codeeditor.py

from PyQt5.QtCore import QRect, QSize, Qt, pyqtSlot
from PyQt5.QtGui import QColor, QFont, QPainter, QTextFormat
from PyQt5.QtWidgets import QApplication, QPlainTextEdit, QTextEdit, QWidget


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

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

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


class CodeEditor(QPlainTextEdit):
    def __init__(self, parent=None):
        QPlainTextEdit.__init__(self, parent)
        self.lineNumberArea = LineNumberArea(self)
        self.blockCountChanged.connect(self.updateLineNumberAreaWidth)
        self.updateRequest.connect(self.updateLineNumberArea)
        self.cursorPositionChanged.connect(self.highlightCurrentLine)
        self.updateLineNumberAreaWidth(0)
        self.highlightCurrentLine()

        self.setFont(QFont('Courier', 14))

    def lineNumberAreaPaintEvent(self, event):
        painter = QPainter(self.lineNumberArea)
        painter.fillRect(event.rect(), Qt.white)

        painter.setFont(QFont('Courier', 14))

        block = self.firstVisibleBlock()
        blockNumber = block.blockNumber()
        top = self.blockBoundingGeometry(block).translated(self.contentOffset()).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(blockNumber + 1)
                painter.setPen(Qt.lightGray)

                painter.drawText(
                    0,
                    int(top),
                    self.lineNumberArea.width(),
                    self.fontMetrics().height(),
                    Qt.AlignRight,
                    number,
                )

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

    def lineNumberAreaWidth(self):
        digits = len(str(self.blockCount()))
        space = 3 + self.fontMetrics().width('9') * digits
        return space

    def resizeEvent(self, event):
        QPlainTextEdit.resizeEvent(self, event)
        cr = self.contentsRect()
        self.lineNumberArea.setGeometry(QRect(cr.left(), cr.top(), self.lineNumberAreaWidth(), cr.height()))

    @pyqtSlot(int)
    def updateLineNumberAreaWidth(self, newBlockCount):
        self.setViewportMargins(self.lineNumberAreaWidth(), 0, 0, 0)

    @pyqtSlot()
    def highlightCurrentLine(self):
        extraSelections = []

        if not self.isReadOnly():
            selection = QTextEdit.ExtraSelection()
            lineColor = QColor(Qt.gray).lighter(150)
            selection.format.setBackground(lineColor)
            selection.format.setProperty(QTextFormat.FullWidthSelection, True)
            selection.cursor = self.textCursor()
            selection.cursor.clearSelection()
            extraSelections.append(selection)

        self.setExtraSelections(extraSelections)

    @pyqtSlot(QRect, int)
    def updateLineNumberArea(self, rect, dy):
        if dy:
            self.lineNumberArea.scroll(0, dy)
        else:
            self.lineNumberArea.update(0, rect.y(), self.lineNumberArea.width(), rect.height())
        if rect.contains(self.viewport().rect()):
            self.updateLineNumberAreaWidth(0)


if __name__ == '__main__':
    import sys

    app = QApplication(sys.argv)
    w = CodeEditor()
    w.show()
    sys.exit(app.exec_())