Python-Dev November 2020

python-dev@python.org

92 participants
50 discussions

When to remove BytesWarning?
by Inada Naoki 11 Nov '20

11 Nov '20

Hi, all. To avoid BytesWarning, the compiler needs to do some hack when they need to store bytes and str constants in one dict or set. BytesWarning has maintenance costs. It is not huge, but significant. When can we remove it? My idea is: 3.10: Deprecate the -b option. 3.11: Make the -b option no-op. Bytes warning never emits. 3.12: Remove the -b option. BytesWarning will be deprecated in the document, but not to be removed. Users who want to use the -b option during 2->3 conversion need to use Python ~3.10 for a while. Regards, -- Inada Naoki <songofacandy(a)gmail.com>

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Re: Who is target reader of tutorial?
by Federico Salerno 07 Nov '20

07 Nov '20

I think it's important to remember that many novice programmers today learn python as their first language. While i don't think the python tutorial is the right place for teaching how to program, i also think it would be best if it didn't make too many assumptions on the reader's knowledge. When it comes to the basics, assuming one already knows another language isn't very beneficial anyway, in my opinion. If all one is looking for is a table comparing a known thing with its python equivalent, there are many of those on the internet already and in much more compact form.

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Who is target reader of tutorial?
by Inada Naoki 07 Nov '20

07 Nov '20

Hi, all. Since "How To" guide is not organized well, it is very tempting to write all details in tutorial. I have seen may pull requests which "improve" tutorial by describe more details and make the tutorial more perfect. But adding more and more information into tutorial makes it slower to learn Python by reading tutorial. 10+ years ago, Python is far less popular in Japan and reading tutorial is the best way to learn Python to me. But now Python is popular and there are many free/paid good books and tutorials on the Web. Some of them would be more new user friendly than official tutorial. Then, should official Python tutorial become detailed guide to Python? Or should we keep new user learning Python as targeted reader? There is ongoing issue for example: https://bugs.python.org/issue42179 Chaining exception was added in tutorial. Current tutorial mention to `__cause__` attribute. https://docs.python.org/3/tutorial/errors.html#exception-chaining bpo-42179 proposes to add mention to `__context__` to make the tutorial more accurate about implicit chaining. And https://github.com/python/cpython/pull/23160 is the pull request to mention `__context__`. On the other hand, I want to remove confusion by removing mention to `__cause__`. Because I don't think `__context__` and `__cause__` is important for new users. See https://github.com/python/cpython/pull/23162 for my proposal. Regards, -- Inada Naoki <songofacandy(a)gmail.com>

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Re: Drop Solaris, OpenSolaris, Illumos and OpenIndiana support in Python
by Garrett D'Amore 06 Nov '20

06 Nov '20

I’m not on this list. But I have offered to help - if there are tasks that need to be done to help this I can help put the weight of a commercial entity behind it whether that involves assigning our developers to work on this, helping pay for external developers to do so, or assisting with access to machine resources. For the record there are multiple illumos distributions and most are both free and run reasonably well in virtual machines. Claiming that developers don’t have access as a reason to discontinue the port is a bit disingenuous. Anyone can get access if they want and if they can figure out how to login and use Linux then this should be pretty close to trivial for them. What’s more likely is that some group of developers aren’t interested in supporting stuff they don’t actively use. I get it. It’s easier to work in a monoculture. But in this case there are many many more users of this that would be impacted than a naive examination of downloads will show. Of course this all presumes that the core Python team still places value on being a cross platform portable tool. I can help solve most of the other concerns - except for this one. - Garrett

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Summary of Python tracker Issues
by Python tracker 06 Nov '20

