Coding Constructors

Now, the first thing we want to do with our Person class is record basic information about people—to fill out record fields, if you will. Of course, these are known as instance object attributes in Python-speak, and they generally are created by assignment to self attributes in class method functions. The normal way to give instance attributes their first values is to assign them to self in the __init__ constructor method, which contains code run automatically by Python each time an instance is created. Let’s add one to our class:

# Add record field initialization

class Person:
    def __init__(self, name, job, pay):      # Constructor takes 3 arguments
        self.name = name                     # Fill out fields when created
        self.job  = job                      # self is the new instance object
        self.pay  = pay

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This is a very common coding pattern: we pass in the data to be attached to an instance as arguments to the constructor method and assign them to self to retain them permanently. In OO terms, self is the newly created instance object, and name, job, and pay become state information—descriptive data saved on an object for later use. Although other techniques (such as enclosing scope references) can save details, too, instance attributes make this very explicit and easy to understand.

Notice that the argument names appear twice here. This code might seem a bit redundant at first, but it’s not. The job argument, for example, is a local variable in the scope of the __init__ function, but self.job is an attribute of the instance that’s the implied subject of the method call. They are two different variables, which happen to have the same name. By assigning the job local to the self.job attribute with self.job=job, we save the passed-in job on the instance for later use. As usual in Python, where a name is assigned (or what object it is assigned to) determines what it means.

Speaking of arguments, there’s really nothing magical about __init__, apart from the fact that it’s called automatically when an instance is made and has a special first argument. Despite its weird name, it’s a normal function and supports all the features of functions we’ve already covered. We can, for example, provide defaults for some of its arguments, so they need not be provided in cases where their values aren’t available or useful.

To demonstrate, let’s make the job argument optional—it will default to None, meaning the person being created is not (currently) employed. If job defaults to None, we’ll probably want to default pay to 0, too, for consistency (unless some of the people you know manage to get paid without having jobs!). In fact, we have to specify a default for pay because according to Python’s syntax rules, any arguments in a function’s header after the first default must all have defaults, too:

# Add defaults for constructor arguments

class Person:
    def __init__(self, name, job=None, pay=0):         # Normal function args
        self.name = name
        self.job  = job
        self.pay  = pay

What this code means is that we’ll need to pass in a name when making Persons, but job and pay are now optional; they’ll default to None and 0 if omitted. The self argument, as usual, is filled in by Python automatically to refer to the instance object—assigning values to attributes of self attaches them to the new instance.