title: How to increase instance variable in class method in Python
body: <p>I am stuck in this question (part two). The question is as follows:
  Create a class called AppleBasket who's constructor accepts two inputs: a string representing a color, and a number representing a quantity of apples. The constructor should initialize two instance variables: apple_color and apple_quantity. Write a class method called increase that increases the quantity by 1 each time it is invoked. You should also write a <strong>str</strong> method for this class that returns a string of the format: "A basket of [quantity goes here] [color goes here] apples." e.g. "A basket of 4 red apples." or "A basket of 50 blue apples."</p>
  
  <p>My code is:</p>
  
  <pre><code>class AppleBasket():
      def __init__(self, apple_color, apple_quantity):
          self.apple_color = apple_color
          self.apple_quantity = apple_quantity
  
      def getC(self):
          return self.apple_color
  
      def getQ(self):
          return self.apple_quantity
  
      def __str__(self):
          return "A basket of {} {} apples.".format(self.apple_quantity, self.apple_color)
  
      def increase(self):
          return self.apple_quantity + 1
  
  
  print(AppleBasket("Red", 4))
  </code></pre>
  
  <p>However,  Quantity still remains at 4 instead of 5.</p>
  
  <p>Can some please advice me what mistake am I doing?  Thank you.</p>