06 Nov '20

ACTIVITY SUMMARY (2020-10-30 - 2020-11-06) Python tracker at https://bugs.python.org/ To view or respond to any of the issues listed below, click on the issue. Do NOT respond to this message. Issues counts and deltas: open 7617 ( -9) closed 46397 (+75) total 54014 (+66) Open issues with patches: 3062 Issues opened (43) ================== #37319: Deprecate using random.randrange() with non-integers https://bugs.python.org/issue37319 reopened by serhiy.storchaka #42160: unnecessary overhead in tempfile https://bugs.python.org/issue42160 reopened by vstinner #42213: Get rid of cyclic GC hack in sqlite3.Connection and sqlite3.Ca https://bugs.python.org/issue42213 opened by erlendaasland #42215: urlparse with a Windows path returns the drive name as the sch https://bugs.python.org/issue42215 opened by rangelspam #42217: compiler: merge co_code and co_lnotab https://bugs.python.org/issue42217 opened by methane #42220: Argparse: wildcards processing https://bugs.python.org/issue42220 opened by Andrew #42222: Modernize integer test/conversion in randrange() https://bugs.python.org/issue42222 opened by rhettinger #42225: Tkinter hangs or crashes when displaying astral chars https://bugs.python.org/issue42225 opened by terry.reedy #42228: Activate.ps1 clears PYTHONHOME https://bugs.python.org/issue42228 opened by mathstuf #42231: test_zipimport failure https://bugs.python.org/issue42231 opened by vstinner #42232: mmap module add Darwin specific madvise options https://bugs.python.org/issue42232 opened by devnexen #42233: GenericAlias does not support union type expressions https://bugs.python.org/issue42233 opened by kj #42234: pathlib relative_to behaviour change https://bugs.python.org/issue42234 opened by armins.bagrats #42235: [macOS] Use --enable-optimizations in build-installer.py https://bugs.python.org/issue42235 opened by ronaldoussoren #42237: test_socket.SendfileUsingSendfileTest fails on illumos https://bugs.python.org/issue42237 opened by wiedi #42238: Deprecate suspicious.py? https://bugs.python.org/issue42238 opened by mdk #42239: IDLE: Restore or keep calltip when needed https://bugs.python.org/issue42239 opened by terry.reedy #42243: Don't access the module dictionary directly https://bugs.python.org/issue42243 opened by erlendaasland #42244: TimedRotatingFileHandler doesn’t handle the switch to/from D https://bugs.python.org/issue42244 opened by Cristian Martinez de Morentin #42245: concurrent.futures.ProcessPoolExecutor freezes depending on co https://bugs.python.org/issue42245 opened by DanilZ #42246: Implement PEP 626 https://bugs.python.org/issue42246 opened by Mark.Shannon #42247: unittest hides traceback frames in chained exceptions https://bugs.python.org/issue42247 opened by dseomn #42248: Raised exception in Enum keeping user objects alive unnecessar https://bugs.python.org/issue42248 opened by efroemling #42252: Embeddable Python indicates that it uses PYTHONPATH https://bugs.python.org/issue42252 opened by teeks99 #42253: xml.dom.minidom.rst missed informations https://bugs.python.org/issue42253 opened by jedie2 #42255: webbrowser.MacOSX is unused, untested and undocumented https://bugs.python.org/issue42255 opened by ronaldoussoren #42256: BaseCookie.__parse_string() doesn't work with expires= between https://bugs.python.org/issue42256 opened by paulie4 #42257: platform.libc_ver() doesn't consider in case of executable is https://bugs.python.org/issue42257 opened by kurochan #42258: argparse: show choices once per argument https://bugs.python.org/issue42258 opened by mendelmaleh #42259: pprint: infinite recursion for saferepr() when using nested ob https://bugs.python.org/issue42259 opened by danbst #42260: [C API] Add PyInterpreterState_SetConfig(): reconfigure an int https://bugs.python.org/issue42260 opened by vstinner #42261: Windows legacy I/O mode mistakenly ignores the device encoding https://bugs.python.org/issue42261 opened by eryksun #42264: Deprecate or remove sqlite3.OptimizedUnicode https://bugs.python.org/issue42264 opened by erlendaasland #42267: Python 3.9 broken installer https://bugs.python.org/issue42267 opened by JackSkellington #42268: ./configure failing when --with-memory-sanitizer specified https://bugs.python.org/issue42268 opened by JaonHax #42269: Add ability