default initialization of configurable traits from os.environ

[rmorshea]

Configurables hooks into a call from the metaclass to setup_class in order to setup default values from os.environ. This in turn calls TraitType.set_default_from_envvar to apply that default value to the TraitType if the key 'envvar' is in its metadata.

[minrk]

Looks like this file was added by mistake.

[rmorshea]

I've included TraitType.set_default_from_envvar to apply default values from environment variables, but I have two questions about how I've implemented that hook:

• Should I check a trait's metadata for 'envvar' (as I am now) or an attribute envvar to determine whether it needs a default value to be set from an environment variable?

Having envvar be an attribute, and an explicit entry in TraitType.__init__ makes it clear that environment variable defaults are a feature, however it might introduce confusion, because it is only a feature of traits attached to configurables.

• Should I apply environment variable defaults through setup_class (as I am now) or setup_instance?

Using setup_class makes sense to me, and would be slightly more efficient, but I'm not really familiar with conventions surrounding how applications are typically configured (i.e. whether configurations should exist at the class level or their instances).

[Carreau]

This seem to be stale a bit, and start to look like over engineering what traitlets can (and should do).
This seem also like it's pretty easy to get by using @default function.

So I'm curious if

• we really want that ?
• is it worked on ?

Because that seem like quite a bit of maintenance for a potentialy really low use case, and we are spread thin, so I'm leaning toward "no new feature" at that point.

[rmorshea]

@Carreau, should probably bring up whether this is needed in #157. As far as @default, it really doesn't add anything different than _<name>_default. However what it does allow for is an @envvar_default(name, envvar) decorator in configurable.py. The decorator could set trait metadata, and pass the envvar string into the default generator.

[rmorshea]

I'll close this one and make a new pr for @envvar_default.