Returns the first environment variable set.

    If none are non-empty, defaults to '' or keyword arg default.

def env(*vars, **kwargs):
    """Returns the first environment variable set.

    If none are non-empty, defaults to '' or keyword arg default.
    """
    for v in vars:
        value = os.environ.get(v)
        if value:
            return value
    return kwargs.get('default', '')