to set __slots__ in dataclasses https://bugs.python.org/issue42269 opened by eric.smith #42270: libregrtest BrokenPipeError on OpenEmbedded builds https://bugs.python.org/issue42270 opened by threexc #42271: Remove the error and the zipfile.ZipFile.BadZipfile aliases https://bugs.python.org/issue42271 opened by PedanticHacker #42272: Warning filter message/module documentation is misleading https://bugs.python.org/issue42272 opened by kevinoid #42273: Using LazyLoader leads to AttributeError https://bugs.python.org/issue42273 opened by KevKeating #42275: Jupyter Lab Terminals not available (error was No module named https://bugs.python.org/issue42275 opened by shakir_juolay #42277: Solaris & PEP 3149: start using ABI version tagged .so files https://bugs.python.org/issue42277 opened by kulikjak #42278: Remove usage of tempfile.mktemp in stdlib https://bugs.python.org/issue42278 opened by epaine Most recent 15 issues with no replies (15) ========================================== #42278: Remove usage of tempfile.mktemp in stdlib https://bugs.python.org/issue42278 #42277: Solaris & PEP 3149: start using ABI version tagged .so files https://bugs.python.org/issue42277 #42273: Using LazyLoader leads to AttributeError https://bugs.python.org/issue42273 #42272: Warning filter message/module documentation is misleading https://bugs.python.org/issue42272 #42271: Remove the error and the zipfile.ZipFile.BadZipfile aliases https://bugs.python.org/issue42271 #42270: libregrtest BrokenPipeError on OpenEmbedded builds https://bugs.python.org/issue42270 #42269: Add ability to set __slots__ in dataclasses https://bugs.python.org/issue42269 #42264: Deprecate or remove sqlite3.OptimizedUnicode https://bugs.python.org/issue42264 #42258: argparse: show choices once per argument https://bugs.python.org/issue42258 #42257: platform.libc_ver() doesn't consider in case of executable is https://bugs.python.org/issue42257 #42255: webbrowser.MacOSX is unused, untested and undocumented https://bugs.python.org/issue42255 #42253: xml.dom.minidom.rst missed informations https://bugs.python.org/issue42253 #42248: Raised exception in Enum keeping user objects alive unnecessar https://bugs.python.org/issue42248 #42247: unittest hides traceback frames in chained exceptions https://bugs.python.org/issue42247 #42244: TimedRotatingFileHandler doesn’t handle the switch to/from D https://bugs.python.org/issue42244 Most recent 15 issues waiting for review (15) ============================================= #42277: Solaris & PEP 3149: start using ABI version tagged .so files https://bugs.python.org/issue42277 #42272: Warning filter message/module documentation is misleading https://bugs.python.org/issue42272 #42264: Deprecate or remove sqlite3.OptimizedUnicode https://bugs.python.org/issue42264 #42260: [C API] Add PyInterpreterState_SetConfig(): reconfigure an int https://bugs.python.org/issue42260 #42258: argparse: show choices once per argument https://bugs.python.org/issue42258 #42257: platform.libc_ver() doesn't consider in case of executable is https://bugs.python.org/issue42257 #42246: Implement PEP 626 https://bugs.python.org/issue42246 #42243: Don't access the module dictionary directly https://bugs.python.org/issue42243 #42239: IDLE: Restore or keep calltip when needed https://bugs.python.org/issue42239 #42237: test_socket.SendfileUsingSendfileTest fails on illumos https://bugs.python.org/issue42237 #42235: [macOS] Use --enable-optimizations in build-installer.py https://bugs.python.org/issue42235 #42233: GenericAlias does not support union type expressions https://bugs.python.org/issue42233 #42222: Modernize integer test/conversion in randrange() https://bugs.python.org/issue42222 #42217: compiler: merge co_code and co_lnotab https://bugs.python.org/issue42217 #42213: Get rid of cyclic GC hack in sqlite3.Connection and sqlite3.Ca https://bugs.python.org/issue42213 Top 10 most discussed issues (10) ================================= #42225: Tkinter hangs or crashes when displaying astral chars https://bugs.python.org/issue42225 20 msgs #42173: Drop Solaris support https://bugs.python.org/issue42173 17 msgs #42179: Clarify chaining exceptions in tutorial/errors.rst https://bugs.python.org/issue42179 10 msgs #1635741: Py_Finalize() doesn't clear