Simple GUI calculator using Tkinter in Python

In this tutorial, we are going to create a simple GUI calculator using the Tkinter module. Tkinter is builtin the Python module for developing the GUI application. It's easy to use and comes with Python. We can visualize our data with GUI applications.
Let's see how to create a simple GUI calculator.
Import everything from the Tkinter using *.
Import everything from the Tkinter using *.
Create an interface for the calculator.
Create an interface for the calculator.
Create an input function that enters a number into the input field.
Create an input function that enters a number into the input field.
Create an apparent function that wipes everything from the input field.
Create an apparent function that wipes everything from the input field.
And finally, evaluate function that computes and gives the result of the expression.
And finally, evaluate function that computes and gives the result of the expression.
# importing everyting from tkinter
from tkinter import *
# expression to access among all the functions
expression = ""
# functions
def input_number(number, equation):
   # accessing the global expression variable
   global expression
   # concatenation of string
   expression = expression + str(number)
   equation.set(expression)
def clear_input_field(equation):
   global expression
   expression = ""
   # setting empty string in the input field
   equation.set("Enter the expression")
def evaluate(equation):
global expression
# trying to evaluate the expression
try:
result = str(eval(expression))
# showing the result in the input field
equation.set(result)
# setting expression to empty string
expression = ""
except:
# some error occured
# showing it to the user equation.set("Enter a valid expression")
expression = ""
# creating the GUI
def main():
   # main window window = Tk()
   # setting the title of GUI window
   window.title("Calculator")
   # set the configuration of GUI window
   window.geometry("325x175")
   # varible class instantiation
   equation = StringVar()
   # input field for the expression
   input_field = Entry(window, textvariable=equation)
   input_field.place(height=100)
   # we are using grid position
   # for the arrangement of the widgets
   input_field.grid(columnspan=4, ipadx=100, ipady=5)
   # settin the placeholder message for users
   equation.set("Enter the expression")
   # creating buttons and placing them at respective positions
   _1 = Button(window, text='1', fg='white', bg='black', bd=0, command=lambda: input_number(1, equation), height=2, width=7)
   _1.grid(row=2, column=0)
   _2 = Button(window, text='2', fg='white', bg='black', bd=0, command=lambda: input_number(2, equation), height=2, width=7)
   _2.grid(row=2, column=1)
   _3 = Button(window, text='3', fg='white', bg='black', bd=0, command=lambda: input_number(3, equation), height=2, width=7)
   _3.grid(row=2, column=2)
   _4 = Button(window, text='4', fg='white', bg='black', bd=0, command=lambda: input_number(4, equation), height=2, width=7)
   _4.grid(row=3, column=0)
   _5 = Button(window, text='5', fg='white', bg='black', bd=0, command=lambda: input_number(5, equation), height=2, width=7)
   _5.grid(row=3, column=1)
   _6 = Button(window, text='6', fg='white', bg='black', bd=0, command=lambda: input_number(6, equation), height=2, width=7)
   _6.grid(row=3, column=2)
   _7 = Button(window, text='7', fg='white', bg='black', bd=0, command=lambda: input_number(7, equation), height=2, width=7)
   _7.grid(row=4, column=0)
   _8 = Button(window, text='8', fg='white', bg='black', bd=0, command=lambda: input_number(8, equation), height=2, width=7)
   _8.grid(row=4, column=1)
   _9 = Button(window, text='9', fg='white', bg='black', bd=0, command=lambda: input_number(9, equation), height=2, width=7)
   _9.grid(row=4, column=2)
   _0 = Button(window, text='0', fg='white', bg='black', bd=0, command=lambda: input_number(0, equation), height=2, width=7)
   _0.grid(row=5, column=0)
   plus = Button(window, text='+', fg='white', bg='black', bd=0, command=lambda: input_number('+', equation), height=2, width=7)
   plus.grid(row=2, column=3)
   minus = Button(window, text='-', fg='white', bg='black', bd=0, command=lambda: input_number('-', equation), height=2, width=7)
   minus.grid(row=3, column=3)
   multiply = Button(window, text='*', fg='white', bg='black', bd=0, command=lambda:  input_number('*', equation), height=2, width=7)
   multiply.grid(row=4, column=3)
   divide = Button(window, text='/', fg='white', bg='black', bd=0, command=lambda: input_number('/', equation), height=2, width=7)
   divide.grid(row=5, column=3)
   equal = Button(window, text='=', fg='white', bg='black', bd=0, command=lambda: evaluate(equation), height=2, width=7)
   equal.grid(row=5, column=2)
   clear = Button(window, text='Clear', fg='white', bg='black', bd=0, command=lambda: clear_input_field(equation), height=2, width=7)
   clear.grid(row=5, column=1)
   # showing the GUI
   window.mainloop()
# start of the program
if __name__ == '__main__':
      main()

If you run the above program, you will get a simple calculator as follows.
Result of the above expression generated after pression = button.
If you have any doubts in the tutorial, mention them in the comment section.