Create entry box in tkinter grid

I want to make a calculator in Tkinter. So far I have made a grid of buttons with the numbers on it. How would I create an entry box at the top but it is centred in the middle of the grid and so creates a row that is just one entry box?

from tkinter import * 
root = Tk()
root.rowconfigure(0, weight=1)
root.columnconfigure(0, weight=1)
button_text = list('1234567890.=')
a = 0
for row_index in range(4):
    Grid.rowconfigure(root, row_index, weight=1)
    for col_index in range(3):
        b = button_text[a]
        Grid.columnconfigure(root, col_index, weight=1)
        btn = Button(root, text = b)
        btn.grid(row=row_index, column=col_index, sticky=N+S+E+W)
        a += 1

Solution

You can use grid(..., columnwidth=3) for the Entry box. To add content to the text, use a StringVar and add a command to the buttons. Note that variables in the callback are evaluated when the callback function is called, not when it is defined, thus the c=char in the lambda.

from tkinter import * 
root = Tk()

var = StringVar()
text = Entry(root, text=var)
text.grid(columnspan=3)

for i, char in enumerate('1234567890.='):
    btn = Button(root, text=char, command=lambda c=char: var.set(var.get()+c))
    btn.grid(row=1+i//3, column=i%3, sticky=N+S+E+W)
    
root.mainloop()

Of course, for an actual calculator the callback will be more complex, with special cases for operators, and should delegate to a proper function def.