Accessing the list while being sorted

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_SET_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".

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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

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).