codeeditor.py

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


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()

	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()

		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(), 
					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.blue).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)

	@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_())

EditorUI.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>1364</width>
    <height>681</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <widget class="CodeEditor" name="textEdit">
    <property name="geometry">
     <rect>
      <x>123</x>
      <y>0</y>
      <width>1241</width>
      <height>681</height>
     </rect>
    </property>
    <property name="styleSheet">
     <string notr="true">background-color: rgb(33, 33, 50);
font: 75 15pt &quot;Consolas&quot;;
color: rgb(255, 255, 255);</string>
    </property>
    <property name="plainText">
     <string/>
    </property>
   </widget>
  </widget>
 </widget>
 <customwidgets>
  <customwidget>
   <class>CodeEditor</class>
   <extends>QPlainTextEdit</extends>
   <header>codeeditor.h</header>
  </customwidget>
 </customwidgets>
 <resources/>
 <connections/>
</ui>

main.py

import sys
from collections import Counter
from PyQt5.QtCore import pyqtRemoveInputHook
from PyQt5.QtWidgets import QMainWindow, QApplication, QAction
from PyQt5 import uic
Ui_MainWindow, QtBaseClass = uic.loadUiType('EditorUI.ui')

class MyApp(QMainWindow):
    def __init__(self):
        self.newLines = 1
        super(MyApp, self).__init__(None)
        menuBar = self.menuBar()
        fileMenu = menuBar.addMenu('&File')

        # New Action
        self.newAction = QAction('&New', self)
        self.newAction.triggered.connect(self.NewCall)
        fileMenu.addAction(self.newAction)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
        
    def NewCall(self):
        print('new')

if __name__ == '__main__':
    pyqtRemoveInputHook()
    app = QApplication(sys.argv)
    window = MyApp()
    window.setWindowTitle('PyEditor')
    window.showMaximized()
    window.show()
    sys.exit(app.exec())