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PEP 353: Drop support for PyArg_ParseTuple() "#" formats when PY_SSIZE_T_CLEAN is not defined #85115

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vstinner opened this issue Jun 10, 2020 · 11 comments

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@vstinner
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@vstinner vstinner commented Jun 10, 2020

BPO 40943
Nosy @vstinner, @methane, @serhiy-storchaka, @miss-islington
PRs
  • #20781
  • #20784
  • #25937
  • #25961
  • Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
    closed_at = <Date 2021-05-07.03:29:07.518>
    created_at = <Date 2020-06-10.16:09:12.080>
    labels = ['expert-C-API', '3.10']
    title = 'PEP 353: Drop support for PyArg_ParseTuple() "#" formats when PY_SSIZE_T_CLEAN is not defined'
    updated_at = <Date 2021-05-10.21:29:50.046>
    user = 'https://github.com/vstinner'

    bugs.python.org fields:

    activity = <Date 2021-05-10.21:29:50.046>
    actor = 'vstinner'
    assignee = 'none'
    closed = True
    closed_date = <Date 2021-05-07.03:29:07.518>
    closer = 'methane'
    components = ['C API']
    creation = <Date 2020-06-10.16:09:12.080>
    creator = 'vstinner'
    dependencies = []
    files = []
    hgrepos = []
    issue_num = 40943
    keywords = ['patch']
    message_count = 11.0
    messages = ['371216', '371218', '371219', '371862', '380656', '393064', '393067', '393068', '393069', '393165', '393434']
    nosy_count = 4.0
    nosy_names = ['vstinner', 'methane', 'serhiy.storchaka', 'miss-islington']
    pr_nums = ['20781', '20784', '25937', '25961']
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = None
    url = 'https://bugs.python.org/issue40943'
    versions = ['Python 3.10']

    @vstinner
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    @vstinner vstinner commented Jun 10, 2020

    Follow-up of bpo-36381: In Python 3.8, PyArg_ParseTuple() and Py_BuildValue() formats using "int" when PY_SSIZE_T_CLEAN is not defined, but Py_ssize_t when PY_SSIZE_T_CLEAN is defined, were deprecated by:

    commit d3c72a2
    Author: Inada Naoki <songofacandy@gmail.com>
    Date: Sat Mar 23 21:04:40 2019 +0900

    bpo-36381: warn when no PY_SSIZE_T_CLEAN defined (GH-12473)
    
    We will remove int support from 3.10 or 4.0.
    

    I propose to drop support for these formats in Python 3.10. It is a backward incompatible change on purpose, to ensure that all C extensions are compatible with objects larger than 2 GB, and that all C extensions behave the same.

    I'm not sure of the effects of this issue on bpo-27499 "PY_SSIZE_T_CLEAN conflicts with Py_LIMITED_API".

    @vstinner vstinner changed the title Drop support for PyArg_ParseTuple() format when PY_SSIZE_T_CLEAN is not defined PEP 353: Drop support for PyArg_ParseTuple() format when PY_SSIZE_T_CLEAN is not defined Jun 10, 2020
    @vstinner vstinner changed the title Drop support for PyArg_ParseTuple() format when PY_SSIZE_T_CLEAN is not defined PEP 353: Drop support for PyArg_ParseTuple() format when PY_SSIZE_T_CLEAN is not defined Jun 10, 2020
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    @vstinner vstinner commented Jun 10, 2020

    New changeset d36cf5f by Victor Stinner in branch 'master':
    bpo-40943: Replace PY_FORMAT_SIZE_T with "z" (GH-20781)
    d36cf5f

    @vstinner
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    @vstinner vstinner commented Jun 10, 2020

    I started to write a long rationale to explain why "#" variants of formats must be deprecated, before I noticed that they are already deprecated! To not lost it, here is my rationale :-)

    The C code base of Python switched from int to Py_ssize_t with PEP-353. For not break the backward compatibility, C extension modules have to opt-in for Py_ssize_t by defining PY_SSIZE_T_CLEAN macro in their code base. It affects a few format PyArg_ParseTuple() and Py_BuildValue():
    https://docs.python.org/dev/c-api/arg.html

    Currently, the documentation says "This behavior will change in a future Python version to only support Py_ssize_t and drop int support. It is best to always define PY_SSIZE_T_CLEAN."

