entry does not respond correctly to buttons' binding in python

I'm trying to program a calculator in python for a while now and there is a problem with my entry that I can't solve it although I don't see any problem with it.

so here is an example for my code:

from Tkinter import *

window =Tk()
window.title("Calculator")
#creating an entry
string=StringVar
entry = Entry(window, width=40,textvariable=string )
entry.grid(row=0, column=0, columnspan=6, ipady=10)
entry.focus()


#basically I have a function for creating buttons but here I will do it the traditional way.

num_one=Button(window,text="1",width=2,height=2,padx=20,pady=20,)
num_one.grid(row=1,column=0,padx=1,pady=1)

#crating an index for the calculator
index=0
#creating a function to insert the number one to the entry in the index position and then add one to the index

def print_one(index):
    entry.insert(index,"1")

binding the num_one button to the function above

num_one.bind("Button-1",print_one(index))

Now the problem is that the string "1" should be entered to the entry only if I click n the num_one button, but when I start the program automatically the number "1" goes into the entry.

Solution

Lots of issues that I noticed in your code -

string=StringVar - You need to call it like StringVar() , otherwise you are just setting StringVar class (not its object) to `string.
When you do -
```
num_one.bind("Button-1",print_one(index))
```
You actually call the function first and bind the return value , you should instead bind the function object (without calling it) , Example -
```
num_one.bind("<Button-1>",print_one)
```
For binding the function to left mouse click, you need to bind to <Button-1> (notice the < and > at the ends) not Button-1 .
In your function, the first parameter you will recieve (the function that is bound) is the event object , not the next index. You can instead use something like -
```
string.set(string.get() + "1")
```