Calculator

from tkinter import (Tk, BOTH, Button, Entry,
    Event, Frame, NSEW, RIGHT, StringVar)

## -- START OF CONFIG --

DISPLAY_FONT = ("Arial", 24)
BUTTON_FONT = ("Arial", 16)
WINDOW_DIMENSIONS = "512x384"

## -- END OF CONFIG --

## Creating the window.
window = Tk()
window.geometry(WINDOW_DIMENSIONS)

## Creating the main frame to expand with resizing.
frame = Frame(window)

## Setting up the statement display.
stream = StringVar(frame, name="stream")

entry = Entry(frame, textvariable=stream, font=DISPLAY_FONT, justify=RIGHT,
    state="readonly", readonlybackground="Blue", foreground="White")
entry.grid(row=0, column=0, columnspan=5, sticky=NSEW)

## Defining functions for handling all key creation and callbacks.
def handle_key(event):
    if event.keysym == "Escape" or event.char == "CLR":
        window.call("set", "stream", "")
    elif event.keysym == "Return" or event.char == '=':
        try:
            window.call("set", "stream", eval(stream.get().replace('x', '*')))
        except:
            window.call("set", "stream", "ERROR")
    elif event.keysym == "BackSpace":
        window.call("set", "stream", stream.get()[:-1])
    elif event.char == '*':
        window.call("append", "stream", 'x')
    else:
        window.call("append", "stream", event.char)

def add_button(key, row, column, binding, **kwargs):
    event = Event()
    event.keysym, event.char = "", key
    button = Button(frame, text=key, font=BUTTON_FONT,
        command=lambda: handle_key(event))
    button.grid(row=row, column=column, sticky=NSEW, **kwargs)
    window.bind(binding, handle_key)

def construct_row_column(num):
    row, column = divmod(num, 3)
    return (row + 1, column - 1) if column != 0 else (row, column + 2)

## Adding number keys into the frame.
for i in range(1, 10):
    row, column = construct_row_column(i)
    add_button(str(i), row, column, str(i))

## Adding operation keys into the frame.
add_button('x', 1, 3, '*')
add_button('+', 2, 3, '+')
add_button('-', 3, 3, '-')

add_button('CLR', 4, 0, "<Escape>")
add_button('0', 4, 1, '0')
add_button('.', 4, 2, '.')
add_button('=', 4, 3, "<Return>", columnspan=2)

add_button('/', 1, 4, '/')
add_button('(', 2, 4, '(')
add_button(')', 3, 4, ')')

window.bind("<BackSpace>", handle_key)

## Configuring cells to autoscale.
frame.pack(fill=BOTH, expand=True)
frame.grid_columnconfigure((0, 1, 2, 3, 4), weight=1)
frame.grid_rowconfigure((0, 1, 2, 3, 4), weight=1)

## Run the frame.
window.mainloop()