all Python objects at exit https://bugs.python.org/issue1635741 10 msgs #42222: Modernize integer test/conversion in randrange() https://bugs.python.org/issue42222 8 msgs #37193: Memory leak while running TCP/UDPServer with socketserver.Thre https://bugs.python.org/issue37193 7 msgs #42195: Inconsistent __args__ between typing.Callable and collections. https://bugs.python.org/issue42195 6 msgs #42245: concurrent.futures.ProcessPoolExecutor freezes depending on co https://bugs.python.org/issue42245 6 msgs #42252: Embeddable Python indicates that it uses PYTHONPATH https://bugs.python.org/issue42252 5 msgs #42260: [C API] Add PyInterpreterState_SetConfig(): reconfigure an int https://bugs.python.org/issue42260 5 msgs Issues closed (69) ================== #8988: import + coding = failure (3.1.2/win32) https://bugs.python.org/issue8988 closed by iritkatriel #11346: Generator object should be mentioned in gc module document https://bugs.python.org/issue11346 closed by methane #12739: read stuck with multithreading and simultaneous subprocess.Pop https://bugs.python.org/issue12739 closed by iritkatriel #14379: Several traceback docs improvements https://bugs.python.org/issue14379 closed by iritkatriel #15128: inspect raises exception when frames are misleading about sour https://bugs.python.org/issue15128 closed by iritkatriel #15626: unittest.main negates -bb option and programmatic warning conf https://bugs.python.org/issue15626 closed by ncoghlan #16855: traceback module leaves off module name in last line of format https://bugs.python.org/issue16855 closed by iritkatriel #24380: Got warning when compiling _scproxy.c on Mac https://bugs.python.org/issue24380 closed by ronaldoussoren #26031: Add stat caching option to pathlib https://bugs.python.org/issue26031 closed by gvanrossum #26377: Tkinter dialogs will not close if root window not packed. https://bugs.python.org/issue26377 closed by terry.reedy #26389: Expand traceback module API to accept just an exception as an https://bugs.python.org/issue26389 closed by pablogsal #26789: logging: Trying to log during Python finalization with NameErr https://bugs.python.org/issue26789 closed by vstinner #27377: Add socket.fdtype() https://bugs.python.org/issue27377 closed by nascheme #29566: binhex() creates files with mixed line endings https://bugs.python.org/issue29566 closed by ronaldoussoren #30384: traceback.TracebackException.format shouldn't format_exc_only( https://bugs.python.org/issue30384 closed by iritkatriel #34106: Add --with-module-config=<file> to 'configure' script https://bugs.python.org/issue34106 closed by nascheme #34704: Do not access ob_type directly, introduce Py_TP https://bugs.python.org/issue34704 closed by nascheme #40511: IDLE: properly handle '(' and ')' within calls https://bugs.python.org/issue40511 closed by terry.reedy #40514: [subinterpreters] Add --experimental-isolated-subinterpreters https://bugs.python.org/issue40514 closed by vstinner #40814: Update typing module documentation based on PEP 585 https://bugs.python.org/issue40814 closed by gvanrossum #40816: Add missed AsyncContextDecorator to contextlib https://bugs.python.org/issue40816 closed by asvetlov #41435: Allow to retrieve ongoing exception handled by every threads https://bugs.python.org/issue41435 closed by serhiy.storchaka #41796: _ast module state should be made per interpreter https://bugs.python.org/issue41796 closed by vstinner #41832: PyType_FromSpec() should accept tp_doc=NULL https://bugs.python.org/issue41832 closed by vstinner #41835: Speed up dict vectorcall creation using keywords https://bugs.python.org/issue41835 closed by methane #42049: Add image/webp to list of media types in mimetypes.py https://bugs.python.org/issue42049 closed by methane #42100: Add _PyType_GetModuleByDef https://bugs.python.org/issue42100 closed by petr.viktorin #42108: HTTPSConnection hangs forever when the percentage completed of https://bugs.python.org/issue42108 closed by Tiger-222 #42126: Optimize BUILD_CONST_KEY_MAP for distinct keys https://bugs.python.org/issue42126 closed by methane #42131: [zipimport] Update zipimport to use specs https://bugs.python.org/issue42131 closed by brett.cannon #42146: subprocess.Popen() leaks cwd in case of uid/gid overflow https://bugs.python.org/issue42146 