TextArea.py

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


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

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

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


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

    def lineNumberAreaWidth(self):
        digits = 1
        max_value = max(1, self.blockCount())
        while max_value >= 10:
            max_value /= 10
            digits += 1
        space = 3 + self.fontMetrics().width('9') * digits
        return space

    def updateLineNumberAreaWidth(self, _):
        self.setViewportMargins(self.lineNumberAreaWidth(), 0, 0, 0)

    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)

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

    def highlightCurrentLine(self):
        extraSelections = []
        if not self.isReadOnly():
            selection = QTextEdit.ExtraSelection()
            lineColor = QColor(Qt.yellow).lighter(160)
            selection.format.setBackground(lineColor)
            selection.format.setProperty(QTextFormat.FullWidthSelection, True)
            selection.cursor = self.textCursor()
            selection.cursor.clearSelection()
            extraSelections.append(selection)
        self.setExtraSelections(extraSelections)

    def lineNumberAreaPaintEvent(self, event):
        painter = QPainter(self.lineNumberArea)

        painter.fillRect(event.rect(), Qt.lightGray)

        block = self.firstVisibleBlock()
        blockNumber = block.blockNumber()
        top = self.blockBoundingGeometry(block).translated(self.contentOffset()).top()
        bottom = top + self.blockBoundingRect(block).height()

        # Just to make sure I use the right font
        height = self.fontMetrics().height()
        while block.isValid() and (top <= event.rect().bottom()):
            if block.isVisible() and (bottom >= event.rect().top()):
                number = str(blockNumber + 1)
                painter.setPen(Qt.black)
                painter.drawText(0, top, self.lineNumberArea.width(), height, Qt.AlignRight, number)

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