Hello.

At first I want to say thank you for all your efforts in python development. I really appreciate it. :)

I am trying to convert custom object to json. 

But, I've found a problem. JSONEncoder has ``default`` method for converting custom objects to some primitives and ``encode`` for converting structures.

But what if I want to return completed json string? I can't do it in ``default``, because JSONEncoder will think it is string and encode it accordingly later in ``iterencode`` method.

Then I tried redefine encode, but it is called with the dict(array_name=default(o)) so I should convert the dict as a whole and basically reimplement all conversions (for int, float, and lists of course).

Did I missed something or there is no way to do it?

I suggest to introduce another method, e.g. encode_obj and call it there:

https://github.com/python/cpython/blob/master/Lib/json/encoder.py#L438
```
   o = _default(o)
   yield from _iterencode(o, _current_indent_level)
```
->
```
   o = _encode_obj(_default(o))
   yield from _iterencode(o, _current_indent_level)
```

If you are agree I would be happy to implement it.

Best Regards,
Artsiom.

See also Issue35111.