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GH-100425: Improve accuracy of builtin sum() for float inputs #100426

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merged 13 commits into from Dec 23, 2022
Merged

GH-100425: Improve accuracy of builtin sum() for float inputs #100426

merged 13 commits into from Dec 23, 2022

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@rhettinger rhettinger commented Dec 22, 2022 •
edited by bedevere-bot

@rhettinger rhettinger added the 3.12 bugs and security fixes label Dec 22, 2022
@bedevere-bot bedevere-bot mentioned this pull request Dec 22, 2022
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Python/bltinmodule.c Outdated
Py_SETREF(result, NULL);
while(result == NULL) {
item = PyIter_Next(iter);
if (item == NULL) {
Py_DECREF(iter);
if (PyErr_Occurred())
return NULL;
if (c) {
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Why check for c being zero? Seems that unconditionally adding 0.0 (if it is 0) to f_result would be cheap and harmless.

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Why check for c being zero?

It does avoid losing the sign on a negative zero result:

>>> sum([-0.0, -0.0], start=-0.0)
-0.0

Admittedly that's a fairly niche corner case; I could live with the behaviour changing.

Thinking about corner cases, the compensation is also going to interfere with infinities and overflow. On this branch:

>>> sum([float("inf"), float("inf")])
nan
>>> sum([1e308, 1e308])
nan

But on main we get infinities as expected:

>>> sum([float("inf"), float("inf")])
inf
>>> sum([1e308, 1e308])
inf

We should try to keep the existing behaviour here. I think it should just be a matter of not adding the correction if it's an infinity or nan.

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In commit 88bdb92 in 2019, Serhiy added these tests:

    self.assertEqual(repr(sum([-0.0])), '0.0')
    self.assertEqual(repr(sum([-0.0], -0.0)), '-0.0')
    self.assertEqual(repr(sum([], -0.0)), '-0.0')

It looks like he thinks there should be an invariant that sign of the start value should be preserved when iterable sum is equal to zero or empty. This seems dubious to me, but I rolled with it and added the "if (c)" test to preserve the invariant.

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I don't think it's dubious: it's consistent with IEEE 754 rules that (under the default ties-to-even rounding mode) a sum of negative zeros is negative zero. It only affects the case where both the sum and the start value are negative zero; if the sum is positive zero it has no effect:

>>> sum([0.0], start=-0.0)  # same as (-0.0) + 0.0
0.0
>>> sum([-0.0], start=-0.0)  # should be the same as (-0.0) + (-0.0)
-0.0

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there should be an invariant that sign of the start value should be preserve

That's fine - but surely something so excruciatingly obscure deserves a comment 😜 .

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We should try to keep the existing behaviour here. I think it should just be a matter of not adding the correction if it's an infinity or nan.

Done.

That's fine - but surely something so excruciatingly obscure deserves a comment

Done.

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@rhettinger rhettinger merged commit 5d84966 into python:main Dec 23, 2022
iritkatriel added a commit to iritkatriel/cpython that referenced this pull request Dec 28, 2022
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Merged main into regmachine branch (#50)
fdc73cb 
* Correct CVE-2020-10735 documentation (python#100306)

* pythongh-94912: Added marker for non-standard coroutine function detection (python#99247)

This introduces a new decorator `@inspect.markcoroutinefunction`,
which, applied to a sync function, makes it appear async to
`inspect.iscoroutinefunction()`.

* Docs: Don't upload CI artifacts (python#100330)

* pythongh-89727: Fix os.walk RecursionError on deep trees (python#99803)

Use a stack to implement os.walk iteratively instead of recursively to
avoid hitting recursion limits on deeply nested trees.