    Continue to use int by default prevents to accept content larger than 2 GB and causes issues with limited C API: bpo-27499.

    I propose to raise a DeprecationWarning on C extension modules built without the PY_SSIZE_T_CLEAN macro defined.

    #warning could be used to emit a warning when building a C extension without PY_SSIZE_T_CLEAN. The problem is that the compiler output is hidden by "pip install", only few developers pay attention to such warnings, and also we should not bother C extensions which don't use PyArg_ParseTuple() and Py_BuildValue().

    I prefer to raise a DeprecationWarning exception at runtime since this warning can be made an error when using -Werror: it eases detection of deprecated modules without preventing to build or use a C extension using the deprecated functions.

    Two releases after the DeprecationWarning is introduced, we can consider to always raise an exception. Again, I dislike the idea of always require PY_SSIZE_T_CLEAN macro when including "Python.h" header file. I suggest to only raise an exception at runtime when the few affected functions are called.

    For example, PyArg_ParseTuple(args, "y*", &buffer) always Py_buffer which uses Py_ssize_t: the behavior does not depend on PY_SSIZE_T_CLEAN.

    An alternative is to even deprecate the few "#" formats which are affected by PY_SSIZE_T_CLEAN.

    @vstinner vstinner changed the title PEP 353: Drop support for PyArg_ParseTuple() format when PY_SSIZE_T_CLEAN is not defined PEP 353: Drop support for PyArg_ParseTuple() "#" formats when PY_SSIZE_T_CLEAN is not defined Jun 10, 2020
    @vstinner vstinner changed the title PEP 353: Drop support for PyArg_ParseTuple() format when PY_SSIZE_T_CLEAN is not defined PEP 353: Drop support for PyArg_ParseTuple() "#" formats when PY_SSIZE_T_CLEAN is not defined Jun 10, 2020
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    @vstinner vstinner commented Jun 19, 2020

    New changeset 37bb289 by Victor Stinner in branch 'master':
    bpo-40943: PY_SSIZE_T_CLEAN required for '#' formats (GH-20784)
    37bb289

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    @vstinner vstinner commented Nov 10, 2020

    @methane
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    @methane methane commented May 6, 2021

    There is a still warning, not error.

    cpython/Python/getargs.c

    Lines 2535 to 2542 in 985ac01

    if (flags & FLAG_SIZE_T)
    (void) va_arg(*p_va, Py_ssize_t *);
    else {
    if (PyErr_WarnEx(PyExc_DeprecationWarning,
    "PY_SSIZE_T_CLEAN will be required for '#' formats", 1)) {
    return NULL;
    }
    (void) va_arg(*p_va, int *);

    @methane methane reopened this May 6, 2021
    @methane methane reopened this May 6, 2021
    @serhiy-storchaka
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    @serhiy-storchaka serhiy-storchaka commented May 6, 2021

    Why PR 20784 has been merged? Was not the plan to emit a deprecation warning first?

    @methane
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    @methane methane commented May 6, 2021

    Why PR 20784 has been merged? Was not the plan to emit a deprecation warning first?

    We had been emitted DeprecationWarning since Python 3.8. See #56682.
    #64983 changed the DeprecationWarning to SystemError.

    @serhiy-storchaka
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    @serhiy-storchaka serhiy-storchaka commented May 6, 2021

    Sorry, I was confused by Victor's long msg371219.

    @miss-islington
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    @miss-islington miss-islington commented May 7, 2021

    New changeset 569ca81 by Miss Islington (bot) in branch '3.10':
    bpo-40943: Fix skipitem() didn't raise SystemError (GH-25937)
    569ca81

    @methane methane closed this as completed May 7, 2021
    @methane methane closed this as completed May 7, 2021
    @vstinner
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    @vstinner vstinner commented May 10, 2021

    commit 4ebf4a6
    Author: Inada Naoki <songofacandy@gmail.com>
    Date: Fri May 7 11:56:48 2021 +0900

    bpo-40943: Fix skipitem() didn't raise SystemError (GH-25937)
    
    `convertitem()` raises `SystemError` when '#' is used without `PY_SSIZE_T_CLEAN`.
    This commit makes `skipitem()` raise it too.
    

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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