How to fix TypeError: unsupported operand type(s) for *: 'instance' and 'float' in a function

Question

This code is basically just a function that uses a value entered by the user. However, when the code multiplies 'population' and 'percent', the user receives the error in the title. What does this error mean? How can the code be modified in order to stop returning this error?

Here is my code:

import random
import math

class populationClass:
    food = 0
    population = 0
    percent = 0

    def populationCount(population, self):    
        while True:
            try:        
                food = int(raw_input("Variety of Foood: "))
                if food > 0: break
                else:
                    print "Enter a Positive Integer!"
            except ValueError:
                print "Enter a Positive Integer!"

        if food == 1:
            percent = 5
        elif food == 2:
            percent = 5
        elif food == 3:
            percent = 7
        elif food == 4:
            percent = 7
        elif food == 5:
            percent = 9
        elif food == 6:
            percent = 9
        elif food >= 7:
            percent = 11
        else:
            print "Enter a Positive Number!"

        rand1 = random.randint(1,6)
        rand2 = random.randint(1,6)
        print rand1
        print rand2
        if rand1 == 1:
            percent -= rand2
        elif rand1 == 2:
            percent += rand2 
        elif rand1 == 3:
            percent -= rand2
        elif rand1 == 4:
            percent += rand2
        elif rand1 == 5:
            percent -= rand2
        elif rand1 == 6:
            percent += rand2
        else:
            print "Something Went Wrong"
        percent = float(percent*.01) + 1.00
        print percent
        population = population*percent
        print "New Population: " + str(int(population))


obj1=populationClass()

while True:
    while True:
        try:
            population = int(raw_input("Population: "))
            if population > 0: break
            else:
                print "Enter a Positive Integer!"
        except ValueError:
            print "Enter a Positive Integer!"

    obj1.populationCount(population)

Here is the traceback:

Traceback (most recent call last):
  File "C:\Users\Andrew\Documents\pyscripts\Board Game Syllabus.py", line 116, in <module>
    obj1.populationCount(population)
  File "C:\Users\Andrew\Documents\pyscripts\Board Game Syllabus.py", line 66, in populationCount
    population = population*percent
TypeError: unsupported operand type(s) for *: 'instance' and 'float'

Answer 1

You have the order of self and population mixed up:

def populationCount(population, self):    

That should be:

def populationCount(self, population):    

Python will bind the arguments in the same order still, so in your method population is the populationClass() instance, and self is bound to the value of population passed in when you called the method.

Answer 2

In your definition of the function populationCount, please change the signature like below:

def populationCount(self, population): 

Python is taking the passed parameter in the call as an instance of self, ie populationClass hence the error.