{% if tag|is_in:'div|p|span' %}is block tag{% endif %}
    {% if tag|is_in:tags_list %}is block tag{% endif %}

def is_in(val, items):
    """
    {% if tag|is_in:'div|p|span' %}is block tag{% endif %}
    {% if tag|is_in:tags_list %}is block tag{% endif %}
    """
    if isinstance(items, basestring):
        items = items.split('|')
    return val in items