def extract_value(proxy):
    value = proxy.namespace.get(proxy.config_key, proxy.default)
    if value is UndefToken:
        raise errors.ConfigurationError("%s is missing value for: %s" %
            (proxy.namespace, proxy.config_key))

    try:
        return proxy.validator(value)
    except errors.ValidationError as e:
        raise errors.ConfigurationError("%s failed to validate %s: %s" %
            (proxy.namespace, proxy.config_key, e))
Given a value proxy type, Retrieve a value from a namespace, raising
dataset/ETHPy150Open dnephin/PyStaticConfiguration/staticconf/proxy.py/extract_value