Is the command in Tkinter button stored in the button?

Question

I am making a simple calculator application using tkinter in python. but firstly i want that what button i press should be printed in the entry.

import tkinter as tk
base = tk.Tk()
base.title("First Calculator")
lst,lst2 = [0,1,2,"+",3,4,5,"-",6,7,8,"*",9,".","=","/"],[]
val = 0
base.configure(bg = "grey")
text_box = tk.Entry(width = 43)

frame = tk.Frame()
text_box.grid(row = 0,column = 0)
for j in range(4):
   for i in range(4):
        a =  lst[val]
        b = tk.Button(master = frame,text = a,height = 3, width = 8, bg = 
   "white",command = lambda: text_box.insert(0,a))
        b.grid(row = j, column = i)
        val += 1
frame.grid()

base.mainloop()

but whatever button i press i gives "/". please help.