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Explanation of list reference


On 2/14/2014 1:08 PM, dave em wrote:

> He is asking a question I am having trouble answering which is how a
> variable containing a value differs from a variable containing a list
> or more specifically a list reference.
>
> I tried the to explain as best I can remember is that a variable is
> assigned to a specific memory location with a value inside of it.

The data model of C and other memory-oriented languages is *different* 
from the object model of Python and similar languages. The concept of 
'memory address' is fundamental to C and absent from Python. To 
understand Python, one must think in terms of its referenced object 
model and not C's memory-address model.

Consider name-binding statements, also called assignment statements. 
The simplest form is
<name> = <expression>

As always, the expression evaluates to a Python object. It may be an 
existing object or it may by a newly created. Either way, the name is 
associated with the object. Subsequent to the name binding, the name, as 
an expression, evaluates to the object it points to.

An object can have 0 to many names associated with it. That set of names 
can change. A name can be associated with only one object at a time, but 
can be rebound to other objects.

All objects have class (type) and a value. The value of some objects can 
be changed (lists, sets, dicts, most user-defined classes), others are 
fixed (numbers, strings, tuples, frozensets).

a = 1  # a points to int object with value 1; a == 1
b = a  # b points to the same int object; b == 1 and b is a
a = '1'  # a now points to a str object with value '1'; a == '1'

c = [1,2,3]  # c points to a list object whose value is the
# given sequence of int objects; c == [1,2,3]
d = c  # d is the same list object as c
d[1] = '2'  # This associates the indicated 'slot' in the
collection object d (which is also c) with a '2' string object.

'Variable' in C (a fixed memory location with a mutable value) is quite 
different from 'variable' in Python. Most people in Python mean a fixed 
name, potentially associated with a sequence of objects. In CPython, 
each object would usually be at a different C location. 'Variable' in 
math has multiple meanings.

-- 
Terry Jan Reedy