PoC of using attrs’s upcoming metadata feature for using declarative application configuration from env variables including envconsul’s HashiCorp Vault support.

app.py

from __future__ import absolute_import, division, print_function


from app_config import (app_config, env_var, env_var_from_vault,
                        env_var_group, environ_to_app_config)


@app_config(prefix="APP", vault_prefix="WHOIS_{env}")
class WhoisConfig(object):
    @app_config
    class Prometheus(object):
        address = env_var(default="127.0.0.1")
        port = env_var(default="0")
        consul_token = env_var_from_vault()

    env = env_var()
    prometheus = env_var_group(Prometheus)


if __name__ == '__main__':
    ac = environ_to_app_config(WhoisConfig)

    print(ac)

app_config.py

import os

import attr


CNF_KEY = object()
RAISE = object()


class ConfigError(Exception):
    pass


class MissingEnvValueError(ConfigError):
    pass


def app_config(maybe_cls=None, vault_prefix=None, prefix="APP"):
    def wrap(cls):
        cls._vault_prefix = vault_prefix
        cls._prefix = prefix
        return attr.s(cls, slots=True, frozen=True)

    if maybe_cls is None:
        return wrap
    else:
        return wrap(maybe_cls)


@attr.s(slots=True, frozen=True)
class _ConfigEntry(object):
    default = attr.ib(default=RAISE)
    sub_cls = attr.ib(default=None)
    from_vault = attr.ib(default=False)


def env_var(default=RAISE):
    return attr.ib(
        default=default,
        metadata={CNF_KEY: _ConfigEntry(default, None, False)}
    )


def env_var_from_vault(default=RAISE):
    return attr.ib(
        default=default,
        metadata={CNF_KEY: _ConfigEntry(default, None, True)}
    )


def env_var_group(cls):
    return attr.ib(
        default=None,
        metadata={CNF_KEY: _ConfigEntry(None, cls, True)}
    )


def environ_to_app_config(cls, environ=os.environ,
                          prefix=None, vault_prefix=None):
    if prefix is None:
        prefix = cls._prefix
    if vault_prefix is None:
        vault_prefix = cls._vault_prefix

    vals = {}
    for a in attr.fields(cls):
        name = a.name.upper()
        cm = a.metadata[CNF_KEY]
        if cm.sub_cls is None:
            if cm.from_vault is False:
                var = prefix + "_" + name
            else:
                var = "SECRET_" + vault_prefix + "_" + prefix + "_" + name

            val = environ.get(var, cm.default)
            if val is RAISE:
                raise MissingEnvValueError(var)

            if name == "ENV" and "{env}" in vault_prefix:
                vault_prefix = vault_prefix.replace("{env}", val.upper())
        else:
            val = environ_to_app_config(
                cm.sub_cls, environ, prefix + "_" + name, vault_prefix,
            )

        vals[a.name] = val
    return cls(**vals)

Example Usage

$ env APP_ENV=dev APP_PROMETHEUS_PORT=7000 SECRET_WHOIS_DEV_APP_PROMETHEUS_CONSUL_TOKEN=abc python app.py
WhoisConfig(env='dev', prometheus=Prometheus(address='127.0.0.1', port='7000', consul_token='abc'))