GUI Calculator Using Tkinter In Python

# importing the tkinter module
from tkinter import *

# initializing the tkinter
root = Tk()

# setting the width and height of the gui
root.geometry("430x500")    # x is small case here

# declaring an empty string variable
expression = ""

# defining function which will set expressions and answers to the  user
def setexpression(num):
    global expression
    expression = expression + str(num)
    value.set(expression)

# defining a function to calculate the expression entered by the user
def calculator():
    try:
        global expression
        answer = str(eval(expression))
        value.set(answer)
    except:
        value.set("Enter correct expression")
        expression = ""

# function to clear everything in expression
def clear():
    global expression
    expression = ""
    value.set(expression)

# declaring font variables as ("Language", size)
large_font = ('Verdana', 15)
small_font = ('Verdana', 10)

# declaring variable to take value of expression entered by the user
value = StringVar(value="Enter expression")

# entry widget to take expression from user and to show 
# calculations 
Entry(root, textvariable=value, font=large_font).grid(row=0, 
    column=0, columnspan=4, ipadx=70)

# Now, there are some most basic buttons which should be present 
# in a calculator
# here, each button is calling the setexpression function which 
# is used to set values in the entry widget entered by the user 
# on pressing the buttons 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, .
Button(root, text="+", fg="red", command=lambda: 
    setexpression("+"), height=4,width=8).grid(row=1,column=0,pady=10)
Button(root, text="-", fg="red", command=lambda:
    setexpression("-"), height=4, width=8).grid(row=2, column=0, pady=10)
Button(root, text="X", fg="red", command=lambda: 
    setexpression("*"), height=4, width=8).grid(row=3, column=0,pady=10)
Button(root, text="/", fg="red", command=lambda: 
    setexpression("/"), height=4, width=8).grid(row=4, column=0,pady=10)
Button(root, text="1", fg="red", command=lambda: 
    setexpression("1"), height=4, width=8).grid(row=1, column=1,pady=10)
Button(root, text="2", fg="red", command=lambda: 
    setexpression("2"), height=4, width=8).grid(row=1, column=2,pady=10)
Button(root, text="3", fg="red", command=lambda: 
    setexpression("3"), height=4, width=8).grid(row=1, column=3,pady=10)
Button(root, text="4", fg="red", command=lambda: 
    setexpression("4"), height=4, width=8).grid(row=2, column=1,pady=10)
Button(root, text="5", fg="red", command=lambda: 
    setexpression("5"), height=4, width=8).grid(row=2, column=2)
Button(root, text="6", fg="red", command=lambda: 
    setexpression("6"), height=4, width=8).grid(row=2, column=3,pady=10)
Button(root, text="7", fg="red", command=lambda: 
    setexpression("7"), height=4, width=8).grid(row=3, column=1,pady=10)
Button(root, text="8", fg="red", command=lambda: 
    setexpression("8"), height=4, width=8).grid(row=3, column=2,pady=10)
Button(root, text="9", fg="red", command=lambda: 
    setexpression("9"), height=4, width=8).grid(row=3, column=3,pady=10)
Button(root, text="0", fg="red", command=lambda: 
    setexpression("0"), height=4, width=8).grid(row=4, column=2,pady=10)
Button(root, text=".", fg="red", command=lambda: 
    setexpression("."), height=4, width=8).grid(row=4, column=1,pady=10)

# "=" button to call the calculator button which will return and 
# show the calculated value in the entry widget
Button(root, text="=", fg="red", command=calculator, height=4,
width=8).grid(row=4, column=3, pady=10)

# "Clear" button to call the clear function which will clear the 
# entry widget so that the user can start clculating again
Button(root, text="Clear", fg="red", command=clear, height=4, 
    width=20).grid(row=5, column=1, pady=10)

# .mainloop() is used when the code is ready to run
root.mainloop()