How to Implement a Custom Lexer Using Tree Sitter in Python

Introduction

Implementing syntax highlighting for your QScintilla-based IDE using Tree Sitter can be both rewarding and challenging. If you're facing issues with color mismatching and incomplete highlight recognition after editing text, you're not alone. This tutorial will walk you through the implementation of a custom lexer that leverages the powerful parsing capabilities of Tree Sitter, ensuring accurate and efficient syntax highlighting for Python code in your application.

Why Syntax Highlighting Issues Occur

Syntax highlighting problems often stem from the way the text is parsed and how styling is applied afterward. When using Tree Sitter, each node in the abstract syntax tree (AST) corresponds to a specific part of your code, like keywords, functions, or comments. However, if the highlights are improperly configured, or if there are issues in your styleText method, mismatched colors can arise, especially as the text is edited.

Setting Up Your Custom Lexer

To implement your custom lexer correctly, follow these organized steps:

Step 1: Import Necessary Libraries

Ensure that you have all the required dependencies set up:

import tree_sitter_python as PYTHON

from tree_sitter import Parser, Node, Language

from PyQt5.QtGui import QColor, QFont

from PyQt5.Qsci import QsciLexerCustom

Step 2: Define Your Lexer Class

Define your lexer class that inherits from QsciLexerCustom:

class PythonLexer(QsciLexerCustom):

    DEFAULT = 0

    KEYWORD = 1

    TYPES = 2

    STRING = 3

    KEYARGS = 4

    BRACKETS = 5

    COMMENTS = 6

    CONSTANTS = 7

    FUNCTIONS = 8

    CLASS_DEF = 9

    FUNCTION_DEF = 10



    def __init__(self, editor: QsciScintilla):

        super().__init__(editor, 'Python')

        self.editor = editor

        self.language_name = 'Python'



        defaults = {'color': '#ffffff', 'paper': '#1e1e1e', 'font': ('JetBrains Mono', 14)}

        self.setDefaultColor(QColor(defaults['color']))

        self.setDefaultPaper(QColor(defaults['paper']))

        self.setDefaultFont(QFont(*defaults['font']))

        self.createStyle()



        self.parser = Parser(Language(PYTHON.language()))

Step 3: Create Styling Colors

Use the createStyle method to configure your different syntax styles. Each style is associated with colors that represent the type of syntax being highlighted:

    def createStyle(self):

        self.setColor(QColor('#abb2bf'), PythonLexer.DEFAULT)

        self.setColor(QColor('#c678dd'), PythonLexer.KEYWORD)

        ...  # Set other styles similarly

Step 4: Style the Text

The styleText method is crucial for parsing the text and applying styles. Here, we will ensure that highlights are built accurately:

    def styleText(self, start, end):

        self.startStyling(start)



        raw_bytes = self.editor.bytes(start, end)

        text = raw_bytes.data().decode('utf-8').replace('\0', '')

        tree = self.parser.parse(bytes(text, 'utf-8'))



        highlights = []

        self.buildHighlights(tree.root_node, highlights)

        highlights.sort(key=lambda h: h[0])  # Sort by start byte



        for start_byte, end_byte, style in highlights:

            self.setStyling(end_byte - start_byte, style)

Here it's important to adjust the lengths correctly for setStyling. Ensure that the parameters are consistent with the actual text length for proper application.

Step 5: Building Highlights

The buildHighlights method traverses the AST and assigns styles based on node types. Ensure that you're accurately identifying the type of each node in your parser:

    def buildHighlights(self, node: Node, highlights: list):

        for child in node.children:

            style = None

            if child.type == 'comment':

                style = PythonLexer.COMMENTS

            elif child.type == 'string':

                style = PythonLexer.STRING

            ...  # Add other node types



            if style:

                highlights.append((child.start_byte, child.end_byte, style))

            self.buildHighlights(child, highlights)

Frequently Asked Questions

What is Tree Sitter?

Tree Sitter is a powerful parser generator for programming languages. It can efficiently parse source code into an AST.

Why Choose Tree Sitter Over Regex?

Tree Sitter offers better accuracy and efficiency for complex languages as it handles nested structures, which can be cumbersome with regular expressions.

Can I Customize the Colors Further?

Yes! Feel free to adjust the color values in the createStyle method to fit your IDE's theme.

Conclusion

Implementing a custom lexer using Tree Sitter offers robust support for syntax highlighting in your PyQt IDE. By following the organized steps above and ensuring careful attention to the parsing and styling methods, you can overcome the issues of incorrect color assignments and incomplete highlighting. Happy coding!