viaenv uses variable annotation to populate values from environment variables.

from datetime import timedelta
from viaenv import populate_from_env


class config:
    port: int = 8080
    log_file: str = '/var/log/server.log'
    timeout: timedelta = timedelta(milliseconds=100)


populate_from_env(
    config, prefix='SRV',
    # If you don't pass env, populate_from_env will use os.environ
    env={'SRV_PORT': '9000', 'SRV_TIMEOUT': '300ms'})

print(config.port)  # 9000
print(cfg.timeout == timedelta(milliseconds=300)  # True

• bool: y, yes, t, true, on, 1 → True, n, no, f, false, off, 0 → False
• date: 2019-05-18
• datetime: 2019-05-18T13:43:12
• dict: {"x": 1, "y": 2} (JSON format)
• float: 1.3, 1e7
• int: 12, 0x12, 0o12, 0b12
• list: [1, 2, 3] (JSON format)
• str: Anything goes
• time: 13:43:12
• timedelta: 10us, 20ms, 30s, 17m, 2h, 7d, 1h20m

You can add your types by calling register_type_parser(typ, parser) where typ is a type (e.g. float) and parser is a one argument function that will get the value as a string.