Inconsistent behavior serializing python numbers and decimals

[ucwillg]

Description

Hello all - we use msgspec to encode json of api call responses.

We recently discovered that there is a discrepancy between how python floats and decimals are encoded.

In particular, NaN and Infinity float values are encoded as "null", but NaN and Infinity Decimal values are interpolated directly, which produces invalid json.

We prefer the behavior of encoding these invalid numeric values as null.

I couldn't find a way to work around this without preprocessing our responses before passing them to msgspec - it doesn't seem like there is a way to add a custom hook for the encoding of known types like decimal. However, it is possible that I missed a way to do so.

Can you all please advise on whether this behavior is expected or not?

Replication code:

import msgspec
import math
import decimal

msgspec_encoder = msgspec.json.Encoder(decimal_format="number")


def encode(arg: object) -> object:
    return msgspec_encoder.encode(arg).decode()


def test(arg: object) -> None:
    wrapped_arg = {"value": arg}
    print(f"`{wrapped_arg}` encodes as `{encode(wrapped_arg)}`")

print(f"Version: {msgspec.__version__}")
test(1)
test(math.inf)
test(math.nan)
test(decimal.Decimal(1))
test(decimal.Decimal("Infinity"))
test(decimal.Decimal("NaN"))

Output:

Version: 0.19.0
`{'value': 1}` encodes as `{"value":1}`
`{'value': inf}` encodes as `{"value":null}`
`{'value': nan}` encodes as `{"value":null}`
`{'value': Decimal('1')}` encodes as `{"value":1}`
`{'value': Decimal('Infinity')}` encodes as `{"value":Infinity}`
`{'value': Decimal('NaN')}` encodes as `{"value":NaN}`