Environs

Features:

- Add support for variable expansion, e.g. `MY_VAR=${MY_OTHER_VAR:-mydefault}` ([164](https://github.com/sloria/environs/issues/164)).
Thanks [gnarvaja](https://github.com/gnarvaja) for the PR.

Deprecations:

- Variable proxying using the `{{VAR}}` syntax is deprecated
and will be removed in environs 9.0.0.
Use variable expansion using `${VAR}` instead.

bash
Before
export MAILGUN_LOGIN=sloria
export SMTP_LOGIN={{MAILGUN_LOGIN}}

After
export MAILGUN_LOGIN=sloria
export SMTP_LOGIN=${MAILGUN_LOGIN}


python
from environs import Env

env = Env(expand_vars=True)

SMTP_LOGIN = env.str("SMTP_LOGIN") => 'sloria'


Bug fixes:

- Fix deferred validation behavior for `dj_db_url`, `dj_email_url`, `dj_cache_url`,
and custom parsers ([121](https://github.com/sloria/environs/issues/121)).
Thanks [hukkinj1](https://github.com/hukkinj1) for reporting.

Other changes:

- Test against Python 3.9.
- Remove usage of implicit `typing.Optional` ([171](https://github.com/sloria/environs/issues/171)).

Bug fixes:

- Fix behavior of recurse=True when custom filepath is passed to `env.read_env`
([100](https://github.com/sloria/environs/issues/100)). Thanks [ribeaud](https://github.com/ribeaud) and [timoklimmer](https://github.com/sloria/environs/pull/157) for the help.

Other changes:

- _Backwards-incompatible_: As a result of the above fix, passing a directory to `env.read_env` is no longer allowed and will raise a `ValueError`.
Only file paths or file names should be passed.

- Add `subcast_key` argument to `env.dict` ([151](https://github.com/sloria/environs/issues/151)).
Thanks [AugPro](https://github.com/AugPro) for the suggestion and PR.

- Fix error when parsing empty list with subcast
[137](https://github.com/sloria/environs/issues/137).
Thanks [sabdouni] for the catch and patch.

- `log_level` accepts lower-cased log level names and rejects invalid
names ([138](https://github.com/sloria/environs/pull/138)).
Thanks [gnarvaja](https://github.com/gnarvaja) for the PR.

- Add `dj_cache_url` for caching Django cache URLs (requires installing with `[django]`)
([126](https://github.com/sloria/environs/issues/126)).
Thanks [epicserve](https://github.com/epicserve) for the suggestion and PR.