原来书中的代码是这样的：

from __future__ import division
import sys
from math import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Form(QDialog):
    def __init__(self, parent=None):
        super(Form, self).__init__(parent)
        self.browser = QTextBrowser()
        self.lineedit = QLineEdit("Type an expression and press Enter")
        self.lineedit.selectAll()
        layout = QVBoxLayout()
        layout.addWidget(self.browser)
        layout.addWidget(self.lineedit)
        self.setLayout(layout)
        self.lineedit.setFocus()
        self.connect(self.lineedit, SIGNAL("returnPressed()"),
                     self.updateUi)
        self.setWindowTitle("Calculate")

    def updateUi(self):
        try:
            text = unicode(self.lineedit.text())
            self.browser.append("%s = %s" % (text, eval(text)))
        except:
            self.browser.append(
                    "%s is invalid!" % text)

app = QApplication(sys.argv)
form = Form()
form.show()
app.exec_()
第一，对于PyQt5来说，还是老问题，要把from PyQt5.QtWidgets import *加进去。
第二，类Form，是QDialog的子类，在Form类中，
self.connect(self.lineedit, SIGNAL("returnPressed()"),
                     self.updateUi)
里面的connect在 PyQt5中已经取消。如果运行的话，会出现：AttributeError: 'Form' object has no attribute 'connect'，这个错误。要改成：
self.lineedit.returnPressed.connect(self.updateUi)
因此，完整的代码为：

from __future__ import division
import sys
from math import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

class Form(QDialog):
    def __init__(self, parent=None):
        super(Form, self).__init__(parent)
        self.browser = QTextBrowser()
        self.lineedit = QLineEdit("Type an expression and press Enter")
        self.lineedit.selectAll()
        layout = QVBoxLayout()
        layout.addWidget(self.browser)
        layout.addWidget(self.lineedit)
        self.setLayout(layout)
        self.lineedit.setFocus()
        self.lineedit.returnPressed.connect(self.updateUi)
        self.setWindowTitle("Calculate")

    def updateUi(self):