8000 GH-96793: Change `FOR_ITER` to not pop the iterator on exhaustion. (G… · python/cpython@22863df · GitHub
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GH-96793: Change FOR_ITER to not pop the iterator on exhaustion. (GH-96801)
Change FOR_ITER to have the same stack effect regardless of whether it branches or not. Performance is unchanged as FOR_ITER (and specialized forms jump over the cleanup code).
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14 files changed

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‎Doc/library/dis.rst

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@@ -413,6 +413,15 @@ The Python compiler currently generates the following bytecode instructions.
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Removes the top-of-stack (TOS) item.
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.. opcode:: END_FOR
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Removes the top two values from the stack.
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Equivalent to POP_TOP; POP_TOP.
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Used to clean up at the end of loops, hence the name.
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.. versionadded:: 3.12
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.. opcode:: COPY (i)
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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.
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TOS is an :term:`iterator`. Call its :meth:`~iterator.__next__` method. If
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this yields a new value, push it on the stack (leaving the iterator below
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it). If the iterator indicates it is exhausted, TOS is popped, and the byte
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it). If the iterator indicates it is exhausted then the byte
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code counter is incremented by *delta*.
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.. versionchanged:: 3.12
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Up until 3.11 the iterator was popped when it was exhausted.
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.. opcode:: LOAD_GLOBAL (namei)
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‎Include/internal/pycore_opcode.h

Lines changed: 20 additions & 20 deletions
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‎Include/opcode.h

Lines changed: 73 additions & 72 deletions
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‎Lib/importlib/_bootstrap_external.py

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

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

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