closed by gregory.p.smith #42162: The license page for Python 3.0.1 is messed up https://bugs.python.org/issue42162 closed by terry.reedy #42177: Improving KeyError exception https://bugs.python.org/issue42177 closed by terry.reedy #42196: Python Windows Installation Error 0x800705aa https://bugs.python.org/issue42196 closed by shakir_juolay #42198: Clean up docs for Union and GenericAlias https://bugs.python.org/issue42198 closed by gvanrossum #42201: Priority queue realization issue https://bugs.python.org/issue42201 closed by rhettinger #42205: Add image/webp to list of non-standard mimetypes https://bugs.python.org/issue42205 closed by methane #42207: Python 3.9 testing fails when building with clang and optimiza https://bugs.python.org/issue42207 closed by vstinner #42208: Using logging or warnings during Python finalization does cras https://bugs.python.org/issue42208 closed by vstinner #42209: Incorrect line reported in syntax error https://bugs.python.org/issue42209 closed by eric.smith #42210: float.hex discards sign from -nan https://bugs.python.org/issue42210 closed by mark.dickinson #42211: CRASH https://bugs.python.org/issue42211 closed by vstinner #42214: Parser/pegen.c:68: _PyPegen_check_barry_as_flufl: Assertion `t https://bugs.python.org/issue42214 closed by pablogsal #42216: Optimize dict.popitem() & insert use case. https://bugs.python.org/issue42216 closed by methane #42218: SystemError in compile builtin function https://bugs.python.org/issue42218 closed by lys.nikolaou #42219: Typo in comment of listobject.h https://bugs.python.org/issue42219 closed by akshayj398p #42221: Potential memory leak in resize_compact() (for unicode objects https://bugs.python.org/issue42221 closed by dralley #42223: List repetition operator gives unexpected results https://bugs.python.org/issue42223 closed by steven.daprano #42224: test_format:test_locale fails when locale does not set groupin https://bugs.python.org/issue42224 closed by lys.nikolaou #42226: imghdr.what is missing docstring, has a logic error, and is ov https://bugs.python.org/issue42226 closed by jph00 #42227: Unexpected sharing of list/set/dict between instances of the s https://bugs.python.org/issue42227 closed by steven.daprano #42229: Fix SQLite warnings on macOS https://bugs.python.org/issue42229 closed by samuelmarks #42230: Document that asyncio's wait() and as_completed() accept arbit https://bugs.python.org/issue42230 closed by asvetlov #42236: os.device_encoding() doesn't respect the UTF-8 Mode https://bugs.python.org/issue42236 closed by vstinner #42240: Add Maxheap version of a heappush into heapq module https://bugs.python.org/issue42240 closed by rhettinger #42241: Backport SQLite trace API v2 https://bugs.python.org/issue42241 closed by corona10 #42242: Backport SQLite trace API v2 https://bugs.python.org/issue42242 closed by corona10 #42249: Plistlib cannot create binary Plist file larger than 4GiB https://bugs.python.org/issue42249 closed by serhiy.storchaka #42250: test_ast leaked [23640, 23636, 23640] references https://bugs.python.org/issue42250 closed by vstinner #42251: Add threading.gettrace and threading.getprofile https://bugs.python.org/issue42251 closed by pablogsal #42254: inspect.getmembers iterator version https://bugs.python.org/issue42254 closed by signing_agreement #42262: [C API] Add Py_NewRef() function to get a new strong reference https://bugs.python.org/issue42262 closed by vstinner #42263: Removing thread reference in thread results in leaked referenc https://bugs.python.org/issue42263 closed by jaraco #42265: Remove binhex module following PEP-594 https://bugs.python.org/issue42265 closed by steven.daprano #42266: LOAD_ATTR cache does not fully replicate PyObject_GetAttr beha https://bugs.python.org/issue42266 closed by pablogsal #42274: Imaplib hangs up infinitely when performing append operation https://bugs.python.org/issue42274 closed by eric.smith #42276: Bug in adfuller test and a suggested fix https://bugs.python.org/issue42276 closed by ronaldoussoren #1633941: for line in sys.stdin: doesn't notice EOF the first time https://bugs.python.org/issue1633941 closed by iritkatriel #1643712: Emphasize buffering issues when sys.stdin is used https://bugs.python.org/issue1643712 closed by iritkatriel

