json encoder different results for json.dump and json.dumps

Question

I had a string in this format, ``` d = {'details': {'hawk_branch': {'tandem': ['4210bnd72']}, 'uclif_branch': {'tandem': ['e2nc712nma89', '23s24212', '12338cm82']}}} ``` I wanted to write it to file in this format, converting lists to dictionaries and adding word `value` as key for each value in the list, so `{'tandem': ['4210bnd72']}` should become ``` "tandem": { "value": "4210bnd72" } ``` Here is the expected output file, ``` { "details": { "hawk_branch": { "tandem": { "value": "4210bnd72" } }, "uclif_branch": { "tandem": { "value": "e2nc712nma89", "value": "23s24212", "value": "12338cm82", } } } } ``` I asked a question here where someone answered to use `json.JSONEncoder`, ``` class restore_value(json.JSONEncoder): def encode(self, o): if isinstance(o, dict): return '{%s}' % ', '.join(': '.join((json.encoder.py_encode_basestring(k), self.encode(v))) for k, v in o.items()) if isinstance(o, list): return '{%s}' % ', '.join('"value": %s' % self.encode(v) for v in o) return super().e…

Accepted Answer

The reason is here:

If you look into the source code of `json.__init__.py` in `CPython/Lib/json` here in github: https://github.com/python/cpython/blob/master/Lib/json/init.py

You'll find that `json.dump` actually use:

```
if (not skipkeys and ensure_ascii and
    check_circular and allow_nan and
    cls is None and indent is None and separators is None and
    default is None and not sort_keys and not kw):
    iterable = _default_encoder.iterencode(obj)
else:
    if cls is None:
        cls = JSONEncoder
    iterable = cls(skipkeys=skipkeys, ensure_ascii=ensure_ascii,
        check_circular=check_circular, allow_nan=allow_nan, indent=indent,
        separators=separators,
        default=default, sort_keys=sort_keys, **kw).iterencode(obj)
# could accelerate with writelines in some versions of Python, at
# a debuggability cost
for chunk in iterable:
    fp.write(chunk)
```

Hence the function you would want to `override` should be `json.JSONEncoder.iterencode` instead of `encode`.

json encoder different results for json.dump and json.dumps

Solution

Problem