GH-96793: Change FOR_ITER to not pop the iterator on exhaustion.

Doc/library/dis.rst

@@ -413,6 +413,15 @@ The Python compiler currently generates the following bytecode instructions.
    Removes the top-of-stack (TOS) item.
 
 
+.. opcode:: END_FOR
+
+   Removes the top two values from the stack.
+   Equivalent to POP_TOP; POP_TOP.
+   Used to clean up at the end of loops, hence the name.
+
+   .. versionadded:: 3.12
+
+
 .. opcode:: COPY (i)
 
    Push the *i*-th item to the top of the stack. The item is not removed from its
@@ -1088,9 +1097,11 @@ iterations of the loop.
 
    TOS 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, TOS is popped, and the byte
+   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.
 
 .. opcode:: LOAD_GLOBAL (namei)
 

Lib/importlib/_bootstrap_external.py

@@ -423,6 +423,7 @@ def _write_atomic(path, data, mode=0o666):
 #     Python 3.12a1 3507 (Set lineno of module's RESUME to 0)
 #     Python 3.12a1 3508 (Add CLEANUP_THROW)
 #     Python 3.12a1 3509 (Conditional jumps only jump forward)
+#     Python 3.12a1 3510 (FOR_ITER leaves iterator on the stack)
 
 #     Python 3.13 will start with 3550
 
@@ -435,7 +436,7 @@ def _write_atomic(path, data, mode=0o666):
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
 
-MAGIC_NUMBER = (3509).to_bytes(2, 'little') + b'\r\n'
+MAGIC_NUMBER = (3510).to_bytes(2, 'little') + b'\r\n'
 
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 

Lib/opcode.py

@@ -78,6 +78,8 @@ def pseudo_op(name, op, real_ops):
 def_op('POP_TOP', 1)
 def_op('PUSH_NULL', 2)
 
+def_op('END_FOR', 4)
+
 def_op('NOP', 9)
 def_op('UNARY_POSITIVE', 10)
 def_op('UNARY_NEGATIVE', 11)