String formatting using Python and Tkinter

Question

I'm trying to get my first program with a gui created. The error I'm getting is as follows:

Exception in Tkinter callback

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/tkinter/__init__.py", line 1487, in __call__
    return self.func(*args)
  File "/Volumes/Mac Storage/Python /letsbuildagui.py", line 7, in calcppp
    labelresult=Label(window,text="The average price per portion is: " % mappp).push()
TypeError: not all arguments converted during string formatting

The following is my code:

from tkinter import *

def calcppp():
    lecost=float(cost.get())
    leportions=float(portions.get())
    mappp=lecost/leportions
    labelresult=Label(window,text="The average price per portion is: " % mappp).push()
    return             

window = Tk()
window.geometry("500x500")
window.title("Price Per Portion")

cost=StringVar()
portions=StringVar()

welcome = Label(text='Please enter your total price of groceries \n and the amount of      meals they yeilded  to find out \n your average price per portion')
welcome.pack()
menubar = Menu(window)
button = Button(window, text="Calculate", command=calcppp)
button.place(x=200,y=450)
myCost=Entry(window,textvariable=cost).pack()
myPortions=Entry(window,textvariable=portions).pack() 
window.config(menu=menubar)
window.mainloop()

Answer 1

There are several errors in your code

1) You can update labels dynamically by using the label.config() option like:

myLabel.config(text="The average price per portion is: %d" %mappp)

2) Label instance has no attribute 'push'.

3) The lines:

menubar = Menu(window)

and

window.config(menu=menubar)

are useless as there is no menu in your program

4) Why are you using place() geometry manger for button and pack() for others. place() is generally used for absolute layout and it is better avoided. Since you are using pack for all other widgets, it is better to use pack, even for your button.

Here's your code modified for all these errors:

from tkinter import *

def calcppp():
    lecost=float(cost.get())
    leportions=float(portions.get())
    mappp=lecost/leportions
    myLabel.config(text="The average price per portion is: %d" %mappp)
    return             

window = Tk()
window.geometry("500x500")
window.title("Price Per Portion")

cost=StringVar()
portions=StringVar()

welcome = Label(text='Please enter your total price of groceries \n and the amount of      meals they yeilded  to find out \n your average price per portion')
welcome.pack()
myCost=Entry(window,textvariable=cost)
myCost.pack()
myPortions=Entry(window,textvariable=portions)
myPortions.pack() 
button = Button(window, text="Calculate", command=calcppp)
button.pack()
myLabel = Label(window)
myLabel.pack()

Answer 2

You are using the string interpolation operator (%) but you aren't giving it anything to interpolate. You need to insert a format tag into the string.

change labelresult=Label(window,text="The average price per portion is: " % mappp).push()

this is giving you error

i think here it should be "The average price per portion is: %g " % mappp