PyYaml based config with environment variables

import os
import re

class ConfigurationError(Exception): pass


def replacer(m):
    var = m.group(2)[2:-1].strip()
    try:
        return f'{m.group(1)}{os.environ[var]}{m.group(3)}'
    except KeyError:
        if var:
            msg = f'Environment variable {var} is not set.'
        else:
            msg = f'Empty environment variable construction in configuration.'
        raise EnvironmentError(msg)


envvar_pat = re.compile(r'(.*?)(\${.*?})(.*?)')


def envvar_constructor(loader, node):
    var = loader.construct_scalar(node)
    try:
        return envvar_pat.sub(replacer, var)
    except EnvironmentError as e:
        raise ConfigurationError(
            f'\n{node.start_mark}\n{node.end_mark}\n' +
            str(e)
        )


yaml.add_implicit_resolver ("!env_var", envvar_pat)
yaml.add_constructor('!env_var', envvar_constructor)


def load_config(configfile):
    with open(configfile) as f:
        return yaml.load(f.read(), Loader=yaml.FullLoader)

Parses Yaml files with ${} templated variables interpreted as environment variables.