Python accessing the list while being sorted

Question

Can I access a list while it is being sorted in the list.sort()

b = ['b', 'e', 'f', 'd', 'c', 'g', 'a']
f = 'check this'

def m(i):
    print i, b, f
    return None

b.sort(key=m)
print b

this returns

b [] check this
e [] check this
f [] check this
d [] check this
c [] check this
g [] check this
a [] check this

Note that individual items of list b is sent to function m. But at m the list b is empty, however it can see the variable f, which has same scope as list b. Why does function m print b as []?

Answer 1

Looking at the source code (of CPython, maybe different behaviour for other implementations) the strange output of your script becomes obvious:

/* The list is temporarily made empty, so that mutations performed
* by comparison functions can't affect the slice of memory we're
* sorting (allowing mutations during sorting is a core-dump
* factory, since ob_item may change).
*/
saved_ob_size = Py_SIZE(self);
saved_ob_item = self->ob_item;
saved_allocated = self->allocated;
Py_SIZE(self) = 0;

The comment says it all: When you begin sorting, the list is emptied. Well, it is "empty" in the eye of an external observer.

I quite like the term "core-dump factory".

---

Compare also:

b = ['b','e','f','d','c','g','a']
f = 'check this'


def m(i):
    print i, b, f
    return None

b = sorted(b, key= m)
print b

Answer 2

This is something you can't rely on in general - not just for lists - unless the documentation for the method you're using explicitly says otherwise. Accessing an object in an intermediate state - ie, after some iteration has been started, but before it has been finished - is a problem that concurrent code runs into a lot. You've found a rare non-concurrent case of it, but the advice is the same: avoid this situation. The intermediate state is not guaranteed to be meaningful to you, and is not guaranteed to be a "valid" state according to the rules of that object (when it tends to be called an "inconsistent" state).