Mailman 3 October 2022 - Python-Dev

Proposal: Allow non-default after default arguments
by Sebastian Rittau Oct. 16, 2023

Oct. 16, 2023

Currently, Python doesn't allow non-default arguments after default arguments: >>> def foo(x=None, y): pass File "<stdin>", line 1 def foo(x=None, y): pass ^ SyntaxError: non-default argument follows default argument I believe that at the time this was introduced, no use cases for this were known and this is is supposed to prevent a source of bugs. I have two use cases for this, one fringe, but valid, the other more important: The fringe use case: Suppose you have a function that takes a 2D coordinate value as separate "x" and "y" arguments. The "x" argument is optional, the "y" argument isn't. Currently there are two ways to do this, none of them particularly great: def foo(y, x): # reverse x and y arguments, confusing ... def foo(x, y=None): # Treat the x argument as y if only one argument is provided if y is None: x, y = y, x ... To me, the "natural" solution looks like this: def foo(x=None, y): ... # Called like this: foo(1, 2) foo(y=2) This could also be useful when evolving APIs. For example there is a function "bar" that takes two required arguments. In a later version, the first argument gains a useful default, the second doesn't. There is no sensible way to evolve the API at the moment. The more important use case involves @overloads. A condensed example of a function where the return type depends on an "encoding" parameter, followed by further parameters that could be called like this: foo(123, "utf-8") # would return bytes foo(encoding="utf-8") foo(123, None) # would return str foo(encoding=None) foo(x=123) # would return str foo() This could ideally be written as: @overload def foo(x: int = ..., encoding: None = ...) -> str: ... @overload def foo(x: int = ..., encoding: str) -> bytes: ... # plus the actual implementation But due to the syntax constraint, this needs to be hacked around with a third overload: @overload def foo(x: int = ... encoding: None = ...) -> str: ... @overload def foo(x: int, encoding: str) -> bytes: ... # for foo(123, "utf-8") @overload def foo(*, encoding: str) -> bytes: ... # for foo(encoding="utf-8") Not only is this hard to read, real examples in typeshed are usually more complex, with many arguments before or after the affected argument or even multiple affected arguments. This often becomes too complex to write or maintain. Here is one example from the wild: https://github.com/python/typeshed/blob/b95b729b9e07ab21d252701af0f5b740467… Allowing non-default arguments after default arguments would solve both use cases above and eliminates a special case. I'm also not sure what exactly the current SyntaxError really protects us from. Adding a non-default after a default argument can't really lead bugs. - Sebastian

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Python multithreading without the GIL
by Sam Gross July 31, 2023

July 31, 2023

Hi, I've been working on changes to CPython to allow it to run without the global interpreter lock. I'd like to share a working proof-of-concept that can run without the GIL. The proof-of-concept involves substantial changes to CPython internals, but relatively few changes to the C-API. It is compatible with many C extensions: extensions must be rebuilt, but usually require small or no modifications to source code. I've built compatible versions of packages from the scientific Python ecosystem, and they are installable through the bundled "pip". Source code: https://github.com/colesbury/nogil Design overview: https://docs.google.com/document/d/18CXhDb1ygxg-YXNBJNzfzZsDFosB5e6BfnXLlej… My goal with the proof-of-concept is to demonstrate that removing the GIL is feasible and worthwhile, and that the technical ideas of the project could serve as a basis of such an effort. I'd like to start a discussion about these ideas and gauge the community's interest in this approach to removing the GIL. Regards, Sam Gross colesbury(a)gmail.com / sgross(a)fb.com

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Mixed Python/C debugging
by Skip Montanaro April 26, 2023

April 26, 2023

Having tried comp.lang.python with no response, I turn here... After at least ten years away from Python's run-time interpreter & byte code compiler, I'm getting set to familiarize myself with that again. This will, I think, entail debugging a mixed Python/C environment. I'm an Emacs user and am aware that GDB since 7.0 has support for debugging at the Python code level. Is Emacs+GDB my best bet? Are there any Python IDEs which support C-level breakpoints and debugging? Thanks, Skip

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PEP 448 review
by Guido van Rossum March 29, 2023

