Extend configparser interpolation to parse environment variables

EnvInterpolation.py

"""
This code expects there's a config.ini file under the same directory.
And the file is expected like below contents

See also below if default values isn't needed
https://stackoverflow.com/questions/26586801/configparser-and-string-interpolation-with-env-variable

-- config.ini --
[testdayo]
envuser=${USERNAME:-defusername}
----------------

>> Expected results <<
$ python EnvInterpolation.py
defusername

$ USERNAME=envuser python EnvInterpolation.py
envuser

"""
import configparser
import os
import re

class EnvInterpolation(configparser.ExtendedInterpolation):
    """Interpolation which expands environment variables in values."""

    def before_get(self, parser, section, option, value, defaults):
        m = re.match(r'\${([A-Z_]+):-(.+)}', value)
        if m:
            return os.environ.get(*m.groups())

        value = os.path.expandvars(value)
        return super().before_get(parser, section, option, value, defaults)

config = configparser.ConfigParser(interpolation=EnvInterpolation())
config.read('config.ini')

print(config['testdayo']['envuser'])