RFC: Use "constantly" for enums

[jml]

Switching to using attrs was a huge help. I'd like to do the same thing for the many, many sets of constants that we have. http://constantly.readthedocs.io/en/latest/ looks like a pretty good library, and has had the advantage of a long period of testing.

e.g.

# Y Axis formats
DURATION_FORMAT = "dtdurations"
NO_FORMAT = "none"
OPS_FORMAT = "ops"
PERCENT_UNIT_FORMAT = "percentunit"
SECONDS_FORMAT = "s"
MILLISECONDS_FORMAT = "ms"
SHORT_FORMAT = "short"
BYTES_FORMAT = "bytes"

which is used as

    yAxes = attr.ib(
        default=attr.Factory(lambda: [YAxis(format=SHORT_FORMAT)] * 2))

would become something like:

from constantly import ValueConstant, Values
class YAxisFormat(Values):
    """Y-axis formats."""
    DURATION = ValueConstant("dtdurations")
    NONE = ValueConstant("none")
    OPS = ValueConstant("ops")
    PERCENT_UNIT = ValueConstant("percentunit")
    SECONDS = ValueConstant("s")
    MILLISECONDS = ValueConstant("ms")
    SHORT = ValueConstant("short")
    BYTES = ValueConstant("bytes")

which would be used as:

    yAxes = attr.ib(
        default=attr.Factory(lambda: [YAxis(format=YAxisFormat.SHORT)] * 2))

We can then write validators that use .iterconstants() or .lookupByName() to validate the constants.

I don't know how this will affect backwards compatibility, or indeed what backwards compatibility goals we should have.

[dmajere]

How do you suppose to use it for shared constants? For example for colors, will it be Color class with fields RED, GREEN etc?

[jml]

Good question! I haven't given it much thought, but I guess something like:

class Colors(Values):
    """Standard colors."""
    RED = ValueConstant(Color(r=255, …))
    # …

[jml]

I think I would rather use enum than constantly at this point.

[jml]

Main concern right now is compatibility. enum would be much easier for maintenance and give better errors. Not sure how much tolerance folk would have for migrating their dashboards.