Objects in Python

[Roy Smith]

In article <mailman.3693.1345697563.4697.python-list at python.org>,
 Evan Driscoll <driscoll at cs.wisc.edu> wrote:

> > In fact, Python doesn't have variables ­ not as C or Java programmers
> > would understand the term. What it has instead are references to objects
> > (with names as one kind of reference).
> 
> OK, I've seen this said a few times, and I have to ask: what do you mean
> by this? I consider myself pretty decent at Python and other languages,
> and I really don't get it.

I'll take a shot at this.

When I execute:

a = 4

I'm doing two things.  The first is to create an object of type int with 
a value of 4.  I think everybody is OK with that part.  The confusing 
part comes with the LHS.

In C, or Java, there's a container called "a" which holds a value.  In 
C, that value is the integer 4, in Java it's an Integer object (well, at 
least I think it is, I've never fully groked how Java handles integers).

In Python, there is no container named "a".  There is, however, a dict 
which exists somewhere in python-space.  You can get a reference to this 
dict by calling globals().  What the assignment does is effectively:

globals()["a"] = 4

In fact, I can even write it that way and everything works:

>>> globals()["a"] = 42
>>> a
42

Even id() thinks they're the same thing:

>>> id(a)
1755402140
>>> id(globals()["a"])
1755402140

But, notice what happens if I now assign something new to a:

>>> a = 123
>>> id(a)
1755403176

The id has changed!  Now, we all know that the id of an object is its 
memory address (that's not guaranteed, but in the standard C 
implementation of Python, that's what it is).

Now, what if I do something similar in C:

#include <stdio.h>

main() {
    int a = 40;
    printf("a = %d, &a = %p\n", a, &a);
    a = 99;
    printf("a = %d, &a = %p\n", a, &a);
}

When I compile and run this, it prints:

a = 40, &a = 0x7fff1911f5bc
a = 99, &a = 0x7fff1911f5bc

Notice that the address of the variable "a" didn't change when I 
assigned it a new value.  That's what people mean when they say C has 
variables and Python doesn't; it just binds names to values.