[issue35111] Make Custom Object Classes JSON Serializable

[Bob Ippolito]

Bob Ippolito  added the comment:

That's what the for_json method is in simplejson, it does not have widespread 
usage.

You can implement that when encoding:

```
def json_default(obj):
try:
return obj.__json__()
except AttributeError:
raise TypeError("{} can not be JSON encoded".format(type(obj)))


json.dumps(foo(), default=json_default)
```

This way, you can choose precisely how the output needs to work when encoding. 
It's not ideal for every use case, but nothing is. The point is that it doesn't 
get in your way, whatever you need to do can be done without any awful tricks, 
so long as you have control over the dumps call site.

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[issue35111] Make Custom Object Classes JSON Serializable

[andrew c]

andrew c  added the comment:

Bob,

I understand what you are saying, but having a built-in is way easier than 
encoder/decoders.  You can still have both actually.  Even if you didn't have 
two way, a one way would be amazingly helpful.  For large development systems, 
a __json__ method would make serialization super easy.

class foo(object):
 a = 1
 b = [1,2,3]
 def __json__(self): return {'a':self.a,'b':self.b}

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[issue35111] Make Custom Object Classes JSON Serializable

[Bob Ippolito]

Bob Ippolito  added the comment:

The trouble with having such a hook is that it would take precedence over any 
customization you might want or need to do to satisfy the protocol you're 
implementing. Other than the limited set of types that are part of the JSON 
specification, there's essentially no standard for encoding of anything else. 
This is why customization is left to the call sites for encoding and decoding, 
and I would recommend using the `default` and `object_pairs_hook` keyword 
arguments to `dumps` and `loads` respectively for that, rather than any of the 
options that you've enumerated.

For what it's worth, simplejson has had a `for_json` method hook for several 
years to encourage some consolidation, but it's opt-in and there hasn't been a 
lot of demand to make it the default. The inverse `from_json` type operation is 
not supported. If you think about it, how *could* you even implement such a 
thing in the general case, in a way that wouldn't have lots of surprises and 
performance issues?

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[issue35111] Make Custom Object Classes JSON Serializable

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> If you can serialize an object using pickle,
> why not have the ability to serialize objects using json?

One reason would be that the JSON spec was intentionally designed to handle a 
limited number of types so that it would have maximum interoperability between 
languages. Another reason is that in order to make other types round-trip 
between encoding and decoding, you would need to control both ends, in which 
case, you might as well use pickle.

--
nosy: +rhettinger
versions:  -Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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[issue35111] Make Custom Object Classes JSON Serializable

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee:  -> bob.ippolito
nosy: +bob.ippolito

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[issue35111] Make Custom Object Classes JSON Serializable

[andrew c]

New submission from andrew c :

When creating a custom class that doesn't inherit from the accepted classes, 
there is no way to serialize it using json.dumps or json.dump.  

I propose that __fromjson__ and __tojson__ that when present will be used by 
the Python's default encoder.  This issue is widely documented on stackoverflow 
as a deficiency. 

You basically have a couple of options to solve this:

1. Implement a custom  json.JSONEncoder/json.JSONDecoder
2. Implement a method like to/from json
3. Monkey patch the json library
4. Use a 3rd party library

These are not very good options.  If you can serialize an object using pickle, 
why not have the ability to serialize objects using json?

Thank you

--
messages: 328897
nosy: andrewchap
priority: normal
severity: normal
status: open
title: Make Custom Object Classes JSON Serializable
type: enhancement
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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