def evaluate(self, evt=None):
    'Evalutes the expression'
    try:
        self.displayStr.set(eval(self.stringContents))
        self.stringContents = str(eval(self.stringContents))
    except Exception as e:
        self.displayStr.set('Error')
        self.stringContents = ''
Evalutes the expression
def evaluate(self, evt=None):
    
    try:
        self.displayStr.set(eval(self.stringContents))
        self.stringContents = str(eval(self.stringContents))
    except Exception as e:
        self.displayStr.set('Error')
        self.stringContents = 