Return a list of PlaceholderNode found in the given template.

    :param template_name: the name of the template file

def get_placeholders(template_name):
    """Return a list of PlaceholderNode found in the given template.

    :param template_name: the name of the template file
    """
    try:
        temp = loader.get_template(template_name)
    except TemplateDoesNotExist:
        return []

    plist, blist = [], []
    try:
        # django 1.8
        _placeholders_recursif(temp.template.nodelist, plist, blist)
    except AttributeError:
        # django 1.7
        # raise
        _placeholders_recursif(temp.nodelist, plist, blist)
    return plist