Takes an --post-filter=... or --storyitem-filter=... filter
     specification and makes a filter_func Callable out of it.

def filterstr_to_filterfunc(filter_str: str, item_type: type):
    """Takes an --post-filter=... or --storyitem-filter=... filter
     specification and makes a filter_func Callable out of it."""

    # The filter_str is parsed, then all names occurring in its AST are replaced by loads to post.<name>. A
    # function Post->bool is returned which evaluates the filter with the post as 'post' in its namespace.

    class TransformFilterAst(ast.NodeTransformer):
        def visit_Name(self, node: ast.Name):
            # pylint:disable=no-self-use
            if not isinstance(node.ctx, ast.Load):
                raise InvalidArgumentException("Invalid filter: Modifying variables ({}) not allowed.".format(node.id))
            if node.id == "datetime":
                return node
            if not hasattr(item_type, node.id):
                raise InvalidArgumentException("Invalid filter: {} not a {} attribute.".format(node.id,
                                                                                               item_type.__name__))
            new_node = ast.Attribute(ast.copy_location(ast.Name('item', ast.Load()), node), node.id,
                                     ast.copy_location(ast.Load(), node))
            return ast.copy_location(new_node, node)

    input_filename = '<command line filter parameter>'
    compiled_filter = compile(TransformFilterAst().visit(ast.parse(filter_str, filename=input_filename, mode='eval')),
                              filename=input_filename, mode='eval')

    def filterfunc(item) -> bool:
        # pylint:disable=eval-used
        return bool(eval(compiled_filter, {'item': item, 'datetime': datetime.datetime}))

    return filterfunc