from django.utils.safestring import SafeData, mark_safe
from django.template import TemplateSyntaxError

@register.filter
def truncate(value, arg):
    """
    Truncates a string after a given number of chars
    Argument: Number of chars to truncate after
    """
    mark_safe_if_safe = lambda v: mark_safe(v) if isinstance(value, SafeData) else v
    try:
        length = int(arg)
    except ValueError: # invalid literal for int()
        raise TemplateSyntaxError, "truncate tag's arg needs to be an integer"
    if not isinstance(value, basestring):
        value = unicode(value)
    if (len(value) > length):
        while value[length] != u' ' and length > 0:
            length -= 1
        return mark_safe_if_safe(value[:length] + u"...")
    return value