consider a choices parser

[willkg]

I've had several situations where I want a parser that requires the value to be one of a list of possible values. I'd like to do something like:

from everett.manager import ConfigManager

config = ConfigManager.basic_config()

CLOUD_PROVIDER = config(
    "CLOUD_PROVIDER",
    default="AWS",
    parser=OneOf(["AWS", "GCP"]),
)

and then have that work for "AWS" and "GCP" and raise aValueError specifying one of the valid values for everything else.

[willkg]

I rethought the API. I want it to support different sub-parsers. Given that, I thought it should mirror ListOf. Examples:

>>> from everett.manager import ChoiceOf, parse_data_size
>>> ChoiceOf(str, choices=["aws", "gcp"])("aws")
"aws"
 >>> ChoiceOf(int, choices=["1", "2", "3"])("1")
1
 >>> ChoiceOf(parse_data_size, choices=["1kb", "1mb", "1gb"])("1mb")
1000000

That gives a lot of flexibility. It forces values to be consistent in the places they show up that the user sees them (documentation, error messages, etc). It also allows the parsed value to be convenient for programming.