[Tutor] Object/Class Beginner Questions

[Steven D'Aprano]

Ben Ganzfried wrote:
> Hey guys,
> 
> I'm using a tutorial geared for a 2.x version of Python and I am currently
> using Python 3.1-- so it is possible that my confusion has to do with
> different notations between them.  But in any case, here is what I have:

> My questions are the following:
> 1) Why is the type for my class Time : >>> type(Time)
>                                                               <class 'type'>
> when the type for their class Point is: <type 'classobj'>

In Python 2.x, objects are divided into two broad families of objects:

1. "classic classes" and their instances;
2. "new-style" types and their instances (including built-ins like str, 
int, float, and similar).

There are some small, but significant differences between them, but in 
general they do mostly the same kind of things.

In Python 2.x, a class definition like:

class Point:
     ...

defines a classic class (or "old style class"), while inheriting from 
"object" or a built-in type defines a new-style class:

class Point(object):  # Tells Python to construct a new-style class
     ...

The type of an instance is the instance's class; the type of a class 
object is "classobj" for old-style classes and (usually) "type" for 
new-style classes and types. Calling type() on old-style classes returns 
<classobj>, and on new-style, <type>.

But in Python 3, old-style classic classes are gone. Point will be a 
"type" object, which is what you are seeing.


> Also, what is the difference between "class" and "classobj" in this case?

Every object has a type, even classes. `class` is the keyword that you 
use for creating new classes, and type() is what you use to find out 
what they are. So:

type(42) -> int  # 42 is an int
type(int) -> type  # int is a new-style class, or "type"

In Python 2.x, the type of a classic class is called "classobj", and the 
type of a new-style class is called "type".


> 2) Why does my t1 object give the following as its type: <class
> '__main__.Time'>
> And in their p object example the type is: <type 'instance'>?

Again, this boils down to old-style vs. new-style.


> 3) What is happening such that when I try to call my print_time(t1) function
> I get the following error:
>>>> t1 = Time()
>>>> t1.hours = 3
>>>> t1.minutes = 30
>>>> t1.seconds = 45
>>>> print_time(t1)
> Traceback (most recent call last):
>   File "<pyshell#77>", line 1, in <module>
>     print_time(t1)
> NameError: name 'print_time' is not defined

You don't have a print_time function, as Python correctly reports. You 
have a print_time *method*, which means it lives inside Time objects. 
You can't access it directly as a global function:

print_time(something)

does not work, because print_time doesn't live in the global namespace. 
You need to call it as a method of a Time instance:

t1.print_time()  # no additional arguments needed



-- 
Steven