On 2006-08-11 07:48:33, Slawomir Nowaczyk wrote:
> But let me try again, please (just one more time, if this doesn't work
> either I am willing to admit I do not see a simple analogy between
> Python and C variables :-)
>
> Python C
> variable: a variable: a
> value of variable: eval("a") dereference operator: *a
> textual representation: "a" address operator: &a
> id of object: id(a) value of variable: a
>
> Thus, equivalent operations would be:
>
> Python C
> a = 1003 a = &three // (1)
> id(a) a
> b = 1004 b = &four
> a == b # False *a == *b // false
> id(a) == id(b) # False a == b // false
> b = a b = a
> a == b # True *a == *b // true
> id(a) == id(b) # True a == b // true
> a = 1001+1002 a = MallocNewIntFromValue( one+two )
> a == b # True *a == *b // true
Probably not True; b refers to the 1003 object, a refers to the 2003
object. But that's just a minor "bug" in your selection of values :) Your
intention was probably to make a = 1001+2 in the line above.
> id(a) == id(b) # False / True (2) a == b // false / true
> a = 1003+1004 a = MallocNewIntFromValue( three+four )
> a == b # False *a == *b // false
> id(a) == id(b) # False a == b // false
>
> (1) one, two, three and four are constants, something like "const int
> one = 1". That is because there is no "literal int object" in C
> - the thing you would write as "1" is likely not to actually
> exist at runtime. And you cannot take an address of it.
>
> (2) is actually True in CPython implementation for small integers, but
> that's a minor detail. MallocNewIntFromValue might cache objects
> as well.
I don't have a problem with that analogy. I guess we also agree that it has
its limits (as all analogies have). However, with this analogy, a and b
/must/ be pointer variables in C; the analogy does not work if a and b are
not pointers.
Which brings me back to my earlier statement: if it is at all possible to
create a useful analogy between Python variables and C variables, it is
between Python variables and C /pointers/. There is still no useful analogy
between Python variables and general C variables (i.e. non-pointer
variables). Note that non-pointer variables can contain objects, not only
simple types. But they still are not analog.
Gerhard
--
http://mail.python.org/mailman/listinfo/python-list
