codeeditor.py

from PySide6.QtCore import Slot, Qt, QRect, QSize
from PySide6.QtGui import QColor, QPainter, QTextFormat
from PySide6.QtWidgets import QPlainTextEdit, QWidget, QTextEdit


class LineNumberArea(QWidget):
    def __init__(self, editor):
        QWidget.__init__(self, editor)
        self.codeEditor = editor

    def sizeHint(self):
        return QSize(self.codeEditor.line_number_area_width(), 0)

    def paintEvent(self, event):
        self.codeEditor.lineNumberAreaPaintEvent(event)


class CodeEditor(QPlainTextEdit):
    def __init__(self):
        QPlainTextEdit.__init__(self)
        self.line_number_area = LineNumberArea(self)

        self.blockCountChanged[int].connect(self.update_line_number_area_width)
        self.updateRequest[QRect, int].connect(self.update_line_number_area)
        self.cursorPositionChanged.connect(self.highlight_current_line)

        self.update_line_number_area_width(0)
        self.highlight_current_line()

    def line_number_area_width(self):
        digits = 1
        max_num = max(1, self.blockCount())
        while max_num >= 10:
            max_num *= 0.1
            digits += 1

        space = 3 + self.fontMetrics().horizontalAdvance('9') * digits
        return space

    def resizeEvent(self, e):
        super().resizeEvent(e)
        cr = self.contentsRect()
        width = self.line_number_area_width()
        rect = QRect(cr.left(), cr.top(), width, cr.height())
        self.line_number_area.setGeometry(rect)

    def lineNumberAreaPaintEvent(self, event):
        painter = QPainter(self.line_number_area)
        painter.fillRect(event.rect(), Qt.lightGray)
        block = self.firstVisibleBlock()
        block_number = block.blockNumber()
        offset = self.contentOffset()
        top = self.blockBoundingGeometry(block).translated(offset).top()
        bottom = top + self.blockBoundingRect(block).height()

        while block.isValid() and top <= event.rect().bottom():
            if block.isVisible() and bottom >= event.rect().top():
                number = str(block_number + 1)
                painter.setPen(Qt.black)
                width = self.line_number_area.width()
                height = self.fontMetrics().height()
                painter.drawText(0, top, width, height, Qt.AlignRight, number)

            block = block.next()
            top = bottom
            bottom = top + self.blockBoundingRect(block).height()
            block_number += 1

    @Slot()
    def update_line_number_area_width(self, newBlockCount):
        self.setViewportMargins(self.line_number_area_width(), 0, 0, 0)

    @Slot()
    def update_line_number_area(self, rect, dy):
        if dy:
            self.line_number_area.scroll(0, dy)
        else:
            width = self.line_number_area.width()
            self.line_number_area.update(0, rect.y(), width, rect.height())

        if rect.contains(self.viewport().rect()):
            self.update_line_number_area_width(0)

    @Slot()
    def highlight_current_line(self):
        extra_selections = []

        if not self.isReadOnly():
            selection = QTextEdit.ExtraSelection()

            line_color = QColor(Qt.yellow).lighter(160)
            selection.format.setBackground(line_color)

            selection.format.setProperty(QTextFormat.FullWidthSelection, True)

            selection.cursor = self.textCursor()
            selection.cursor.clearSelection()

            extra_selections.append(selection)

        self.setExtraSelections(extra_selections)

main.py

import sys
from PySide6.QtWidgets import QApplication
from codeeditor import CodeEditor

"""PySide6 port of the widgets/codeeditor example from Qt5"""

if __name__ == "__main__":
    app = QApplication([])
    editor = CodeEditor()
    editor.setWindowTitle("Code Editor Example")
    editor.show()
    sys.exit(app.exec_())