在Python课程设计中，设计一个简易计算器，根据课本内容所写

import tkinter
import tkinter.messagebox
import re

root = tkinter.Tk()
root.geometry('300x270+400+100') # 设置窗口大小和位置
root.resizable(False,False)  # 不允许改变窗口大小
root.title('计算器')

# 放置用来显示信息的文本框，并设置为只读
contentVar = tkinter.StringVar(root,'')
contentEntry = tkinter.Entry(root, textvariable=contentVar, state='readonly')
contentEntry.place(x=10, y=10, width=280, height=20)

# 按钮通用代码
def buttonClick(btn):

    content = contentVar.get()
    # 如果已有内容是以小数点开头的，前面加0
    if content.startswith('.'):
        content = '0' + content
  	# 根据不同按钮做出相应的处理
    if btn in '0123456789':
        content += btn
    elif btn == '.':
        lastPast = re.split(r'\+|-|\*|/]',content)[-1]
        if '.' in lastPast:
            tkinter.messagebox.showerror('错误','小数点太多了')
            return
        else:
            content += btn
    elif btn == 'C':
        content = ''
    elif btn == '=':
        try:
        	# 对输入的表达式求值
            content = str(eval(content))
        except:
            tkinter.messagebox.showerror('错误','表达式错误')
            return
    elif btn in operators:
        if content.endswith(operators):
            tkinter.messagebox.showerror('错误','不允许存在连续的运算符')
            return
        content += btn
    elif btn =='Sqrt':
        n = content.split('.')
        if all(map(lambda x:x.isdigit(),n)):
            content = eval(content) ** 0.5
        else:
            tkinter.messagebox.showerror('错误','表达式错误')
            return
    contentVar.set(content)
# 放置clear按钮和”=“按钮
btnClear = tkinter.Button(root, text='C', command=lambda:buttonClick('C'))
btnClear.place(x=40, y=40, width=80, height=20)
btnCompute = tkinter.Button(root, text='=', command=lambda:buttonClick('='))
btnCompute.place(x=170, y=40, width=80, height=20)

# 放置10个数字，小数点，和计算平方根的按钮
digits = list('0123456789.') + ['Sqrt']
index = 0
for row in range(4):
    for col in range(3):
        d = digits[index]
        index += 1
        btnDigit = tkinter.Button(root, text=d, command=lambda x=d: buttonClick(x))
        btnDigit.place(x=20 + col * 70, y=80 + row * 50, width=50, height=20)

# 放置运算符按钮
operators = ('+', '-', '*', '/', '**', '//')
for index, operator in enumerate(operators):
    btnOperator = tkinter.Button(root, text=operator, command=lambda x=operator: buttonClick(x))
    btnOperator.place(x=230, y=80 + index * 50, width=50, height=20)


root.mainloop()