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Drop Solaris, OpenSolaris, Illumos and OpenIndiana support in Python
by Victor Stinner 06 Nov '20

06 Nov '20

Hi, I propose to drop the Solaris support in Python to reduce the Python maintenance burden: https://bugs.python.org/issue42173 I wrote a draft PR to show how much code could be removed (around 700 lines in 65 files): https://github.com/python/cpython/pull/23002/files In 2016, I asked if we still wanted to maintain the Solaris support in Python, because Solaris buildbots were failing for longer than 6 months and nobody was able to fix them. It was requested to find a core developer volunteer to fix Solaris issues and to set up a Solaris buildbot. https://mail.python.org/archives/list/python-dev@python.org/thread/NOT2RORS… Four years later, nothing has happened. Moreover, in 2018, Oracle laid off the Solaris development engineering staff. There are around 25 open Python bugs specific to Solaris. I see 3 options: * Current best effort support (no change): changes only happen if a core dev volunteers to review and merge a change written by a contributor. * Schedule the removal in 2 Python releases (Python 3.12) and start to announce that Solaris support is going to be removed * Remove the Solaris code right now (my proposition): Solaris code will have to be maintained outside the official Python code base, as "downstream patches" Solaris has a few specific features visible at the Python level: select.devpoll, os.stat().st_fstype and stat.S_ISDOOR(). While it's unclear to me if Oracle still actively maintains Solaris (latest release in 2018, no major update since 2018), Illumos and OpenSolaris (variants or "forks") still seem to be active. In 2019, a Solaris blog post explains that Solaris 11.4 still uses Python 2.7 but plans to migrate to Python 3, and Python 3.4 is also available. These two Python versions are no longer supported. https://blogs.oracle.com/solaris/future-of-python-on-solaris The question is if the Python project has to maintain the Solaris specific code or if this code should now be maintained outside Python. What do you think? Should we wait 5 more years? Should we expect a company will offer to maintain the Solaris support? Is there a motivated core developer to fix Solaris issue? As I wrote, nothing has happened in the last 4 years... Victor -- Night gathers, and now my watch begins. It shall not end until my death.

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Contribution to github repos.
by Manak Wadhwa 06 Nov '20

06 Nov '20

Hey Guys, I am Manak Wadhwa. I have a good hand in python language, ML Algorithms, Web Dev (HTML5, CSS3, JS, PHP, AJAX, Bootstrap, JQuery). I want to contribute to python org can someone help as which repository should i start with or work on.

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[RELEASE] Python 3.10.0a2 available for testing
by Pablo Galindo Salgado 04 Nov '20

04 Nov '20

The engines of the secret release manager machine have finished producing a new pre-release. Go get it here: https://www.python.org/downloads/release/python-3100a2/ *Major new features of the 3.10 series, compared to 3.9* Python 3.10 is still in development. This releasee, 3.10.0a2 is the second of six planned alpha releases. Alpha releases are intended to make it easier to test the current state of new features and bug fixes and to test the release process. During the alpha phase, features may be added up until the start of the beta phase (2021-05-03) and, if necessary, may be modified or deleted up until the release candidate phase (2021-10-04). Please keep in mind that this is a preview release and its use is not recommended for production environments. Many new features for Python 3.10 are still being planned and written. Among the new major new features and changes so far: PEP 623 -- Remove wstr from Unicode PEP 604 -- Allow writing union types as X | Y PEP 612 -- Parameter Specification Variables PEP 626 -- Precise line numbers for debugging and other tools. (Hey, fellow core developer, if a feature you find important is missing from this list, let Pablo know.) The next pre-release of Python 3.10 will be 3.10.0a3, currently scheduled for 2020-12-07. *And now for something completely different * The cardinality (the number of elements) of infinite sets can be one of the most surprising results of set theory. For example, there are the same amount of even natural numbers than natural numbers (which can be even or odd). There is also the same amount of rational numbers than natural numbers. But on the other hand, there are more real numbers between 0 and 1 than natural numbers! All these sets have infinite cardinality but turn out that some of these infinities are bigger than others. These infinite cardinalities normally are represented using aleph numbers. Infinite sets are strange beasts indeed. Regards from cold London, Pablo Galindo Salgado

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Re: PEP 642: Constraint Pattern Syntax for Structural Pattern Matching
by Federico Salerno 03 Nov '20

03 Nov '20

Re: symbol for lookup Whatever happened to the proposal of using . as prefix? If memory serves, the main objection was about it being hard to see, but is it really? We use fixed width fonts for a reason, and there are other places a dot is quite critical (has any php programmer ever mistaken a .= for a = ?) without it's size ever causing issues. I think . is visible enough while being aesthetically inoffensive. Am i missing some problem or important past objection to it?

