def get_env_attr(env, attr, default='__ERROR__', max_depth=100):
    """
    Get the given attribute from a potentially wrapped environment.

    Note that the wrapped envs are traversed from the outside in. Once the
    attribute is found, the search stops. This means that an inner wrapped env
    may carry the same (possibly conflicting) attribute. This situation is
    *not* resolved by this function.

    Parameters
    ----------
    env : gym environment

        A potentially wrapped environment.

    attr : str

        The attribute name.

    max_depth : positive int, optional

        The maximum depth of wrappers to traverse.

    """
    e = env
    for i in range(max_depth):
        if hasattr(e, attr):
            return getattr(e, attr)
        if not hasattr(e, 'env'):
            break
        e = e.env
    if default == '__ERROR__':
        raise AttributeError('env is missing attribute: {}'.format(attr))
    return default
