Question

(In regards to Python 3.2)

I’m trying to make a statement along the lines of:

In Python, an object is…

According to the doc (http://docs.python.org/py3k/reference/datamodel.html#objects-values-and-types):

Every object has an identity, a type and a value

But where do attributes fall into that? If I do something like a = 3; print(a.__class__) I get <class 'int'> I assume that is the type of the object a references, meaning that "type" is an "attribute" of an object. So in that sense we can say a sufficient set of "things" an object has would be its identity, value and attributes. However, looking through the attributes of a using dir(a), I do not see anything resembling identity (even though I know the id() function will tell me that information).

So my question is are any of the following minimal statements to sufficiently describe the notion of a Python object?

1. In Python an object has attributes, of which always include an identity, type and value.

2. In Python an object has an identity and attributes, of which always include its type and value.

3. In Python an object has an identity, value and attributes, of which always include its type, among other things.

If not, could someone give me a definition that conveys the relationships attributes, identity, type and value for an object?

(I would prefer number 1 to be true. :P)

Answer 1

While you can access the type of an object through an attribute, its type isn’t just an attribute — the type defines how the object was created before it had any attributes at all. By that fact alone none of those statements is sufficient to describe a Python object.

I’d say it this way:

In Python, everything is an object.

An object is a block of information, which has a type, which defines its creation and how it interacts with other objects, an identity, which differentiates it from all other objects, and a value, which is the information in the block. Attributes are other objects associated with a given object, including the object that is its type.

You should then give some examples of things people might not expect to be objects, like functions.

A paragraph on "What is an object" can be found in Dive Into Python:

Everything in Python is an object, and almost everything has attributes and methods. All functions have a built-in attribute __doc__, which returns the doc string defined in the function’s source code. The sys module is an object which has (among other things) an attribute called path. And so forth.

Still, this begs the question. What is an object? Different programming languages define “object” in different ways. In some, it means that all objects must have attributes and methods; in others, it means that all objects are subclassable. In Python, the definition is looser; some objects have neither attributes nor methods (more on this in Chapter 3), and not all objects are subclassable (more on this in Chapter 5). But everything is an object in the sense that it can be assigned to a variable or passed as an argument to a function (more in this in Chapter 4).