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Re: Pattern matching reborn: PEP 622 is dead, long live PEP 634, 635, 636
by Tin Tvrtković 02 Nov '20

02 Nov '20

Hi, first of all, I'm a big fan of the changes being proposed here since in my code I prefer the 'union' style of logic over the OO style. I was curious, though, if there are any plans for the match operator to support async stuff. I'm interested in the problem of waiting on multiple asyncio tasks concurrently, and having a branch of code execute depending on the task. Currently this can be done by using asyncio.wait, looping over the done set and executing an if-else chain there, but this is quite tiresome. Go has a select statement (https://tour.golang.org/concurrency/5) that looks like this: select { case <-ch1: fmt.Println("Received from ch1") case <-ch2: fmt.Println("Received from ch2") } Speaking personally, this is a Go feature I miss a lot when writing asyncio code. The syntax is similar to what's being proposed here. Although it could be a separate thing added later, async match, I guess. > Message: 2 > Date: Thu, 22 Oct 2020 09:48:54 -0700 > From: Guido van Rossum <guido(a)python.org> > Subject: [Python-Dev] Pattern matching reborn: PEP 622 is dead, long > live PEP 634, 635, 636 > To: Python-Dev <python-dev(a)python.org> > Message-ID: > <CAP7+vJKzn+y8gqAFUr1g= > jOCk2if496ST00Ke8Mg921Yp5ZitQ(a)mail.gmail.com> > Content-Type: multipart/alternative; > boundary="00000000000039e83905b2453edf" > > --00000000000039e83905b2453edf > Content-Type: text/plain; charset="UTF-8" > > After the pattern matching discussion died out, we discussed it with the > Steering Council. Our discussion ended fairly positive, but there were a > lot of problems with the text. We decided to abandon the old PEP 622 and > break it up into three parts: > > - PEP 634: Specification > - PEP 635: Motivation and Rationale > - PEP 636: Tutorial > > This turned out to be more work than I had expected (basically we wrote all > new material) but we've finally made it to a point where we can request > feedback and submit the new version to the SC for approval. > > While the text of the proposal is completely different, there aren't that > many substantial changes: > > - We changed walrus patterns ('v := p') to AS patterns ('p as v'). > - We changed the method of comparison for literals None, False, True to use > 'is' instead of '=='. > - SyntaxError if an irrefutable case[1] is followed by another case block. > - SyntaxError if an irrefutable pattern[1] occurs on the left of '|', e.g. > 'x | [x]'. > - We dropped the `@sealed` decorator and everything aimed at static type > checkers. > > [1] An irrefutable pattern is one that never fails, notably a wildcard or a > capture. An irrefutable case has an irrefutable pattern at the top and no > guard. Irrefutability is defined recursively, since an '|' with an > irrefutable pattern on either side is itself irrefutable, and so is an AS > pattern with an irrefutable pattern before 'as'. > > The following issues were specifically discussed with the SC: > > - Concerns about side effects and undefined behavior. There's now some > specific language about this in a few places (giving the compiler freedom > to optimize), and a section "Side Effects and Undefined Behavior". > > - Footgun if `case NAME:` is followed by another case. This is now a > SyntaxError. > > - Adding an 'else' clause. We decided not to add this; motivation in PEP > 635. > > - Alternative 'OR' symbol. Not changed; see PEP 635. > > - Alternative wildcard symbol. Not changed, but Thomas wrote PEP 640 which > proposes '?' as a general assignment target. PEP 635 has some language > against that idea. > > - Alternative indentation schemes. We decided to stick with the original > proposal; see PEP 635. > > - Marking all capture variables with a sigil. We all agreed this was a bad > idea; see PEP 635. > > -- > --Guido van Rossum (python.org/~guido) > *Pronouns: he/him **(why is my pronoun here?)* >

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Python-Dev November 2020