Question

What is the highest number Python 2.6’s x86 id() function can return?

I presume the ceiling must be the amount of memory any 32-bit application can see which must be: 2³², i.e. 4294967296?

(Which would be fine as I have to fit this value in a C# type for which UInt32 or the CLS-compliant Int64 would suffice, which it the reason for my concern, though irrelevant to the question.)

But what if a I am running on Windows x64 with more than 2GB of memory say 32GB of memory – is it possible for Python’s id() function, even though the Python interpreter itself is x86, to return a value higher then 2³²???

I am guessing it comes down to the Python interpreter’s view of the machine – I imagine WoW64 translates 64-bit memory addresses to 32-bit addresses – but I am only guessing – I need to be sure!

Answer 1

Assuming the documentation for id() is correct and CPython (x86) returns the address of the object, the maximum value that can be returned is 2³²-1 (4294967295). This can be demonstrated in a simple C program:

#include <stdio.h>

int main(void) {
    void* p = 0;          // specify a pointer at 0
    p = ~p;               // invert all bits
    uintptr_t x = p;      // convert to an integer type fo the same size
    printf("%lu\n", x);
    return 0;
}

This is true regardless of the underlying OS, an application compiled with a 32-bit pointer type can only specify addresses between 0 and 2³²-1. Note that the address visible in the application may not correspond to a particular physical memory address, due to the use of virtual memory.