# Python 3
# -*- coding: utf-8 -*-
import sys
from PyQt4 import QtCore, QtGui
from PyQt4.QtGui import (QWidget, qApp, QAction, QApplication, QHBoxLayout, QVBoxLayout,
                             QGridLayout, QLabel, QLineEdit, QTextEdit, QPushButton, QComboBox,
                             QCheckBox, QRadioButton, QFrame, QScrollArea, QTabWidget, QSizePolicy,
                             QGroupBox, QFileDialog, QPlainTextEdit)
from PyQt4.QtGui import QIcon, QPixmap, QPalette, QTextCursor
from PyQt4.QtCore import QSize
# ЦВЕТА ПОЛЕЙ
sss_vivod = ("background-color: #456173; color: #f2f2f0; font: 14pt 'Courier New'")
# ГРАФИКА
class Window(QWidget):
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.resize(200, 200) # шир / выс окна
        self.setWindowTitle('Заголовок') # Заголовок
        
        # БЛОК РАЗМЕТКИ
        #grid_os = QGridLayout()
        vbox = QVBoxLayout()
        # --- ---
        self.pole_1 = QTextEdit()
        # ---
        self.pole_2 = QPlainTextEdit()
        # --- ---
        self.tab = QTabWidget()
        self.tab.currentChanged.connect(self.on_tab)
        self.tab.addTab(self.pole_1, 'Читать')
        self.tab.addTab(self.pole_2, 'Править')
        self.tab.setCurrentIndex(1)
        vbox.addWidget(self.tab)
        
        # ---
        self.setLayout(vbox)
    # ЛОГИКА
    
    def on_tab(self):
        tab_index = self.tab.currentIndex()
        if tab_index == 0:
            text = self.pole_2.toPlainText()
            print(text)
            self.pole_1.clear()
            self.pole_1.append('<br>'.join(text))
# КОНЕЦ
if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = Window()   
    window.show()
    sys.exit(app.exec_())
    
 

 self.pole_1.append(''.join(text))

 self.pole_1.append(text + '\n')

 text = self.pole_2.toPlainText().split('\n')