* pythongh-69929: re docs: Add more specific definition of \w (python#92015)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

* pythongh-89051: Add ssl.OP_LEGACY_SERVER_CONNECT (python#93927)

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Fixes python#89051

* pythongh-88211: Change lower-case and upper-case to match recommendations in imaplib docs (python#99625)

* pythongh-100348: Fix ref cycle in `asyncio._SelectorSocketTransport` with `_read_ready_cb` (python#100349)

* pythongh-99925: Fix inconsistency in `json.dumps()` error messages (pythonGH-99926)

* Clarify that every thread has its own default context in contextvars (python#99246)

* pythongh-99576: Fix cookiejar file that was not truncated for some classes (pythonGH-99616)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

* pythongh-100188: Reduce misses in BINARY_SUBSCR_(LIST/TUPLE)_INT (python#100189)

Don't specialize if the index is negative.

* pythongh-99991: improve docs on str.encode and bytes.decode (python#100198)

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>

* pythongh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry (python#91499)

Co-authored-by: Pieter Eendebak <P.T.eendebak@tudelft.nl>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

* pythongh-85267: Improvements to inspect.signature __text_signature__ handling (python#98796)

This makes a couple related changes to inspect.signature's behaviour
when parsing a signature from `__text_signature__`.

First, `inspect.signature` is documented as only raising ValueError or
TypeError. However, in some cases, we could raise RuntimeError.  This PR
changes that, thereby fixing python#83685.

(Note that the new ValueErrors in RewriteSymbolics are caught and then
reraised with a message)

Second, `inspect.signature` could randomly drop parameters that it
didn't understand (corresponding to `return None` in the `p` function).
This is the core issue in python#85267. I think this is very surprising
behaviour and it seems better to fail outright.

Third, adding this new failure broke a couple tests. To fix them (and to
e.g. allow `inspect.signature(select.epoll.register)` as in python#85267), I
add constant folding of a couple binary operations to RewriteSymbolics.

(There's some discussion of making signature expression evaluation
arbitrary powerful in python#68155. I think that's out of scope. The
additional constant folding here is pretty straightforward, useful, and
not much of a slippery slope)

Fourth, while python#85267 is incorrect about the cause of the issue, it turns
out if you had consecutive newlines in __text_signature__, you'd get
`tokenize.TokenError`.

Finally, the `if name is invalid:` code path was dead, since
`parse_name` never returned `invalid`.

* pythonGH-100363: Speed up `asyncio.get_running_loop` (python#100364)

* pythonGH-100133: fix `asyncio` subprocess losing `stderr` and `stdout` output (python#100154)

* pythongh-100374: Fixed a bug in socket.getfqdn() (pythongh-100375)

* pythongh-100129: Add tests for pickling all builtin types and functions (pythonGH-100142)

* Remove unused variable from `dis._find_imports` (python#100396)

* pythongh-78878: Fix crash when creating an instance of `_ctypes.CField` (python#14837)

* pythonGH-69564: Clarify use of octal format of mode argument in help(os.chmod) (python#20621)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

* pythonGH-99554: Pack location tables more effectively (pythonGH-99556)

* Correct typo in typing.py (python#100423)

In the docstring of `ParamSpec`, the name of `P = ParamSpec('P')` was
mistakenly written as `'T'`.

* pythongh-99761: Add `_PyLong_IsPositiveSingleDigit` function to check for single digit integers  (python#100064)

* pythonGH-99770: Make the correct call specialization fail kind show up in the stats (pythonGH-99771)

* pythongh-78997: fix bad rebase of moved test file (python#100424)

* pythongh-100344: Add C implementation for `asyncio.current_task` (python#100345)

Co-authored-by: pranavtbhat

* pythonGH-99554: Trim trailing whitespace (pythonGH-100435)



Automerge-Triggered-By: GH:brandtbucher

* pythongh-85432: Harmonise parameter names between C and pure-Python implementations of `datetime.time.strftime`, `datetime.datetime.fromtimestamp` (python#99993)

* pythongh-57762: fix misleading tkinter.Tk docstring (python#98837)

Mentioned as a desired change by terryjreedy on the corresponding issue,
since Tk is not a subclass of Toplevel.

* pythongh-48496: Added example and link to faq for UnboundLocalError in reference (python#93068)

* Fix typo in 3.12 What's New (python#100449)

* pythongh-76963: PEP3118 itemsize of an empty ctypes array should not be 0 (pythonGH-5576)

The itemsize returned in a memoryview of a ctypes array is now computed from the item type, instead of dividing the total size by the length and assuming that the length is not zero.