March 29, 2023

I'm back, I've re-read the PEP, and I've re-read the long thread with "(no subject)". I think Georg Brandl nailed it: """ *I like the "sequence and dict flattening" part of the PEP, mostly because itis consistent and should be easy to understand, but the comprehension syntaxenhancements seem to be bad for readability and "comprehending" what the codedoes.The call syntax part is a mixed bag on the one hand it is nice to be consistent with the extended possibilities in literals (flattening), but on the other hand there would be small but annoying inconsistencies anyways (e.g. the duplicate kwarg case above).* """ Greg Ewing followed up explaining that the inconsistency between dict flattening and call syntax is inherent in the pre-existing different rules for dicts vs. keyword args: {'a':1, 'a':2} results in {'a':2}, while f(a=1, a=2) is an error. (This form is a SyntaxError; the dynamic case f(a=1, **{'a': 1}) is a TypeError.) For me, allowing f(*a, *b) and f(**d, **e) and all the other combinations for function calls proposed by the PEP is an easy +1 -- it's a straightforward extension of the existing pattern, and anybody who knows what f(x, *a) does will understand f(x, *a, y, *b). Guessing what f(**d, **e) means shouldn't be hard either. Understanding the edge case for duplicate keys with f(**d, **e) is a little harder, but the error messages are pretty clear, and it is not a new edge case. The sequence and dict flattening syntax proposals are also clean and logical -- we already have *-unpacking on the receiving side, so allowing *x in tuple expressions reads pretty naturally (and the similarity with *a in argument lists certainly helps). From here, having [a, *x, b, *y] is also natural, and then the extension to other displays is natural: {a, *x, b, *y} and {a:1, **d, b:2, **e}. This, too, gets a +1 from me. So that leaves comprehensions. IIRC, during the development of the patch we realized that f(*x for x in xs) is sufficiently ambiguous that we decided to disallow it -- note that f(x for x in xs) is already somewhat of a special case because an argument can only be a "bare" generator expression if it is the only argument. The same reasoning doesn't apply (in that form) to list, set and dict comprehensions -- while f(x for x in xs) is identical in meaning to f((x for x in xs)), [x for x in xs] is NOT the same as [(x for x in xs)] (that's a list of one element, and the element is a generator expression). The basic premise of this part of the proposal is that if you have a few iterables, the new proposal (without comprehensions) lets you create a list or generator expression that iterates over all of them, essentially flattening them: >>> xs = [1, 2, 3] >>> ys = ['abc', 'def'] >>> zs = [99] >>> [*xs, *ys, *zs] [1, 2, 3, 'abc', 'def', 99] >>> But now suppose you have a list of iterables: >>> xss = [[1, 2, 3], ['abc', 'def'], [99]] >>> [*xss[0], *xss[1], *xss[2]] [1, 2, 3, 'abc', 'def', 99] >>> Wouldn't it be nice if you could write the latter using a comprehension? >>> xss = [[1, 2, 3], ['abc', 'def'], [99]] >>> [*xs for xs in xss] [1, 2, 3, 'abc', 'def', 99] >>> This is somewhat seductive, and the following is even nicer: the *xs position may be an expression, e.g.: >>> xss = [[1, 2, 3], ['abc', 'def'], [99]] >>> [*xs[:2] for xs in xss] [1, 2, 'abc', 'def', 99] >>> On the other hand, I had to explore the possibilities here by experimenting in the interpreter, and I discovered some odd edge cases (e.g. you can parenthesize the starred expression, but that seems a syntactic accident). All in all I am personally +0 on the comprehension part of the PEP, and I like that it provides a way to "flatten" a sequence of sequences, but I think very few people in the thread have supported this part. Therefore I would like to ask Neil to update the PEP and the patch to take out the comprehension part, so that the two "easy wins" can make it into Python 3.5 (basically, I am accepting two-thirds of the PEP :-). There is some time yet until alpha 2. I would also like code reviewers (Benjamin?) to start reviewing the patch <http://bugs.python.org/issue2292>, taking into account that the comprehension part needs to be removed. -- --Guido van Rossum (python.org/~guido)

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PEP 638: Syntactic macros
by Mark Shannon Feb. 5, 2023

Feb. 5, 2023

Hi everyone, I've submitted my PEP on syntactic macros as PEP 638. https://www.python.org/dev/peps/pep-0638/ All comments and suggestions are welcome. Cheers, Mark

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Switching to Discourse
by Petr Viktorin Jan. 26, 2023

Jan. 26, 2023

Hello, Currently development discussions are split between multiple communication channels, for example: - python-dev and discuss.python.org for design discussions, - GitHub Issues and Pull Requests for specific changes, - IRC, Discord and private chats for real-time discussions, - Topic-specific channels like typing-sig. While most of these serve different needs, there is too much overlap between python-dev and discuss.python.org. It seems that for most people, this situation is worse than sticking to either one platform – even if we don't go with that person's favorite. The discuss.python.org experiment has been going on for quite a while, and while the platform is not without its issues, we consider it a success. The Core Development category is busier than python-dev. According to staff, discuss.python.org is much easier to moderate.. If you're following python-dev but not discuss.python.org, you're missing out. The Steering Council would like to switch from python-dev to discuss.python.org. Practically, this means: - Moving the required PEP announcements to discuss.python.org - Moving discuss.python.org up in the devguide communications page (https://devguide.python.org/communication/) - And that's it? I imagine that the mailing list will stay around for continuing past discussion threads and for announcements, eventually switching to auto-reject incoming messages with a pointer to discuss.python.org. To be clear, discuss.python.org allows editing posts, which is frankly handy for typos and clarifications. Editing alone should not be used for adding new info -- we should cultivate a culture of being friendly to mail users & notification watchers. This probably bears repeating in a few places. We're aware not everyone wants to use the discuss.python.org website, but there are some ways to avoid it: - For new PEPs, you can point your RSS client to https://www.python.org/dev/peps/peps.rss – it's not e-mail, but many email clients have RSS support. You can also watch the Steering Council issues on GitHub (https://github.com/python/steering-council/issues/) for important questions and discussions. - You can use discuss.python.org's “mailing list mode” (which subscribes you to all new posts), possibly with filtering and/or categorizing messages locally. However, we would like to know if this will pose an undue burden to anyone, if there are workflows or usage problems that we are not aware of. As mentioned, this is something the Steering Council thinks is a good idea, but we want to make sure we're aware of all the impact when we make the final decision. – Petr, on behalf of the Steering Council

