Python2.7 PyQt4 AttributeError：＆＃39; MyWindowClass＆＃39;对象没有属性＆＃39; input_1＆＃39;

Question

当我运行此代码并按下“预览条形码”按钮时，出现以下错误：

self.input_1.toPlainText（） AttributeError：'MyWindowClass'对象没有属性'input_1'

为什么我收到此错误？

import sys

from PyQt4 import QtCore, QtGui, uic

_fromUtf8 = QtCore.QString.fromUtf8

class MyWindowClass(QtGui.QMainWindow):

    def __init__(self):
        super(MyWindowClass, self).__init__()
        self.setupUi()

    def setupUi(self):
        self.setObjectName(_fromUtf8("Form"))
        self.resize(559, 340)
        previewButton = QtGui.QPushButton(self)
        previewButton.setText(QtGui.QApplication.translate("form","Preview\nBarcode", None))
        previewButton.setGeometry(QtCore.QRect(140, 20, 151, 101))
        sizePolicy = QtGui.QSizePolicy(QtGui.QSizePolicy.Expanding, QtGui.QSizePolicy.Expanding)
        previewButton.setSizePolicy(sizePolicy)
        font = QtGui.QFont()
        font.setFamily(_fromUtf8("Papyrus"))
        font.setPointSize(18)
        font.setBold(True)
        font.setWeight(75)
        previewButton.setFont(font)
        previewButton.setObjectName(_fromUtf8("previewButton"))
        input_1 = QtGui.QPlainTextEdit(self)
        input_1.setFocus()
        input_1.setGeometry(QtCore.QRect(10, 10, 104, 31))
        input_1.setObjectName(_fromUtf8("input_1"))

        previewButton.clicked.connect(self.previewButton_clicked)
        input_1.cursorPositionChanged.connect(input_1.selectAll)

        self.setWindowTitle('Barcode Generator')
        self.show()

    def previewButton_clicked(self):
        self.input_1.toPlainText()

        sum = 0.0
        if float(addend1) > 0:
            sum = sum + addend1
        strSum = str('%.3f' % sum)
        print(strSum)


def main():

    app = QtGui.QApplication(sys.argv)

    Form = MyWindowClass()

    sys.exit(app.exec_())

if __name__ == '__main__':
        main()