Python BMI program using Tkinter

Question

I just started using Tkinter and so far haven't had too many problems. The main issue I am having is that I am supposed to use tk__init__(self) in the program, which I have and it calculates everything fine. However when I run the program, I'm getting two pop up boxes and not just 1. I am also trying to stretch the main box so that it shows the title completely. Here is my code:

from Tkinter import *

class App(Tk):
    def __init__(self):
        self.root = Tk()
        self.root.title("BMI Calculator")
        Tk.__init__(self)


        # Sets a label and entry field into the window for weight, height in
        # feet, and height in inches
        self.label = Label(self.root, text="Enter your weight in pounds.").pack()
        self.lbs = StringVar()
        Entry(self.root, textvariable=self.lbs).pack()

        self.label = Label(self.root, text="Enter your height in feet.").pack()
        self.feet = StringVar()
        Entry(self.root, textvariable=self.feet).pack()

        self.label = Label(self.root, text="Enter your height in inches.").pack()
        self.inches = StringVar()
        Entry(self.root, textvariable=self.inches).pack()

        # Sets a button and label to click and calculate BMI
        self.buttontext = StringVar()
        Button(self.root, textvariable=self.buttontext, command=self.calculate).pack()
        self.buttontext.set("Calculate")

        # Sets bmi_num to a StringVar so that when it is changed, the label will
        # update
        self.bmi_num = StringVar()
        Label(self.root, textvariable=self.bmi_num).pack()

        # Same thing here
        self.bmi_text = StringVar()
        Label(self.root, textvariable=self.bmi_text).pack()

        self.root.mainloop()

    def calculate(self):
        # Retrieves all necessary information to calculate BMI
        weight = float(self.lbs.get())
        feet = float(self.feet.get())
        inches = float(self.inches.get())
        height = (feet*12)+inches
        bmi = float((weight*703)/(height**2))
        # Updates the status label
        self.bmi_num.set("Your BMI is %.2f" % bmi)
        if bmi < 18.5:
            self.bmi_text.set("You are underweight")
        if 18.5 <= bmi < 25:
            self.bmi_text.set("You are normal")
        if 25 <= bmi < 30:
            self.bmi_text.set("You are overweight")
        if 30<= bmi > 30:
            self.bmi_text.set("You are obese")

App()

Any suggestions as to better write this or just fix this issues would be great. Thanks.

Answer 1

It seems like you're confusing inheritance and encapsulation here. Your App class is inheriting from Tk, which means it IS a Tk. However, you're also encapsulating a Tk object inside App and assigning it to self.root. So you've got App, which is a subclass of Tk, encapsulating App.root, which is also a Tk.

You're actually building your GUI on the self.root Tk instance, but then you're also calling Tk.__init__(self), which is initializing a blank Tk object (hence the second window). You need to pick one approach or the other; either (1) Don't inherit from Tk, don't call Tk.__init__(self), and just use self.root, or (2) Don't create a self.root object at all, call Tk.__init__(self), and simply use self everywhere you're currently using self.root.

Answer 2

Please note that this appears to be a homework assignment for a class I teach. If you are tempted to use this example as your own please note two things.

• The code has some problems so it's not the best thing to copy
• The TAs and I are very aware that this code is online so you'll probably get busted