Core Python Programming (2nd Edition)

13.6. Instance Attributes
Instances have only data attributes (methods are strictly class attributes) and are simply data values
that you want to be associated with a particular instance of any class and are accessible via the familiar
dotted-attribute notation. These values are independent of any other instance or of the class it was
instantiated from. When an instance is deallocated, so are its attributes.
13.6.1. "Instantiating" Instance Attributes (or Creating a Better Constructor)
Instance attributes can be set any time after an instance has been created, in any piece of code that has
access to the instance. However, one of the key places where such attributes are set is in the
constructor, __init__().
Core Note: Instance attributes
Being able to create an instance attribute "on-the-fly" is one of the
great features of Python classes, initially (but gently) shocking those
coming from C++ or Java in which all attributes must be explicitly
defined/ declared first.
Python is not only dynamically typed but also allows for such dynamic
creation of object attributes during run-time. It is a feature that once
used may be difficult to live without. Of course, we should mention to
the reader that one much be cautious when creating such attributes.
One pitfall is when such attributes are created in conditional clauses: if
you attempt to access such an attribute later on in your code, that
attribute may not exist if the flow had not entered that conditional
suite. The moral of the story is that Python gives you a new feature
you were not used to before, but if you use it, you need to be more
careful, too.
Constructor First Place to Set Instance Attributes
The constructor is the earliest place that instance attributes can be set because __init__() is the first
method called after instance objects have been created. There is no earlier opportunity to set instance
attributes. Once __init__() has finished execution, the instance object is returned, completing the
instantiation process.
Default Arguments Provide Default Instance Setup
One can also use __init__() along with default arguments to provide an effective way of preparing an
instance for use in the real world. In many situations, the default values represent the most common
cases for setting up instance attributes, and such use of default values precludes them from having to
be given explicitly to the constructor. We also outlined some of the general benefits of default