Accessing xrange internal structure

Question

I'm trying to use ctypes to extract data from internal python structures. Namely, I'm trying to read the 4 fields in an xrange:

typedef struct {
    PyObject_HEAD
    long    start;
    long    step;
    long    len;
} rangeobject;

Is there any standard way of getting at such fields within python itself?

Answer 1

The ctypes module isn't meant for accessing Python internals. ctypes lets you deal with C libraries in C terms, but coding in Python.

You probably want a C extension, which in many ways is the opposite of ctypes. With a C extension, you deal with Python code in Python terms, but code in C.

UPDATED: Since you want pure Python, why do you need to access the internals of a built-in xrange object? xrange is very simple: create your own in Python, and do what you want with it.

Answer 2

You can access data you need without ctypes:

>>> obj = xrange(1,11,2)
>>> obj.__reduce__()[1]
(1, 11, 2)
>>> len(obj)
5

Note, that __reduce__() method is exactly for serialization. Read this chapter in documentation for more information.

Update: But sure you can access internal data with ctypes too:

from ctypes import *

PyObject_HEAD = [
    ('ob_refcnt', c_size_t),
    ('ob_type', c_void_p),
]

class XRangeType(Structure):
    _fields_ = PyObject_HEAD + [
        ('start', c_long),
        ('step', c_long),
        ('len', c_long),
    ]

range_obj = xrange(1, 11, 2)

c_range_obj = cast(c_void_p(id(range_obj)), POINTER(XRangeType)).contents
print c_range_obj.start, c_range_obj.step, c_range_obj.len