Abstract Syntax Trees (ASTs)

By default, an AST is either:

  • a value, for simple elements such as token, pattern, or constant
  • a tuple, for closures, gatherings, and the right-hand-side of rules with more than one element but without named elements
  • a dict-derived object (AST) that contains one item for every named element in the grammar rule, with items can be accessed through the standard dict syntax (ast['key']), or as attributes (ast.key).

AST entries are single values if only one item was associated with a name, or tuple if more than one item was matched. There’s a provision in the grammar syntax (the +: operator) to force an AST entry to be a tuple even if only one element was matched. The value for named elements that were not found during the parse (perhaps because they are optional) is None.

When the parseinfo=True keyword argument has been passed to the Parser constructor or enabled with the @@parseinfo directive, a parseinfo item is added to AST nodes that are dict-like. The item contains a collections.namedtuple with the parse information for the node:

ParseInfo = namedtuple(

With the help of the Tokenizer.line_info() method, it is possible to recover the line, column, and original text parsed for the node. Note that when ParseInfo is generated, the Tokenizer used during parsing is kept in memory for the lifetime of the AST.

Generation of parseinfo can also be controlled using the @@parseinfo :: True grammar directive.