* pythonGH-100459: fix copy-paste errors in specialization stats (pythonGH-100460)

* pythongh-99110: Initialize `frame->previous` in init_frame to fix segmentation fault when accessing `frame.f_back` (python#100182)

* pythongh-98712: Clarify "readonly bytes-like object" semantics in C arg-parsing docs (python#98710)

* pythongh-92216: improve performance of `hasattr` for type objects (pythonGH-99979)

* pythongh-100288: Specialise LOAD_ATTR_METHOD for managed dictionaries (pythonGH-100289)

* Revert "pythongh-100288: Specialise LOAD_ATTR_METHOD for managed dictionaries (pythonGH-100289)" (python#100468)

This reverts commit c3c7848.

* pythongh-94155: Reduce hash collisions for code objects (python#100183)

* Uses a better hashing algorithm to get better dispersion and remove commutativity.

* Incorporates `co_firstlineno`, `Py_SIZE(co)`, and bytecode instructions.

* This is now the entire set of criteria used in `code_richcompare`, except for `_PyCode_ConstantKey` (which would incorporate the types of `co_consts` rather than just their values).

* pythongh-83076: 3.8x speed improvement in (Async)Mock instantiation (python#100252)

* pythongh-99482: remove `jython` compatibility parts from stdlib and tests (python#99484)

* bpo-40447: accept all path-like objects in compileall.compile_file (python#19883)

Signed-off-by: Filipe Laíns <lains@archlinux.org>
Signed-off-by: Filipe Laíns <lains@riseup.net>
Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

* pythonGH-100425: Improve accuracy of builtin sum() for float inputs (pythonGH-100426)

* pythongh-68320, pythongh-88302 - Allow for private `pathlib.Path` subclassing (pythonGH-31691)

Users may wish to define subclasses of `pathlib.Path` to add or modify
existing methods. Before this change, attempting to instantiate a subclass
raised an exception like:

    AttributeError: type object 'PPath' has no attribute '_flavour'

Previously the `_flavour` attribute was assigned as follows:

    PurePath._flavour        = xxx not set!! xxx
    PurePosixPath._flavour   = _PosixFlavour()
    PureWindowsPath._flavour = _WindowsFlavour()

This change replaces it with a `_pathmod` attribute, set as follows:

    PurePath._pathmod        = os.path
    PurePosixPath._pathmod   = posixpath
    PureWindowsPath._pathmod = ntpath

Functionality from `_PosixFlavour` and `_WindowsFlavour` is moved into
`PurePath` as underscored-prefixed classmethods. Flavours are removed.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Brett Cannon <brett@python.org>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Eryk Sun <eryksun@gmail.com>

* pythongh-99947: Ensure unreported errors are chained for SystemError during import (pythonGH-99946)

* Add "strict" to dotproduct(). Add docstring. Factor-out common code. (pythonGH-100480)

* pythongh-94808: improve test coverage of number formatting (python#99472)

* pythongh-100454: Start running SSL tests with OpenSSL 3.1.0-beta1 (python#100456)

* pythongh-100268: Add is_integer method to int (python#100439)

This improves the lives of type annotation users of `float` - which type checkers implicitly treat as `int|float` because that is what most code actually wants. Before this change a `.is_integer()` method could not be assumed to exist on things annotated as `: float` due to the method not existing on both types.

* pythongh-77771: Add enterabs example in sched (python#92716)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

* pythonGH-91166: Implement zero copy writes for `SelectorSocketTransport` in asyncio (python#31871)

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>

* pythonGH-91166: Implement zero copy writes for `SelectorSocketTransport` in asyncio (python#31871)

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>

* Misc Itertools recipe tweaks (pythonGH-100493)

* pythongh-100357: Convert several functions in `bltinsmodule` to AC (python#100358)

* Remove wrong comment about `repr` in `test_unicode` (python#100495)

* pythongh-99908: Tutorial: Modernize the 'data-record class' example (python#100499)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

* pythongh-100474: Fix handling of dirs named index.html in http.server (pythonGH-100475)



If you had a directory called index.html or index.htm within a directory, it would cause http.server to return a 404 Not Found error instead of the directory listing. This came about due to not checking that the index was a regular file.

