substre = re.compile('\$\{([^:]+):([^\}]+)\}')

class MyConfigParser(SafeConfigParser):
    def get(self,*a,**kw):
        value = super().get(*a,**kw)
        for m in substre.finditer(value):
            envvar = m.group(1)
            default = m.group(2)
            match = m.group()
            if envvar in os.environ:
                value = value.replace(match,os.environ[envvar])
        return value