Support encoding decimals as numbers

[jcrist]

Previously we always encoded decimal.Decimal values as strings. While this is the best format for these types (prevents precision loss, widely supported), sometimes there is a need to encode Decimal objects the same as numbers. This adds a new decimal_format option to all Encoder classes. This value defaults to "string" (to encode decimals as strings), but may be set to "number" to encode them the same as the protocol's numeric types.

We opt not to support this configuration in the top-level encode functions for now, since this is a less-common setting.

Fixes #440.

[jcrist]

A quick demo:

In [1]: import msgspec, decimal

In [2]: x = decimal.Decimal("1.3")

In [3]: msgspec.json.encode(x)  # by default encodes as a string
Out[3]: b'"1.3"'

In [4]: encoder = msgspec.json.Encoder(decimal_format="number")  # create an encoder to override

In [5]: encoder.encode(x)  # now encodes as a number
Out[5]: b'1.3'

In [6]: import json

In [7]: decimal.Decimal(json.loads(b'1.3'))  # downstream tools may have trouble loading decimals from numbers without precision loss
Out[7]: Decimal('1.3000000000000000444089209850062616169452667236328125')

In [8]: msgspec.json.decode(b'1.3', type=decimal.Decimal)  # msgspec has no issues with this though, but other tools may
Out[8]: Decimal('1.3')

[jcrist]

If you wish to try this out before the next release, please see our instructions for installing from GitHub.

[s7clarke10]

Brilliant, thank you very much for taking this on board and providing the ability to output as a decimal. I do take you point that there may be a loss of precision. I will utilize configuration to select whether we wish to output as a string or decimal using this feature.