I have also added a test case for this situation.

Automerge-Triggered-By: GH:merwok

* pythongh-100287: Fix unittest.mock.seal with AsyncMock (python#100496)

* pythongh-99535: Add test for inheritance of annotations and update documentation (python#99990)

* pythongh-100428: Make float documentation more accurate (python#100437)

Previously, the grammar did not accept `float("10")`.
Also implement mdickinson's suggestion of removing the indirection.

* [Minor PR] Quotes in documentation changed into code blocks (python#99536)

Minor formatting fix in documentation

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

* pythongh-100472: Fix docs claim that compileall parameters could be bytes (python#100473)

* pythongh-100519: simplification to `eff_request_host` in cookiejar.py (python#99588)

`IPV4_RE` includes a `.`, and the `.find(".") == -1` included here is already testing to make sure there's no dot, so this part of the expression is tautological. Instead use more modern `in` syntax to make it clear what the check is doing here. The simplified implementation more clearly matches the wording in RFC 2965.

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

* pythongh-99308: Clarify re docs for byte pattern group names (python#99311)

* pythongh-92446: Improve argparse choices docs; revert bad change to lzma docs (python#94627)

Based on the definition of the collections.abc classes, it is more accurate to use "sequence" instead of "container" when describing argparse choices.

A previous attempt at fixing this in python#92450 was mistaken; this PR reverts that change.

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

* Fix name of removed `inspect.Signature.from_builtin` method in 3.11.0a2 changelog (python#100525)

* pythongh-100520: Fix `rst` markup in `configparser`  docstrings (python#100524)

* pythongh-99509: Add `__class_getitem__` to `multiprocessing.queues.Queue` (python#99511)

* pythongh-94603: micro optimize list.pop (pythongh-94604)

* Remove `NoneType` redefinition from `clinic.py` (python#100551)

* pythongh-100553: Improve accuracy of sqlite3.Row iter test (python#100555)

* pythonGH-98831: Modernize a ton of simpler instructions (python#100545)

* load_const and load_fast aren't families for now
* Don't decref unmoved names
* Modernize GET_ANEXT
* Modernize GET_AWAITABLE
* Modernize ASYNC_GEN_WRAP
* Modernize YIELD_VALUE
* Modernize POP_EXCEPT (in more than one way)
* Modernize PREP_RERAISE_STAR
* Modernize LOAD_ASSERTION_ERROR
* Modernize LOAD_BUILD_CLASS
* Modernize STORE_NAME
* Modernize LOAD_NAME
* Modernize LOAD_CLASSDEREF
* Modernize LOAD_DEREF
* Modernize STORE_DEREF
* Modernize COPY_FREE_VARS (mark it as done)
* Modernize LIST_TO_TUPLE
* Modernize LIST_EXTEND
* Modernize SET_UPDATE
* Modernize SETUP_ANNOTATIONS
* Modernize DICT_UPDATE
* Modernize DICT_MERGE
* Modernize MAP_ADD
* Modernize IS_OP
* Modernize CONTAINS_OP
* Modernize CHECK_EXC_MATCH
* Modernize IMPORT_NAME
* Modernize IMPORT_STAR
* Modernize IMPORT_FROM
* Modernize JUMP_FORWARD (mark it as done)
* Modernize JUMP_BACKWARD (mark it as done)

Signed-off-by: Filipe Laíns <lains@archlinux.org>
Signed-off-by: Filipe Laíns <lains@riseup.net>
Co-authored-by: Jeremy Paige <ucodery@gmail.com>
Co-authored-by: Carlton Gibson <carlton@noumenal.es>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
Co-authored-by: Jon Burdo <jon@jonburdo.com>
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neuralstring added a commit to neuralstring/cpython that referenced this pull request Feb 12, 2023
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Update tutorial docs related to sum() accuracy
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Related to python#100426
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