def create_dummy_func(func, dependency):
    """
    When a dependency of a function is not available, create a dummy function which throws ImportError when used.

    Args:
        func (str): name of the function.
        dependency (str or list[str]): name(s) of the dependency.

    Returns:
        function: a function object
    """
    assert not building_rtfd()

    if isinstance(dependency, (list, tuple)):
        dependency = ','.join(dependency)

    def _dummy(*args, **kwargs):
        raise ImportError("Cannot import '{}', therefore '{}' is not available".format(dependency, func))
    return _dummy

    When a dependency of a function is not available, create a dummy function which throws ImportError when used.

    Args:
        func (str): name of the function.
        dependency (str or list[str]): name(s) of the dependency.

    Returns:
        function: a function object
    
tensorpack/utils/develop.py