Using grabKeyboard() in the following manner throws an X error to the console,
but the program works without issue:

the error:

X Error: BadWindow (invalid Window parameter) 3
  Major opcode: 31 (X_GrabKeyboard)
  Resource id:  0x0

==============
the code:

import sys
from PySide import QtCore, QtGui
from test34_backend import Ui_Form

class CalculatorForm(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)

        self.ui = Ui_Form()

        self.ui.setupUi(self)
        self.grabKeyboard()

    @QtCore.Slot()
    def on_pushButton_clicked(bool):
        print "Hurray!!"

    @QtCore.Slot()
    def keyPressEvent(self, event):
        kP = event.text()
        if kP in ' ': #spacebar pressed
            print event.text()



if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    calculator = CalculatorForm()
    calculator.show()
    sys.exit(app.exec_())