# Mapping Tree-Sitter trees to Lezer trees
**URL:** https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362
**Category:** Lezer
**Created:** [April 3, 2020, 11:50pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362 "2020-04-03T23:50:15Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@wcrichto](https://discuss.codemirror.net/u/wcrichto)
#### Post date: [April 3, 2020, 11:50pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/1 "2020-04-03T23:50:15Z")
I’m trying to use codemirror.next for Python. It seems like writing the Python grammar in Lezer is not [currently support](https://discuss.codemirror.net/t/dedent-tokens-in-lezer/2266), so in the meantime I was planning on using [tree-sitter-python](https://github.com/tree-sitter/tree-sitter-python) as a fill-in. I’m implementing a [`Syntax`](https://codemirror.net/6/docs/ref/#state.Syntax) service that translates from a tree-sitter Tree into a Lezer Tree. My immediate goal is to get Python syntax highlighting working.
It’s easy enough to derive a [`NodeType`](https://lezer.codemirror.net/docs/ref/#tree.NodeType) for each tree-sitter node, but I’m having trouble creating a Lezer [`Tree`](https://lezer.codemirror.net/docs/ref/#tree.Tree). Right now, I’m fine with something’s that inefficient since my use case is primarily read-only. I was trying to create a `Tree` for each node, but that doesn’t work because `Tree` only works at the toplevel (`start` and `end` are fixed to 0 and length). Presumably the inner nodes have to be `NodeSubtree`.
But there I’m also in a bind. `NodeSubtree` expects a parent `Subtree` to its constructor. But the parent `Tree` expects. a list of `children` to its constructor. So it’s not clear how to satisfy both dependencies.
What is the recommended way to do this?
---
### Author:  [@marijn](https://discuss.codemirror.net/u/marijn)
#### Post date: [April 4, 2020, 6:31pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/2 "2020-04-04T18:31:22Z")
> [@wcrichto](#):
>
> but that doesn’t work because `Tree` only works at the toplevel ( `start` and `end` are fixed to 0 and length)
No, that’s not how that works—each individual Tree instance only knows its length, but their parent will know their position. This is necessary to allow trees to be reused across different parents, where they might sit at a different position.
You don’t need to create subtree instances, if you’re just trying to model a tree. It might also be workable to look at the `fromBuffer` interface—if you can iterate the original tree-sitter tree, it might be quite easy to emit an array of numbers that you can convert to a Lezer tree.
A Lezer Python grammar is relatively high on my list of priorities, but I can’t promise any concrete timeline yet.
---
### Author:  [@wcrichto](https://discuss.codemirror.net/u/wcrichto)
#### Post date: [April 4, 2020, 6:52pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/3 "2020-04-04T18:52:40Z")
Thanks for the clarifications. By `fromBuffer` do you mean [`Tree.build`](https://lezer.codemirror.net/docs/ref/#tree.Tree%5Ebuild)? Do you have any documentation for the format of the buffer? The `build` docs just mention " a postfix-ordered buffer of node information" but I’m not sure what exactly that means.
---
### Author:  [@marijn](https://discuss.codemirror.net/u/marijn)
#### Post date: [April 4, 2020, 8:11pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/4 "2020-04-04T20:11:16Z")
Oops, yeah, I did mean `Tree.build`, and I assumed I had documented the input format, which I apparently haven’t. Each node is represented as 4 numbers, with child nodes coming before parent nodes. The numbers are, in order, the node’s type id, its start position, its end position, and its size—including children—in the array (so that is (1 + total child count) \* 4, where the total child count includes also the children of the node’s direct children).
So to encode a node of type 2 with a child of type 3, both of which span position 0 to 1, you’d need an array like
```
[3, 0, 1, 4, 2, 0, 1, 8]
```
---
### Author:  [@wcrichto](https://discuss.codemirror.net/u/wcrichto)
#### Post date: [April 6, 2020, 11:51pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/5 "2020-04-06T23:51:09Z")
Perfect, got it working. Here’s the gist I came up with in case anyone else finds it useful.
>
Thanks @marijn!
---
### Author:  [@qbane](https://discuss.codemirror.net/u/qbane)
#### Post date: [April 11, 2020, 10:52am UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/6 "2020-04-11T10:52:45Z")
Works perfectly for me. However, I noticed there is a subtle quirk: a node type with id 0 seems always reserved for the top node. For example, suppose there is a `LezerTree` instance:
```auto
import {parser as parserExpr} from './parsers/expr'
const tree = parserExpr.parse('(3+4)*5')
```
with grammar
```auto
@precedence { times left, plus left }
@top Expression { expression }
expression {
Number |
BinaryExpression
}
BinaryExpression {
expression !times "*" expression |
expression !plus "+" expression |
"(" expression ")"
}
@tokens {
Number { std.digit+ }
}
```
The value `tree.type` always equals to the type with id 0 (`Expression` in this case), **even when that type is not the one of the top node**. A quick fix is to explicitly include it before collecting node types:
```auto
build() {
const rootNode = this.tree.rootNode;
this.node_types[rootNode.type] = new _LezerTree.NodeType(rootNode.type, {}, 0);
const buffer = this.build_node(this.tree.rootNode);
/* ... */
}
```
---
### Author:  [@marijn](https://discuss.codemirror.net/u/marijn)
#### Post date: [April 14, 2020, 9:58pm UTC](https://discuss.codemirror.net/t/mapping-tree-sitter-trees-to-lezer-trees/2362/7 "2020-04-14T21:58:02Z")
> [@marijn](#):
>
> A Lezer Python grammar is relatively high on my list of priorities, but I can’t promise any concrete timeline yet.
This exists now: [GitHub - lezer-parser/python: A Python parser for Lezer](https://github.com/lezer-parser/python/)