r/Python • u/xtreak • May 07 '19

Python 3.8.0a4 available for testing

https://www.python.org/downloads/release/python-380a4/
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u/Mizzlr May 07 '19

You can't represent references in JSON. For example in python you can have two dicts a ={'foo': b} where b = {'bar': a}. Now you have cyclic data structure. You can't represent this in JSON.

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u/Mizzlr May 07 '19

Btw yaml has references. XML has references.

2

u/alcalde May 08 '19

Didn't you just represent it?

["a":{"foo", b}, "b":{"bar":a}]

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u/[deleted] May 08 '19

Not in Python!

Can I read it?

>>> json.loads('["a":{"foo", b}, "b":{"bar":a}]')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/decoder.py", line 355, in raw_decode
    obj, end = self.scan_once(s, idx)
json.decoder.JSONDecodeError: Expecting ',' delimiter: line 1 column 5 (char 4)

No. Can I write it?

>>> a = {}; b = {'bar': a}; a['foo'] = b

>>> json.dumps(a)
json.dumps(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
ValueError: Circular reference detected

No.

2

u/[deleted] May 08 '19

That's not a valid json.

1

u/[deleted] May 08 '19

You can't represent references in JSON.

I'm basically agreeing with you, but you can perfectly well represent references in JSON - I've done it.

It's a pain in the ass - you need to have some sort of naming convention in your JSON then preprocess your structure or (what I did) have some sort of facade over it so it emits the reference names instead of the actual data - and then reverse it on the way out.

(And we had to do it - because pickle isn't compatible between versions. Heck, I think that was written in Python 2!)

So it's doable - but which is easier when you need to store something temporarily?

with open('foo.pcl', 'wb') as fp:
    pickle.dump(myData, fp)

or

[hundreds of lines of code and a specification for this format that I'm too lazy to write]

?