IndexError:
                raise StopIteration(idx) from e
            if nextchar == '+' and string[idx:idx + 9] == '+Infinity':
                return self.parse_constant('Infinity'), idx + 9
            raise e
        except Exception as e:
            print(f"Got an unexpected exception from default (type: {type(e)}) - {e}")
            raise e

print(json.loads('+Infinity', cls=PositiveInfinityJSONDecoder))

The result of this is that (apparently) a StopIteration exception is thrown, although its traceback looks like a JSONDecodeError

Output:
$ python3 confusion.py 
Custom _scan_once entered; trying default first...
Got an unexpected exception from default (type: <class 'StopIteration'>) - 0
Traceback (most recent call last):
  File "confusion.py", line 28, in <module>
    print(json.loads('+Infinity', cls=PositiveInfinityJSONDecoder))
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 361, in loads
    return cls(**kw).decode(s)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

I'm baffled because the stack trace clearly shows a JSONDecodeError being raised, but at runtime when I try to catch it, it appears to have a different type. In case the class is doing something funky, I also tried catching a ValueError (its superclass) but that didn't work either.

What does work is catching a StopIteration exception, but that doesn't seem right. I'm afraid that there's something weird going on here and if I catch the wrong exception, there will be some surprise down the line. Does anyone know what's going on here?