Gareth Rees added the comment:
The JSON implementation uses these tests to determine how to serialize a Python
object:
isinstance(o, (list, tuple))
isinstance(o, dict)
So any subclasses of list and tuple are serialized as a list, and any subclass
of dict is serialized as an object. For example:
>>> json.dumps(collections.defaultdict())
'{}'
>>> json.dumps(collections.OrderedDict())
'{}'
>>> json.dumps(collections.namedtuple('mytuple', ())())
'[]'
When deserialized, you'll get back a plain dictionary or list, so there's no
round-trip property here.
The tests could perhaps be changed to:
isinstance(o, collections.abc.Sequence)
isinstance(o, collections.abc.Mapping)
I'm not a JSON expert, so I have no informed opinion on whether this is a good
idea or not, but in any case, this change wouldn't help with deques, as a deque
is not a Sequence. That's because deques don't have an index method (see
issue10059 and issue12543).
----------
nosy: +Gareth.Rees
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<http://bugs.python.org/issue20774>
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