Extend `json.dumps` to encode namedtuples as maps (not arrays)

namedtuple-json.py

from collections import namedtuple
from json import dumps
from datetime import datetime

def fix_default(default=None):
    
    class TupleButNotNamedTupleMeta(type):
        def __instancecheck__(cls, obj):
            return isinstance(obj, tuple) and not hasattr(obj, '_fields')
    class TupleButNotNamedTuple(metaclass=TupleButNotNamedTupleMeta):
        pass
    
    def default(obj, default=default):
        if isinstance(obj, tuple) and not isinstance(obj, TupleButNotNamedTuple): # i.e., it IS a namedtuple...
            return obj._asdict()
        if default is not None:
            return default(obj)
        raise TypeError('cannot serialize')
        
    # patch `json` module
    from json.encoder import _make_iterencode as mi
    import json.encoder as enc
    mi.__defaults__ = (*mi.__defaults__[:8], TupleButNotNamedTuple, *mi.__defaults__[9:])
    enc.c_make_encoder = None # disable use of C encoder, which cannot be patched in this way
    
    # check
    try:
        from collections import namedtuple
        from json import dumps
        
        obj = namedtuple('_', 'x')(0)
        actual, expected = dumps(obj, default=default), '{"x": 0}'
        if actual != expected: raise Exception(f'Expected {expected!r}, got {actual!r}')
        
        obj = (0, 1, 2)
        actual, expected = dumps(obj, default=default), '[0, 1, 2]'
        if actual != expected: raise Exception(f'Expected {expected!r}, got {actual!r}')
    except Exception as e:
        raise Exception('JSON encoding hack broke') from e
        
    return default

if __name__ == '__main__':
    @fix_default
    def default(obj):
        if isinstance(obj, datetime):
            return obj.isoformat()
        raise TypeError("cannot serialize")
    
    class Foo(namedtuple('Foo', 'a b c, d')):
        pass

    class Bar(namedtuple('Bar', 'x y')):
        pass

    obj = Foo(Bar(1, 2), (Bar(3, 4), Bar(5, 6)), [Bar(7, 9), Bar(9, 10)], datetime.now())
    print(dumps(obj, default=default))