# Abstract Syntax Trees (ASTs)¶

By default, and AST is either a list (for closures and rules without named elements), or dict-derived object that contains one item for every named element in the grammar rule. 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 lists 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 list 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, a parseinfo element is added to AST nodes that are dict-like. The element contains a collections.namedtuple with the parse information for the node:

ParseInfo = namedtuple(
'ParseInfo',
[
'buffer',
'rule',
'pos',
'endpos',
'line',
'endline',
]
)


With the help of the Buffer.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 Buffer 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.