GUI_calc

from tkinter import *
from tkinter import messagebox
from tkinter import ttk

root = Tk()
root.title('Calculator')


# logic of calc
def calc(key):
    global memory
    if key == "=":
        str1 = '-+/*0123456789.'
# here we exclude letters
        if calc_entry.get()[0] not in str1:
            #calc_entry.insert(END, 'First symb is not a num!')
            messagebox.showerror(' Error!', 'Entered not a num')
# countings
        try:
            result = eval(calc_entry.get())
            calc_entry.insert(END, "=" + str(result))
        except:
            #calc_entry.insert(END, "Error!")
            messagebox.showerror("Error!", "Check data validation")
# clear field
    elif key == "C":
        calc_entry.delete(0, END)
# change +-
    elif key == "-/+":
        if "=" in calc_entry.get():
            calc_entry.delete(0, END)
        try:
            if calc_entry.get()[0] == "-":
                calc_entry.delete(0)
            else:
                calc_entry.insert(0, "-")
        except IndexError:
            pass
    else:
        if "=" in calc_entry.get():
            calc_entry.delete(0, END)
        calc_entry.insert(END, key)


# here we create buttons
bttn_list = [
    '7', '8', '9', '+', '-',
    '4', '5', '6', '*', '/',
    '3', '2', '1', '-/+', '=',
    '0', '.', 'C', '', ''
]

r = 1
c = 0

for i in bttn_list:
    rel = ""
    cmd = lambda x=i: calc(x)
    ttk.Button(root, text=i, command=cmd).grid(row=r, column=c)
    c += 1
    if c > 4:
        c = 0
        r += 1

calc_entry = Entry(root, width=33)
calc_entry.grid(row=0, column=0, columnspan=5)

root.mainloop()