Allow customization of NaN and Infinity serialization in json module

[krzyszti]

Feature or enhancement

Proposal:

Body:
The json module currently serializes special floating-point values such as NaN, Infinity, and -Infinity as unquoted identifiers when allow_nan=True, which is non-standard JSON. While this behavior is useful in some contexts, there is currently no built-in way to override how these values are serialized when using json.dumps() or json.encoder.JSONEncoder.

Use Case:
Some applications may need to emit null, a fixed numeric value (like 0), or a quoted string (e.g., "NaN") instead of the default NaN, Infinity, and -Infinity. This is particularly important when interoperating with systems expecting strict or customized JSON.

Proposal:
Introduce a way to override the serialization of these special float values via a floatstr parameter, enabling developers to supply a callable that determines how to stringify such values.

Code of floatstr will not change, but it would be moved to the class, therefore it would allow the function to be overriten with custom value for NaN, infinite and -infinite.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-134717: Allow overriding serialization of NaN and Infinity in json.encoder #134715

[nineteendo]

@picnixz, could you add this to JSON issues?

[nineteendo]

Wouldn't a hook be more flexible?

import jsonyx as json
from math import inf, isfinite, nan

def float_hook(obj):
    if isinstance(obj, float) and not isfinite(obj):
        return None
    return obj

json.dump([nan, inf, -inf], hook=float_hook) # [null, null, null]