How do I attach callbacks to terminal nodes?

[salimfadhley]

Continuing from the previous example - I'd like to be able to define some kind of callback which will be called any time the parser encounters a terminal.

So continuing on from my previous example, supposing the parser had successfully parsed the expression:

"go north, south"

And supposing that matches a particular rule for 'movement instruction' - I'd like a particular callback to be invoked any time we have one of those.

Can it be done?

[erikrose]

Parsimonious divides the problem into 2 phases: parsing and tree walking. They're decoupled for various reason (https://github.com/erikrose/parsimonious#why-no-streaming-tree-processing). Parsimonious does the parsing for you, emitting a syntax tree—a tree of Node instances. Then you can do whatever you want with the tree. If I were going to make an adventure game, I'd do something along these lines…

Let's build on the example grammar I suggested in #26. I imagine there'd be more than one type of instruction: maybe a go_instruction and also a get_instruction that adds an item to your inventory, for example. You'd have a top-level rule like this, which dispatches between them:

instruction = go_instruction / get_instruction

You make that rule the top one in your grammar so it's the default entrypoint. Then you do something like this:

grammar = Grammar("""
    instruction = go_instruction / get_instruction
    ...
    """)
instruction_tree = grammar.parse(user_input)

That would give you a syntax tree representing the user's input. If you were a masochist, you could use it directly, if-then-ing your way down its branches to figure out what in-game thing to do.

To make it easier on yourself, you can first transform the tree a bit to make it easier to work with. First, tweak my #26 grammar a bit by breaking out the "additional directions" part:

go_instruction = "go " _ directions
directions = direction additional_directions*
additional_directions = "," _ direction
direction = "north" / "south" / "east" / "west"
_ = ~r"\s*"

Then you can write a fairly straightforward NodeVisitor subclass with a visit_additional_directions method which replaces an additional_directions node with its direction component (discarding commas and whitespace). Next, add a visit_directions method which flattens out all the directions into a list. Then you can walk the transformed tree pretty easily to figure out where to go. Remember, there's no need to return Nodes from your visitor methods; you can transform the tree into whatever data structure you find handiest.

I plan to add some common tree transformations to the grammar proper so you don't have to write visitors at all in common cases: https://github.com/erikrose/parsimonious#niceties. #23 has a good discussion of these and also some ideas for weaving grammar and tree-walker together without losing decoupling.

Does this help?

[erikrose]

Say, while we're on the subject, have you seen https://pypi.python.org/pypi/blessings/ ? It might make your output easier, if you're targeting the terminal.