Homework: The __str__ Method

Read sections 17.6 and 17.7 in the book and try out the code samples.

There is another special method that you've been indirectly been using in many classes: __str__. The purpose of this method is to return a string version of the subject. It is automatically called anytime you use str to turn something into a string. Let's add it to our Recipe class:

class Recipe(object):
    """Models a recipe to make food."""
    
    def __init__(self, name):
        self.name = name
        self.ingredients = []
        self.steps = []
        
    def __str__(self):
        """Returns a string version of self."""
        string = "Recipe: " + str(self.name)  #start the string to build the output
        string += "\n\n" #add a line break and a blank line
        string += "Ingredients:\n"
        for ingredient in self.ingredients:
            string += "  * " + str(ingredient) + "\n"
        string += "\n"  #another blank line
        string += "Steps:\n"
        for i in range(len(self.steps)):
            step = self.steps[i]
            string += " " + str(i) + ": " + str(step) + "\n"
        return string
        

Now we can use this by calling the str function (different from the __str__ method) on a Recipe:

>>> cup = Recipe("PB Cup")
>>> cup.add_ingredient("Peanut Butter Cup")
>>> cup.add_step("Unwrap it and eat.")
>>> str(cup)
'Recipe: PB Cup

Ingredients:
  * Peanut Butter Cup

Steps:
  0: Unwrap it and eat.
'
>>>
        

Now that we have the __str__ method, we don't need our clunky print_recipe function. Why not? Because the str function is automatically applied to each argument in a print statement:

>>> cup = Recipe("PB Cup")
>>> cup.add_ingredient("Peanut Butter Cup")
>>> cup.add_step("Unwrap it and eat.")
>>> print(cup)
Recipe: PB Cup

Ingredients:
  * Peanut Butter Cup

Steps:
  0: Unwrap it and eat.

>>>
        

Whenever you create a new class in Python, you should always immediately write two methods:

• First, the constructor, so that you can easily create new instances of your class.
• Next, define the __str__ method so you can easily see what's going on with the state of the object.