Extend dict keys to allow for string literals?

Currently, string literals can’t be used as dict keys in a Struct. See example below:

from typing import Literal
from msgspec import Struct, json

AorB = Literal["a", "b"]

class Test(Struct):
    d: dict[AorB, float]

data = Test(d={'a': 1.0, 'b': 2.0})
encoded = json.encode(data)
print(encoded)

decoded = json.decode(encoded, type=Test) # TypeError: JSON doesn't support dicts with non-string keys - type `dict[typing.Literal['a', 'b'], float]` is not supported

I would think it should be allowed - but I’d like to hear your thoughts.

[jcrist]

With the new rename config option (https://jcristharif.com/msgspec/structs.html#renaming-field-names) in 0.6.0, this could also be represented as a struct with field names renamed to "1" and "2". For example:

In [1]: import msgspec

In [2]: class ODRaw(msgspec.Struct, rename=lambda f: f[-1:]):
   ...:     """The rename kwarg is used to rename chan_1 to 1 and chan_2 to 2"""
   ...:     chan_1: dict = {}  # Should use your ODReading type instead
   ...:     chan_2: dict = {}
   ...: 

In [3]: decoder = msgspec.json.Decoder(ODRaw)

In [4]: msg = decoder.decode(b'{"1": {"test": "data"}}')  # decoder uses the renamed fields

In [5]: msg
Out[5]: ODRaw(chan_1={'test': 'data'}, chan_2={})

In [6]: msg.chan_1  # Python uses the original field names
Out[6]: {'test': 'data'}

One reason to prefer this over a dict with literal keys is it allows you to mark fields as required, or provide defaults if they’re optional (there’s no way to do this for a dict with literal keys). This should also be a bit faster than decoding a dict with literal keys, but whether that matters is application dependent. No reason to change what you have now if it’s working for you, just demonstrating another way this could be handled.

[CamDavidsonPilon]

👍

My example does look silly to write this way, but my IRL use case looks like:

PdChannel = Literal["1", "2"]

class ODReadings(Struct):
    timestamp: str
    od_raw: dict[pt.PDChannel, ODReading] 

Because the PdChannels are numbers, I can’t use them as keys / attrs!