通过检查按钮小部件在Tkinter中分配全局变量

Question

I've been working on a crude Tkinter population growth calculator type project, and it works fine with just the population consistently growing with no decay or lowered birth rate. I have been trying to get a variable to be considered global so it can be read in my function that calculates the population.

However I can't seem to figure out how to assign a value to the variable when the checkbutton widget is checked or not. Here is the code for you guys to see:

'''shows human population growth rate through a certain year.
   It also has an option to include a list of prepared disasters
   that can lower the population.'''

from math import *
from Tkinter import *
import random

class App(Tk):
    def __init__ (self):
        Tk.__init__(self)
        '''Creates the set up for the Tkinter widgets with text on the left side, and the entry values on the right'''
        self.title("Population Growth")

        self.LblOutput = Label (self, foreground = "blue", font = "arial", text = "Enter the year you wish to see the world's population: ")
        self.LblOutput.grid(row = 1, column = 0)

        self.TxtInput = Entry(self)
        self.TxtInput.grid(row = 1, column = 1)

        self.LblOutput2 = Label (self, foreground = "blue", font = "arial", text = "Enter the desired Starting population: ")
        self.LblOutput2.grid(row = 2, column = 0)

        self.TxtInput2 = Entry(self)
        self.TxtInput2.grid(row = 2, column = 1)

        Label(self, foreground = "blue", font = "arial", text = "Turn on Natural Disasters?").grid(row = 3, column = 0)
        self.checkVar = IntVar()
        self.chkCheck = Checkbutton(self, text = "Natural Disasters", variable = self.checkVar, onvalue = 1, offvalue = 0, command = self.mortality)
        self.chkCheck.grid(row = 3, column = 1)

        self.StrtBtn = Button(self, foreground = "black", font = "arial", text = "Begin!", command = self.population_growth)
        self.StrtBtn.grid (row = 6, column = 0)

        self.TxtOutput = Label(self, foreground = "red", font = "gothica" , text = "The Population Is:").grid(row = 7, column = 0)
        self.LblTotal = Label(self)
        self.LblTotal.grid(row = 7, column = 1)

        self.mainloop()

    def mortality(self):
    '''checks to see if the checkmark is checked, if so assigns a random number to deathrate from the range, otherwise deathrate is 0'''
        if self.checkVar.get() == 1:
            deathrate = random.uniform(0.001, 0.00009903)
        elif self.checkVar.get() == 0:
            deathrate = 0
        global deathrate

    def population_growth(self):
        #Using the following equation P=P(subscript = 0)e^rt
        #P(subscript = 0)=initial population, 0 = time; for simplicity = p
        #e = euler's number [approximation]
        #rate_of_growth = rate of growth (found by subtracting crude death rate from crude birth rate
        #time_desired = time (years) that the population is calculated for
        time_desired = int(self.TxtInput.get())
        population_beginning = int(self.TxtInput2.get())
        e_mans_number = 2.71828182845904523536028747135266249775724709369995
        rate_of_growth = 0.01113 - int(deathrate)
        P = population_beginning * ((e_mans_number)**((rate_of_growth)*(time_desired)))
        self.LblTotal["text"] = " %d  people" % P 

def main():
    a = App()

if __name__ == "__main__":
    main()

It seems to work fine if I check the box, but once unchecked the value does not change. If I start it with the value unchecked it will give me an error, but once I check it, no error. After checking it, and deselecting it, no error, any help???

Answer 1

Initialise deathrate before class with the imports

Whenever changing the value of deathrate do:

global deathrate

before changing the value. Rest of your code is working fine

Answer 2

Why make a global variable out of it? Global variables are a bad habit most of the time.

Try the following:

class App(Tk):

def __init__ (self):
    Tk.__init__(self)
    '''Creates the set up for the Tkinter widgets with text on the left side, and the          entry values on the right'''
    self.title("Population Growth")
    self.deathRate = random.uniform(0.001, 0.00009903)
    [...]

Now you can access the variable deathRate within your mortality and population growth function via:

self.deathrate

For example:

    def mortality(self):
        if self.checkVar.get() == 1:
            self.deathrate = random.uniform(0.001, 0.00009903)
        elif self.checkVar.get() == 0:
            self.deathrate = 0

    def population_growth(self):
        time_desired = int(self.TxtInput.get())
        population_beginning = int(self.TxtInput2.get())
        e_mans_number = 2.71828182845904523536028747135266249775724709369995
        rate_of_growth = 0.01113 - int(self.deathrate)
        P = population_beginning * ((e_mans_number)**((rate_of_growth)*(time_desired)))
        self.LblTotal["text"] = " %d  people" % P 

This is called an instance variable. Please refer to http://www.python.org/doc/essays/ppt/acm-ws/sld051.htm or the python tutorial( http://docs.python.org/2/tutorial/classes.html ).

Greetings

Answer 3

You don't need to create a global variable, and you don't need to do anything when the checkbutton is checked. Just use the checkbutton variable at the time that you need it:

def population_growth(self):
    ...
    deathrate = 0
    if self.checkVar.get() == 1:
        deathrate = random.uniform(0.001, 0.00009903)
    rate_of_growth = 0.01113 - deathrate

If you don't want to put that logic in the function, just declare another instance variable. Instead of using a global, use self.deathrate:

def mortality(self):
    if self.checkVar.get() == 1:
        self.deathrate = random.uniform(0.001, 0.00009903)
    else:
        self.deathrate = 0

def population_growth(self):
    ...
    rate_of_growth = 0.01113 - self.deathrate