Proxy env var to prefixed env vars

[Kamforka]

When I proxy a non-prefixed env var to a prefixed one and try to load them an error is thrown.

Example:

.env

FLASK_ENV=development
APP_ENV={{FLASK_ENV}}

settings.py

from environs import Env

env = Env()
env.read_env()


with env.prefixed('APP_'):
    ENV = env.str('ENV')

Output:

Traceback (most recent call last):
  File "settings.py", line 8, in <module>
    ENV = env.str('ENV')
  File "/.../venv/lib/python3.6/site-packages/environs.py", line 47, in method
    'Environment variable "{}" not set'.format(proxied_key or parsed_key)
environs.EnvError: Environment variable "FLASK_ENV" not set

Is it intentional to only allow proxying variables within the prefix scope?

Obviously there are many possible workarounds to fix the error, however it feels unintuitive to restrict proxying to a prefix scope.

[sloria]

This appears to be a bug. I won't have time to look at this soon, so I would definitely review and merge a PR.

[Kamforka]

@sloria Got it. I'll make a PR. Took a look at it and seems like a quick fix.