Is there a way to override python's json handler?

Question

I'm having trouble encoding infinity in json. `json.dumps` will convert this to `"Infinity"`, but I would like it do convert it to `null` or another value of my choosing. Unfortunately, setting `default` argument only seems to work if dumps does't already understand the object, otherwise the default handler appears to be bypassed. Is there a way I can pre-encode the object, change the default way a type/class is encoded, or convert a certain type/class into a different object prior to normal encoding?

Accepted Answer

Look at the source here: http://hg.python.org/cpython/file/7ec9255d4189/Lib/json/encoder.py

If you subclass JSONEncoder, you can override just the `iterencode(self, o, _one_shot=False)` method, which has explicit special casing for Infinity (inside an inner function).

To make this reusable, you'll also want to alter the `__init__` to take some new options, and store them in the class.

Alternatively, you could pick a json library from pypi which has the appropriate extensibility you are looking for: https://pypi.python.org/pypi?%3Aaction=search&term=json&submit=search

Here's an example:

```
import json

class FloatEncoder(json.JSONEncoder):

def __init__(self, nan_str = "null", **kwargs):
        super(FloatEncoder,self).__init__(**kwargs)
    self.nan_str = nan_str

# uses code from official python json.encoder module.
    # Same licence applies.
    def iterencode(self, o, _one_shot=False):
        """Encode the given object and yield each string
        representation as available.

For example::

for chunk in JSONEncoder().iterencode(bigobject):
                mysocket.write(chunk)
        """
        if self.check_circular:
            markers = {}
        else:
            markers = None
        if self.ensure_ascii:
            _encoder = json.encoder.encode_basestring_ascii
        else:
            _encoder = json.encoder.encode_basestring
        if self.encoding != 'utf-8':
            def _encoder(o, _orig_encoder=_encoder,
                         _encoding=self.encoding):
                if isinstance(o, str):
                    o = o.decode(_encoding)
                return _orig_encoder(o)

def floatstr(o, allow_nan=self.allow_nan,
                     _repr=json.encoder.FLOAT_REPR,
                     _inf=json.encoder.INFINITY,
                     _neginf=-json.encoder.INFINITY,
                     nan_str = self.nan_str):
            # Check for specials. Note that this type of test is 
            # processor and/or platform-specific, so do tests which
            # don't depend on the internals.

if o != o:
                text = nan_str
            elif o == _inf:
                text = 'Infinity'
            elif o == _neginf:
                text = '-Infinity'
            else:
                return _repr(o)

if not allow_nan:
                raise ValueError(
                    "Out of range float values are not JSON compliant: " +
                    repr(o))

return text

_iterencode = json.encoder._make_iterencode(
                markers, self.default, _encoder, self.indent, floatstr,
                self.key_separator, self.item_separator, self.sort_keys,
                self.skipkeys, _one_shot)
        return _iterencode(o, 0)

example_obj = {
    'name': 'example',
    'body': [
        1.1,
        {"3.3": 5, "1.1": float('Nan')},
        [float('inf'), 2.2]
    ]}

print json.dumps(example_obj, cls=FloatEncoder)
```

ideone: http://ideone.com/dFWaNj

Is there a way to override python's json handler?

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