Calculator GUI App

import PySimpleGUI as sg

btn_size = (3, 1)
sg.theme("LightBlue5")
sg.set_options(font=("Rockwell", 25))

layout = [
    [sg.Push(), sg.Text("0", font="Consolas 20", key="-FORMULA-")],
    [sg.Push(), sg.Text("0", font="Consolas 40", key="-DISPLAY-")],
    [
        sg.Button("*", size=btn_size),
        sg.Button("del", size=btn_size, button_color="black on red"),
        sg.Button("clear", expand_x=True, button_color="black on orange"),
    ],
    [
        sg.Button("7", size=btn_size),
        sg.Button("8", size=btn_size),
        sg.Button("9", size=btn_size),
        sg.Button("+", size=btn_size),
    ],
    [
        sg.Button("4", size=btn_size),
        sg.Button("5", size=btn_size),
        sg.Button("6", size=btn_size),
        sg.Button("-", size=btn_size),
    ],
    [
        sg.Button("1", size=btn_size),
        sg.Button("2", size=btn_size),
        sg.Button("3", size=btn_size),
        sg.Button("/", size=btn_size),
    ],
    [
        sg.Button("0", size=btn_size),
        sg.Button(".", size=btn_size),
        sg.Button("=", expand_x=True, button_color="green3"),
    ],
]

window = sg.Window("My Calculator", layout)
eval("print('hello')")

current_num = []
formula = []
# Main Event Loop
while True:
    event, values = window.read()

    if event == sg.WIN_CLOSED:
        break

    if event.isdigit() or (event == "." and "." not in current_num):
        # Skip first 0
        if len(current_num) == 0 and event == "0":
            continue
        # Extend current number
        current_num.append(event)
        num_str = "".join(current_num)
        window.Element("-DISPLAY-").update(num_str)

    if event in ["+", "-", "*", "/"] and len(current_num):
        # Append Operation
        formula.append(num_str)
        if current_num[-1] == ".":
            formula.append("0")
        formula.append(event)
        window["-FORMULA-"].update("".join(formula))
        current_num = []

    if event == "del" and len(current_num):
        current_num.pop()
        num_str = "".join(current_num) if len(current_num) else "0"
        window["-DISPLAY-"].update(num_str)

    if event == "clear":
        current_num = []
        formula = []
        window["-DISPLAY-"].update(0)
        window["-FORMULA-"].update(0)

    # Calculating the result
    if event == "=" and len(current_num) and len(formula):
        formula.append(num_str)
        formula_string = "".join(formula)
        result = eval(formula_string)
        window["-DISPLAY-"].update(result)
        window["-FORMULA-"].update(formula_string)
        current_num = []
        formula = []


window.close()