[Edu-sig] How does Python do Pointers?

[kirby urner]

Useful discussion for me too, although my teenager students usually
aren't that familiar with C, so mapping that shoptalk to Python is
less of the issue.

On the other hand, understanding that the same objects might have many
names remains critical, also the idea of immutability, with reference
to strings, tuples and such (a good counter to having so many names,
each potentially a source of changes, as the name also "anchors" the
"api" to its object i.e. any name might change objects' values as seen
by others, via the various syntax triggers such as dot notation).

I like that we call them "names" in Python and not variables so much,
per the cited http://effbot.org/zone/python-objects.htm

People new to programming know about objects, and that objects have
names (maybe multiple, aliases and such), whereas the term "variable"
is maybe not so accessible, sounds more like old algebra, plus its not
the names that vary, but what they stick to.  To say "the identity of
x has changed" is to undo the work we just did, to distinguish
x-the-name from what it points to (its object, not its identity).

We also like calling them names because its namespaces they live in
(have currency and defined meaning within) -- which is why a given
name might reference different objects because defined in different
namespaces (still a possibility for name collisions if not careful
with importing -- why from "module import * " is often not a good
idea).  A key verb:  "to disambiguate" (a purpose of namespaces).

And having no names at all, in the sense of no references, no sticky
notes, pointers from anywhere:  that's what makes an object a
candidate for garbage collection (reference count zero **), something
else C doesn't provide out of the box either (but then CPython is C
after all, made to do trix, behave like a snake (very agile)).

Kirby


On Mon, May 5, 2008 at 3:06 PM, David MacQuigg <macquigg at ece.arizona.edu> wrote:
> Many thanks to Michael, Anna and John for the very thorough answers to this question.  I especially like John's sticky-note analogy.  I have been using a similar analogy with "labels", but that gets confused with the other more common uses of the word "label".  Sticky Note is unique and more memorable.
>
>  I'm disturbed that there doesn't seem to be any agreement on simple definitions for "call by reference" and "call by value".  A Google search for [python "call by value"] shows there is widespread confusion in the Python world.  I'm not a computer scientist, but somewhere I did learn these concepts as being simply copying the value of a variable vs passing just a reference to the original.  I see that my simple understanding agrees with Kernighan and Ritchie and with http://en.wikipedia.org/wiki/Call_by_value. (Note that "call-by-reference", as defined in this article, clearly includes Python's calling method.)
>
>  Luckily, we can answer the question without wading into this swamp.  Here is my latest revision:
>
>  '''
>  Python doesn't need pointers because of the simple relationship between variables and the objects they refer to.  A variable has a name and a pointer to an object.  An object has a type, a value, and an address in memory.  Unlike C, the type is with the object, not the variable.  A variable is "bound" to an object much like a sticky note on a box.  Notes can be moved from one box to another, and a box can have more than one note, or no notes at all.
>
>  Python's pointers are "under the hood", not something the programmer sees or ever needs to worry about.  This part of what makes Python a higher level language than C.  You gain a lot in simplicity and readability, while losing the ability to work directly with memory addresses.  You also lose a little memory, because objects are more complex than raw data values.
>
>  The sticky-note analogy has a flaw.  You can't stick one note on top of another.  When you say x = y = z, all three variables now point to the object originally pointed to by z.  Then when you say y = 8, y now points to an integer object 8, but x doesn't move with y.  Python's "sticky notes" have a special glue that sticks only to an object, not to another variable.
>
>  See http://effbot.org/zone/python-objects.htm for more on Python objects and variables.
>
>  Note:  You can get the address of an object x with the function id(x), but this is used only to provide a unique identity for the object, not to access it.  If id(x) == id(y), you know that variables x and y both point to the same object.
>  '''
>
>  -- Dave
>
>
>
>
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