Somewhat inspired by the philosophical thickets in the depths of one of the more fundamental discussions on python-list aka comp.lang.python, I wrote a little function in C that grossly violates the Python object model's integrity and swaps two object structures in-place. What I wasn't expecting is that this can be used to illustrate some interesting facets of the CPython internals.
The original version looked like this:
static PyObject *
swap(PyObject *self, PyObject *args)
{
PyObject *obj1, *obj2;
Py_ssize_t len;
PyObject *temp;
if (!PyArg_ParseTuple(args, "OO", &obj1, &obj2)) {
return NULL;
}
len = obj1->ob_type->tp_basicsize;
if (obj2->ob_type->tp_basicsize != len) {
PyErr_SetString(PyExc_TypeError, "types have different sizes (incompatible)");
return NULL;
}
temp = PyMem_Malloc(len);
memcpy(temp, obj1, len);
memcpy(obj1, obj2, len);
memcpy(obj2, temp, len);
obj2->ob_refcnt = obj1->ob_refcnt;
obj1->ob_refcnt = temp->ob_refcnt;
Py_INCREF(Py_None);
return Py_None;
}
Simple: get the object size in memory, and swap using a temporary variable. This sort of works — but not quite.
Python 3.1.2 (release31-maint, Jul 8 2010, 09:18:08)
[GCC 4.4.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from hell import swap
>>> a = "this is the first string"
>>> b = "this is the second string!"
>>> swap(a,b)
>>> a
'this is the second string!'
>>> b
'this is the first string'
>>> t1 = (1,2,3)
>>> t2 = (a,)
>>> swap(t1, t2)
>>> t1
(1,)
>>> t2
zsh: segmentation fault python3
As you can see, it swapped the strings without any problems (I'll show you some problems further below), but it behaved strangely with the tuples: the new t1 does have only one element, like the old t2, but that one element is the first element of the old t1! Also, what the flip happens when you try to access t2?
Turns out tuple is a variable-size type. That means it can be created with any number of items, and have an according size in memory depending on how large it has to be. My original code only respected the “basic size” of the type, meaning that, in the case of tuples, it copied the information on how many items there are, but not the actual items. When trying to print t2, Python reads beyond the end of the tuple structure, probably dereferences an invalid pointer, and dies a painful death.
On a side note, Python's list type is, contrary to what you might expect, not a variable-size type — it cannot be, since, in the case of variable-size types, the length must be known when the object is created (and allocated), and can never change. (The reason is that realloc(3)-ing an object might move it, which would invalidate pointers, which is when all hell would break loose). Lists don't keep their items in the actual object structure, they simply keep a pointer.
Armed with the knowledge of variable-size types, we can fix hell.swap to work for tuples:
len1 = obj1->ob_type->tp_basicsize
+ ((PyVarObject*)obj1)->ob_size * obj1->ob_type->tp_itemsize;
len2 = obj2->ob_type->tp_basicsize
+ ((PyVarObject*)obj2)->ob_size * obj2->ob_type->tp_itemsize;
if (len1 != len2) {
PyErr_SetString(PyExc_TypeError, "objects have different sizes (incompatible)");
return NULL;
}
temp = PyMem_Malloc(len1);
memcpy(temp, obj1, len1);
memcpy(obj1, obj2, len1);
memcpy(obj2, temp, len1);
obj2->ob_refcnt = obj1->ob_refcnt;
obj1->ob_refcnt = temp->ob_refcnt;
PyMem_Free(temp);
Recompile, and we're ready for more apocalyptic idiocy. This time, after checking that tuples actually work as expected, we will be swapping strings in the wrong place to the great detriment of our sanity.
Python 3.1.2 (release31-maint, Jul 8 2010, 09:18:08)
[GCC 4.4.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from hell import swap
>>> t1, t2, t3 = (1,2,3), (None,), ("a", "b", "erm...")
>>> swap(t1, t2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: objects have different sizes (incompatible)
>>> swap(t1, t3)
>>> t1
('a', 'b', 'erm...')
>>> t3
(1, 2, 3)
>>> s = set(t1)
>>> swap(t1[0], t1[1])
>>> s
{'b', 'erm...', 'a'}
>>> 'b' in s
False
>>> 'a' in s
False
>>> 'erm...' in s
True
>>> 'a' in list(s)
True
>>> 'b' in list(s)
True
>>>
Okay, erm, what? It looks like it's there, but it's not, but then it is? There is, of course, a simple explanation for this:
sets (like dicts) are, for speed, implemented as a hash table. When you look up something in a set or dict, it first calculates a hash, and then searches for that. However, since it's possible for two objects to have the same hash, it also checks for equality. You will only get a result when there is an object around with both the same hash and is equal.
So, what happens here is: when you execute 'a' in s, the hash of 'a' is calculated, and all the items of the set that are referred to by that hash are checked whether they actually are 'a'. Since swapping, however, the hash of 'a' is associated with 'b' and vice versa — the set is corrupted because it correctly assumes that the hash of an object either never changed or does not, in fact, exist (lists, for example, aren't hashable at all, since they're mutable, and the hash would have to change when the object changes, which would defeat the whole point).
There you have it: that's what you get when you muck around in Python's memory.
I've uploaded the source code to JollyBOX code. Use it wisely.
>> from hell import swap
>>> swap(str, int)
zsh: segmentation fault python3
% ]]>
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