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NEWLINE sentinel behavior in CPython's PEG grammar
by David J W Nov. 3, 2022

Nov. 3, 2022

I am writing a Rust version of Python for fun and I am at the parser stage of development. I copied and modified a PEG grammar ruleset from another open source project and I've already noticed some problems (ex Newline vs NL) with how they transcribed things. I am suspecting that CPython's grammar NEWLINE is a builtin rule for the parser that is something like `(Newline+ | NL+ ) {NOP}` but wanted to sanity check if that is right before I figure out how to hack in a NEWLINE rule and update my grammar ruleset.

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Possible bug in `re` module in Python 3.11?
by Piotr Waszkiewicz Oct. 26, 2022

Oct. 26, 2022

Hi, I would like to ask your guidance as I'm entirely sure whether the problem I'm experiencing should be posted in CPython's repo as a bug issue. I've tried using newly released Python 3.11 interpreter in some of my projects and one of them failed to start with "RuntimeError: invalid SRE code" error. Looking into implementation of one of the dependencies I've found out that the issue has been found out by a maintainer and fixed (https://github.com/pydicom/pydicom/issues/1658). It looks like this particular regexp caused the `re.compile()` method to raise: ``` re.compile( r"(?P<h>^([01][0-9]|2[0-3]))" r"((?P<m>([0-5][0-9]))?" r"(?(5)(?P<s>([0-5][0-9]|60))?)" r"(?(7)(\.(?P<ms>([0-9]{1,6})?))?))$" ) ``` I've checked and this hasn't been an issue in all previous Python interpreter versions, starting from 3.6 (the oldest I've checked). What's more the regex i correctly recognized and does not cause any issues in other regexp implementations, e.g. the online tool https://regex101.com/ Could somebody help me decide whether this is indeed a bug? Best regards, Piotr Waszkiewicz

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Thank you for your contributions to Python 3.11!
by Pablo Galindo Salgado Oct. 26, 2022

Oct. 26, 2022

Hi everyone, Now that the 3.11.0 release is finally done and I can relax a bit, I just wanted to thank you all for your fantastic work that has made Python 3.11 such a fantastic release. No matter if you committed code to 3.11 or opened a bug, helped with the documentation, reviewed pull requests, participated in discussions, made a PEP or help writing one, fixed a bug or one hundred or make optimizations to the interpreter or any other of the many ways to contribute. Your work makes a huge difference and Python 3.11 is much better because of that :) Also, I want to especially thank all core devs and contributors that have helped me and the release team take care of release blockers, buildbot failures, CVE patches, and any other form of release crisis. Thank you! Finally, a huge thanks to my colleagues in the release team that make these releases possible and help to make sure that my mistakes are not too obvious to end users :P Being your release manager for 3.11 and 3.10 has been a privilege and an honor (and it will continue for a couple of years of bugfixes and security releases, I'm not going anywhere). Regards from rainy London, Pablo Galindo Salgado

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problem with Distributed File System Replication and Namespacing and different versions of Python 3
by rainonthescarecrowhumanwheels＠gmail.com Oct. 26, 2022

Oct. 26, 2022

Hi, I have a question about a problem we are facing in different Python versions, like Python 3.7.7, Python 3.8, Python 3.6 and Python 3.9. This apparently doesnt happen in Python 3.10.2 and we were wondering how we could achieve the same behaviours without going deep into the nuts and bolts of the python import system. We have python libraries that are distributed in a networked environment that when imported for the first time they are correctly imported but something happens in the meantime that prevents it from loading anymore. This networked paths are domain paths that are bound to a final "real" path in a specific file server from a specific site in the world. What happens is, when injecting into the sys.path the domain names it doesnt import but when injecting into the sys.path the "real" file server path it works, generally speaking. We have been facing this issue in such different python 3 versions and i was wondering what makes this work in Python 3.10.2. Apparently, in my opinion it is a mix of two things involved: one, something the DFS-R DFS-N does at a very low level, that python 3 versions dont like when importing and the other one, the fact that is a network path. Can anybody shed some light about the improvements regarding this? We will be tied to all these python versions for a very long time and we need to give solution to this. One of the possibilities would be to distribute the libraries into C:/ local disc, this eliminating the network path variable. But it would be nice to know what is causing this in Python < 3.10.2. Thanks in advance.

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