IsString

[Tom Anderson]

On Wed, 14 Dec 2005, Steven D'Aprano wrote:

> On Tue, 13 Dec 2005 15:28:32 +0000, Tom Anderson wrote:
>
>> On Tue, 13 Dec 2005, Steven D'Aprano wrote:
>>
>>> On Mon, 12 Dec 2005 18:51:36 -0600, Larry Bates wrote:
>>>
>>> [snippidy-doo-dah]
>>>
>>>> I had the same thought, but reread the post.  He asks "if a given
>>>> variable is a character or a number".  I figured that even if he is
>>>> coming from another language he knows the difference between "a given
>>>> variable" and the "contents of a give variable".  I guess we will
>>>> see.... ;-).  This list is so good, he gets BOTH questions answered.
>>>
>>> The problem is, Python doesn't have variables (although it is
>>> oh-so-tempting to use the word, I sometimes do myself). It has names in
>>> namespaces, and objects.
>>
>> In what sense are the names-bound-to-references-to-objects not variables?
>
> Because saying "Python has variables" leads to nonsense like the following:
>
> [snip]
>>> That's why, for instance, Python is neither call by reference nor call
>>> by value, it is call by object.
>>
>> No, python is call by value, and it happens that all values are
>> pointers.
>
> All values in Python are pointers???

Right.

> So when I write:
>
> name = "spam spam spam spam"
>
> the value of the variable "name" is a pointer, and not a string. Riiight.

Right.

> Call by value and call by reference have established meanings in 
> computer science,

Right.

> and Python doesn't behave the same as either of them.

Wrong. Python behaves exactly like call by value, just like Smalltalk, 
Objective C, LISP, Java, and even C.

> Consider the following function:
>
> def modify(L):
>    "Modify a list and return it."
>    L.append(None); return L
>
> If I call that function:
>
> mylist = range(10**10) # it is a BIG list
> anotherlist = modify(mylist)
>
> if the language is call by value, mylist is DUPLICATED before being
> passed to the function.

Wrong. The value of mylist is a pointer to a list, and that's what's 
passed to the function. The same analysis applies to the rest of your 
example.

> The conceptual problem you are having is that you are conflating the 
> object model of Python the language with the mechanism of the underlying 
> C implementation, which does simply pass pointers around.

No, i'm not, i'm really not. Thinking in terms of variables, pointers and 
objects is a simple, consistent and useful abstract model of computation 
in python. If you like, we can use the word 'reference' instead of 
'pointer' - i guess a lot of people who came from C (which i didn't) are 
hung up on the idea that a pointer is a memory address, rather than just a 
conceptual thing which goes from a variable to an object; the trouble is 
that then we remind people of 'call by reference', and it all goes to pot.

I think the background thing is the kicker here. I'm guessing you come 
from C, where pointers are physical and explicit, you can have a variable 
which really does contain an object, etc, and so for you, applying those 
terms to python is awkward. I come from java, where all pointers are 
abstract (in the sense of being opaque) and implicit, and variables only 
ever contain pointers (unless they're primitive - but that's an 
implementation detail), so the terminology carries over to python quite 
naturally.

>> I'm sure this has been argued over many times here, and we still all 
>> have our different ideas, so please just ignore this post!
>
> I'd love to, but unfortunately I've already hit send on my reply.

Fair enough. Sorry about all this. In future, i'm going to send posts 
which i *know* will generate heat but no light straight to /dev/null ...

tom

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