def backSpace(self, evt=None):
    'Backspace on expression'
    self.stringContents = self.stringContents[:(- 1)]
    self.displayStr.set(self.stringContents)
Backspace on expression
def backSpace(self, evt=None):
    
    self.stringContents = self.stringContents[:(- 1)]
    self.displayStr.set(self.stringContents)