Why do we need dis.Positions? #90580

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1. sobolevn commented on Jan 18, 2022

   

   sobolevn

   on Jan 18, 2022

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   While working on #30058 I've noticed that Positions namedtuple is kinda strange, source:
   https://github.com/python/cpython/blame/8c2fd09f365e082cfceb29afdf38953cdd670946/Lib/dis.py#L204-L213

   Why?

   1. It is not used. grep shows that:

   » ag Positions .  
   Misc/HISTORY
   21894:  treated as being relative to the end of the input string. Positions
   
   Objects/codeobject.c
   1021:static PyTypeObject PositionsIterator = {
   1067:    positionsiterator* pi = (positionsiterator*)PyType_GenericAlloc(&PositionsIterator, 0);
   
   Mac/PythonLauncher/English.lproj/MainMenu.nib/info.nib
   7:      <key>IBEditorPositions</key>
   
   Lib/dis.py
   204:Positions = collections.namedtuple(
   205:    'Positions',
   239:_Instruction.positions.__doc__ = "dis.Positions object holding the span of source code covered by this instruction"
   259:         positions - Optional dis.Positions object holding the span of source code
   
   Lib/test/test_compile.py
   1013:class TestSourcePositions(unittest.TestCase):
   
   Doc/library/re.rst
   1579:Finding all Adverbs and their Positions
   

   2. Commenting it out does not make test_dis to fail (again, because it does not seem to be used)
   3. It is documented (in docstrings only, not in dis.rst) that Instruction.positions is dis.Positions, but this is not true. Because Instruction is created as:

   co_positions = co_positions or iter(())
   
   try:
     positions = next(co_positions)
   except StopIteration:
     positions = None
   
   Instruction(positions=positions)

   So, it is at best is tuple | None. Never Positions.

   What to do with it?

   1. Should it be removed as unused, undocumented, and unreleased?
   2. Should it be used as documented? Here: https://github.com/python/cpython/blame/8c2fd09f365e082cfceb29afdf38953cdd670946/Lib/dis.py#L453-L455
   3. Any other ideas?

   In my opinion, it is not required, because we already have codeobject.co_positions which is pretty much the same thing: https://docs.python.org/3.11/reference/datamodel.html?highlight=co_positions#codeobject.co_positions

   I would like to work on it after a final decision is made :)
2. sobolevn

   added

   3.11only security fixesonly security fixes

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jan 18, 2022
3. isidentical commented on Jan 19, 2022

   

   isidentical

   on Jan 19, 2022

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   The initial aim of the dis.Positions was to provide an interface like AST nodes. So you could do

   for instr in dis.Bytecode(source):
       print("located in: ", instr.positions.lineno)

   instead of

   for instr in dis.Bytecode(source):
       if instr.positions:
           lineno = instr.positions[0]
       else:
           lineno = None
       print("located in: ", lineno)

   I think this is a bug that we are not currently using it, I'd say we should use it properly and go with option 2.
4. sobolevn commented on Jan 19, 2022

   

   sobolevn

   on Jan 19, 2022

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   Ok then, I will send my option 2 proposal to fix this later today.
5. isidentical commented on Jan 24, 2022

   

   isidentical

   on Jan 24, 2022

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   New changeset 58f3d98 by Nikita Sobolev in branch 'main':
   bpo-46422: use dis.Positions in dis.Instruction (GH-30716)
   58f3d98
6. sobolevn

   closed this as completedon Jan 24, 2022
7. sobolevn

   closed this as completedon Jan 24, 2022
8. ezio-melotti

   transferred this issue fromon Apr 10, 2022