dis: FOR_ITER says it no longer pops the stack in 3.12 but it still does when the iterator ended normally

frigus02

Documentation

The FOR_ITER docs in the dis module say "Up until 3.11 the iterator was popped when it was exhausted". This sounds like in 3.12+ the iterator is not popped anymore:

	.. opcode:: FOR_ITER (delta)

	``STACK[-1]`` is an :term:`iterator`. Call its :meth:`~iterator.__next__` method.
	If this yields a new value, push it on the stack (leaving the iterator below
	it). If the iterator indicates it is exhausted then the byte code counter is
	incremented by delta.

	.. versionchanged:: 3.12
	Up until 3.11 the iterator was popped when it was exhausted.

Instead there is a new opcode END_FOR, which takes care of popping the iterator off the stack. This surprised me because in 3.12 END_FOR is supposed to remove 2 elements from the top of the stack. But if the iterator ends normally, there will only be the iterator at the top. I tried to document my thought process in this godbolt repro.

Reading the generated code for FOR_ITER, specifically:

/* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */

	STACK_SHRINK(1);
	/* Jump forward oparg, then skip following END_FOR and POP_TOP instruction */
	JUMPBY(oparg + 2);

it looks like there are 2 cases:

If the iterator ends normally, FOR_ITER pops the iterator off the stack, then it skips the next END_FOR (and in 3.13 POP_TOP) instructions.
Otherwise (I'm not sure when that happens?), the iterator ends with both the iter and iter() on the stack, which are both popped by END_FOR (and in 3.13 POP_TOP).

Is it worth documenting the 2 different cases?