I am trying to add the math.sqrt function in my py 2.7 calculator. Here is the function I am using:

```
from Tkinter import *
from math import *

class calculator:
    def __init__(self, master):
        master.title("Simple Calculator")
        self.display = Entry(master)
        self.display.grid(columnspan = 8, ipadx = 60, ipady = 10)
        Button(master, text = "sqrt", fg = 'white', bg = 'black', 
               command = lambda:self.sqroot(), height =1, width = 10).grid(row = 6, column = 1)

def sqroot(self):
    try:
        self.s=self.display.get()
        self.sqroot=sqrt(eval(self.s))
        self.display.delete(0,END)
        self.display.insert(0,self.sqroot)
    except:
        self.display.delete(0,END)
        self.display.insert(0,'Invalid operation')

```

In the resulting GUI, I can use the 'sqrt' button correctly only once. I am getting the following error message from second time:

```none
TypeError: 'float' object is not callable

```