Homework: Constructors

Read section 17.5 in the book and try out the code samples.

Constructors are methods that are automatically called to help create objects. We've already used them:

world = TurtleWorld()
michelangelo = Turtle()
        

These two lines call the constructors in the TurtleWorld and Turtle classes, but they don't call them directly. Instead:

0. First, the new object is created, without setting any fields.
1. Then, if a method named __init__ is defined in the class, it is called, with the object itself as the first (self) parameter.

The __init__ method is one of many special methods, meaning it has some pre-defined use in Python. (Here it's the constructor.) Let's do an example by updating our Recipe class:

class Recipe(object):
    """Models a recipe to make food."""
    
    def __init__(self):
        self.name = ""
        self.ingredients = []
        self.steps = []
        

Add this constructor to your code from before, now it is automatically invoked when we create a Recipe object:

>>> fake_chx_parm = Recipe()
>>>
        

As a general rule, you should never invoke special methods directly! They will automatically get called by Python when you do something else.

Three fields are already set!

>>> fake_chx_parm = Recipe()
>>> fake_chx_parm.steps
[]
>>> fake_chx_parm.name
''
>>>
        

The name field isn't correct, so we need to set this separately, probably right after we create the recipe.

>>> fake_chx_parm = Recipe()
>>> fake_chx_parm.name = "Microwave Chicken Parm"
        

There's a much better way to initialize fields, though! Instead of setting them after, we can generalize the constructor by adding parameters. Let's do this to Recipe's constructor:

    def __init__(self, name):
        self.name = name
        self.ingredients = []
        self.steps = []
        

Now we can initialize all fields in one line:

>>> fake_chx_parm = Recipe("Microwave Chicken Parm")
>>> fake_chx_parm.add_ingredient("5 frozen chicken nuggets") ...