def paste_if(head, args=None, closure='', args_separator='|'):
    """
    Paste only if argument exists, otherwise returns None.

    :param head: Text to be inserted before the argument
    :param args: Value to be inserted after the head, if the value is None, the title will not be shown
    :param closure: Text to be inserted after the argument
    :param args_separator: Character string to separate the results
    :return: a string or None
    """
    if isinstance(args, dict):
        raise Exception('Dictionary not accept in paste_if')
    elif isinstance(args, list):
        args_with_sep = ''
        for arg in args[0:len(args) - 1]:
            args_with_sep = args_with_sep + str(arg) + args_separator
        return head + args_with_sep + args[-1] + closure
    elif isinstance(args, str):
        return head + args + closure
    else:
        return None
