for i in range(21):
    if cbList[i]:
        cb'i'.setChecked=True
    else:
        cb'i'.setChecked=False

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
        ...
        for i in range(21):
            checkbox = getattr(self.ui, 'cb%s' % i)
            if cbList[i]:
                checkbox.setChecked(True)
            else:
                checkbox.setChecked(False)

cb = defaultdict(dict)
for i in range(21):
    if cbList[i]:
        cb[i]['setChecked'] = True
    else:
        cb[i]['setChecked'] = False

cb["cb%s"%(i)]

cb = {}
for i in range(21):
    cb['cb%s'%i] = getattr(self.ui, 'cb%s'%i)

cb['cb3'].setChecked(True)

import sys
from PyQt4.QtGui import *

app = QApplication(sys.argv)

listWidget = QListWidget()

for i in range(10):
    item = QListWidgetItem("Item %i" % i)
    listWidget.addItem(item)

listWidget.show()
sys.exit(app.exec_())