python

how do I fix this error I am getting

code:

from tkinter import *
expression = ""

def press(num):
global expression
expression = expression + str(num)
equation.set(expression)

def equalpress():
try:
global expression
total = str(eval(expression))
equation.set(total)
expression = ""

except:
equation.set(" error ")
expression = ""

def clear():
global expression
expression = ""
equation.set("")


equation.set("")

if __name__ == "__main__":
gui = Tk()



gui.geometry("270x150")

equation = StringVar()

expression_field = Entry(gui, textvariable=equation)

expression_field.grid(columnspan=4, ipadx=70)


buttonl = Button(gui, text=' 1', fg='black', bg='white',command=lambda: press(1), height=l, width=7)
buttonl.grid(row=2, column=0)

button2 = Button(gui, text=' 2', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button2.grid(row=2, column=1)

button3 = Button(gui, text=' 3', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button3.grid(row=2, column=2)

button4 = Button(gui, text=' 4', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button4.grid(row=3, column=0)
button5 = Button(gui, text=' 5', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button5.grid(row=3, column=1)
button6 = Button(gui, text=' 6', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button6.grid(row=3, column=2)
button7 = Button(gui, text=' 7', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button7.grid(row=4, column=0)
button8 = Button(gui, text=' 8', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button8.grid(row=4, column=1)
button9 = Button(gui, text=' 9', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button9.grid(row=4, column=2)
button0 = Button(gui, text=' 0', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
button0.grid(row=5, column=0)


Add = Button(gui, text=' +', fg='black', bg='white',command=lambda: press("+"), height=l, width=7)
Add.grid(row=2, column=3)

Sub = Button(gui, text=' -', fg='black', bg='white',command=lambda: press(2), height=l, width=7)
Sub.grid(row=3, column=3)

Div = Button(gui, text=' /', fg='black', bg='white',command=lambda: press("/"), height=l, width=7)
Div.grid(row=5, column=3)

Mul = Button(gui, text=' *', fg='black', bg='white',command=lambda: press("*"), height=l, width=7)
Mul.grid(row=4, column=3)

Equal = Button(gui, text=' =', fg='black', bg='white',command=equalpress, height=l, width=7)
Equal.grid(row=5, column=2)

Clear = Button(gui, text=' Clear', fg='black', bg='white',command=clear, height=l, width=7)
Clear.grid(row=5, column=1)

Decimal = Button(gui, text=' .', fg='black', bg='white',command=lambda: press("."), height=l, width=7)
buttonl.grid(row=6, column=0)

gui.mainloop()
​