StyleSheet = """
QComboBox { color: darkblue; }
QLineEdit { color: darkgreen; }
QLineEdit[mandatory="true"] { #mandatory="true"时，QLineEdit的样式会变化
 background-color: rgb(255, 255, 127);
 color: darkblue;
}

#!/usr/bin/env python3

import sys
from PyQt5.QtWidgets import (QApplication, QComboBox, QDialog,
  QDialogButtonBox, QGridLayout, QLabel, QLineEdit, QVBoxLayout)


class ContactDlg(QDialog):

 StyleSheet = """
QComboBox { color: darkblue; }
QLineEdit { color: darkgreen; }
QLineEdit[mandatory="true"] {
 background-color: rgb(255, 255, 127);
 color: darkblue;
}
"""

 def __init__(self, parent=None):
  super(ContactDlg, self).__init__(parent)

  forenameLabel = QLabel("&Forename:")
  self.forenameEdit = QLineEdit()
  forenameLabel.setBuddy(self.forenameEdit)
  surnameLabel = QLabel("&Surname:")
  self.surnameEdit = QLineEdit()
  surnameLabel.setBuddy(self.surnameEdit)
  categoryLabel = QLabel("&Category:")
  self.categoryComboBox = QComboBox()
  categoryLabel.setBuddy(self.categoryComboBox)
  self.categoryComboBox.addItems(["Business", "Domestic",
          "Personal"])
  companyLabel = QLabel("C&ompany:")
  self.companyEdit = QLineEdit()
  companyLabel.setBuddy(self.companyEdit)
  addressLabel = QLabel("A&ddress:")
  self.addressEdit = QLineEdit()
  addressLabel.setBuddy(self.addressEdit)
  phoneLabel = QLabel("&Phone:")
  self.phoneEdit = QLineEdit()
  phoneLabel.setBuddy(self.phoneEdit)
  mobileLabel = QLabel("&Mobile:")
  self.mobileEdit = QLineEdit()
  mobileLabel.setBuddy(self.mobileEdit)
  faxLabel = QLabel("Fa&x:")
  self.faxEdit = QLineEdit()
  faxLabel.setBuddy(self.faxEdit)
  emailLabel = QLabel("&Email:")
  self.emailEdit = QLineEdit()
  emailLabel.setBuddy(self.emailEdit)
  self.buttonBox = QDialogButtonBox(QDialogButtonBox.Ok|
           QDialogButtonBox.Cancel)
  addButton = self.buttonBox.button(QDialogButtonBox.Ok)
  addButton.setText("&Add")
  addButton.setEnabled(False)

  grid = QGridLayout()
  grid.addWidget(forenameLabel, 0, 0)
  grid.addWidget(self.forenameEdit, 0, 1)
  grid.addWidget(surnameLabel, 0, 2)
  grid.addWidget(self.surnameEdit, 0, 3)
  grid.addWidget(categoryLabel, 1, 0)
  grid.addWidget(self.categoryComboBox, 1, 1)
  grid.addWidget(companyLabel, 1, 2)
  grid.addWidget(self.companyEdit, 1, 3)
  grid.addWidget(addressLabel, 2, 0)
  grid.addWidget(self.addressEdit, 2, 1, 1, 3)
  grid.addWidget(phoneLabel, 3, 0)
  grid.addWidget(self.phoneEdit, 3, 1)
  grid.addWidget(mobileLabel, 3, 2)
  grid.addWidget(self.mobileEdit, 3, 3)
  grid.addWidget(faxLabel, 4, 0)
  grid.addWidget(self.faxEdit, 4, 1)
  grid.addWidget(emailLabel, 4, 2)
  grid.addWidget(self.emailEdit, 4, 3)
  layout = QVBoxLayout()
  layout.addLayout(grid)
  layout.addWidget(self.buttonBox)
  self.setLayout(layout)

  self.lineedits = (self.forenameEdit, self.surnameEdit,
    self.companyEdit, self.phoneEdit, self.emailEdit)
  for lineEdit in self.lineedits:
   lineEdit.setProperty("mandatory", True)
   lineEdit.textEdited.connect(self.updateUi)
  self.categoryComboBox.activated.connect(self.updateUi)

  self.buttonBox.accepted.connect(self.accept)
  self.buttonBox.rejected.connect(self.reject)
  self.setStyleSheet(ContactDlg.StyleSheet)
  self.setWindowTitle("Add Contact")


 def updateUi(self):
  mandatory = bool(self.companyEdit.property("mandatory"))
  if self.categoryComboBox.currentText() == "Business":
   if not mandatory:
    self.companyEdit.setProperty("mandatory", True)
  elif mandatory:
   self.companyEdit.setProperty("mandatory", False)
  if (mandatory !=
   bool(self.companyEdit.property("mandatory"))):
   self.setStyleSheet(ContactDlg.StyleSheet)

  enable = True
  for lineEdit in self.lineedits:
   if (bool(lineEdit.property("mandatory")) and
    not lineEdit.text()):
    enable = False
    break
  self.buttonBox.button(QDialogButtonBox.Ok).setEnabled(enable)


if __name__ == "__main__":
 app = QApplication(sys.argv)
 form = ContactDlg()
 form.show()
 app.exec_()