Python-Dev October 2020

python-dev@python.org

108 participants
53 discussions

Boundaries between numbers and identifiers

by Serhiy Storchaka

In Python 2.5 `0or[]` was accepted by the Python parser. It became an error in 2.6 because "0o" became recognizing as an incomplete octal number. `1or[]` still is accepted. On other hand, `1if 2else 3` is accepted despites the fact that "2e" can be recognized as an incomplete floating point number. In this case the tokenizer pushes "e" back and returns "2". Shouldn't it do the same with "0o"? It is possible to make `0or[]` be parseable again. Python implementation is able to tokenize this example: $ echo '0or[]' | ./python -m tokenize 1,0-1,1: NUMBER '0' 1,1-1,3: NAME 'or' 1,3-1,4: OP '[' 1,4-1,5: OP ']' 1,5-1,6: NEWLINE '\n' 2,0-2,0: ENDMARKER '' On other hand, all these examples look weird. There is an assymmetry: `1or 2` is a valid syntax, but `1 or2` is not. It is hard to recognize visually the boundary between a number and the following identifier or keyword, especially if numbers can contain letters ("b", "e", "j", "o", "x") and underscores, and identifiers can contain digits. On both sides of the boundary can be letters, digits, and underscores. I propose to change the Python syntax by adding a requirement that there should be a whitespace or delimiter between a numeric literal and the following keyword.

3 weeks

PEP 624: Remove Py_UNICODE encoder APIs

by Inada Naoki

Hi, folks. Since the previous discussion was suspended without consensus, I wrote a new PEP for it. (Thank you Victor for reviewing it!) This PEP looks very similar to PEP 623 "Remove wstr from Unicode", but for encoder APIs, not for Unicode object APIs. URL (not available yet): https://www.python.org/dev/peps/pep-0624/ --- PEP: 624 Title: Remove Py_UNICODE encoder APIs Author: Inada Naoki <songofacandy(a)gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 06-Jul-2020 Python-Version: 3.11 Abstract ======== This PEP proposes to remove deprecated ``Py_UNICODE`` encoder APIs in Python 3.11: * ``PyUnicode_Encode()`` * ``PyUnicode_EncodeASCII()`` * ``PyUnicode_EncodeLatin1()`` * ``PyUnicode_EncodeUTF7()`` * ``PyUnicode_EncodeUTF8()`` * ``PyUnicode_EncodeUTF16()`` * ``PyUnicode_EncodeUTF32()`` * ``PyUnicode_EncodeUnicodeEscape()`` * ``PyUnicode_EncodeRawUnicodeEscape()`` * ``PyUnicode_EncodeCharmap()`` * ``PyUnicode_TranslateCharmap()`` * ``PyUnicode_EncodeDecimal()`` * ``PyUnicode_TransformDecimalToASCII()`` .. note:: `PEP 623 <https://www.python.org/dev/peps/pep-0623/>`_ propose to remove Unicode object APIs relating to ``Py_UNICODE``. On the other hand, this PEP is not relating to Unicode object. These PEPs are split because they have different motivation and need different discussion. Motivation ========== In general, reducing the number of APIs that have been deprecated for a long time and have few users is a good idea for not only it improves the maintainability of CPython, but it also helps API users and other Python implementations. Rationale ========= Deprecated since Python 3.3 --------------------------- ``Py_UNICODE`` and APIs using it are deprecated since Python 3.3. Inefficient ----------- All of these APIs are implemented using ``PyUnicode_FromWideChar``. So these APIs are inefficient when user want to encode Unicode object. Not used widely --------------- When searching from top 4000 PyPI packages [1]_, only pyodbc use these APIs. * ``PyUnicode_EncodeUTF8()`` * ``PyUnicode_EncodeUTF16()`` pyodbc uses these APIs to encode Unicode object into bytes object. So it is easy to fix it. [2]_ Alternative APIs ================ There are alternative APIs to accept ``PyObject *unicode`` instead of ``Py_UNICODE *``. Users can migrate to them. ========================================= ========================================== Deprecated API Alternative APIs ========================================= ========================================== ``PyUnicode_Encode()`` ``PyUnicode_AsEncodedString()`` ``PyUnicode_EncodeASCII()`` ``PyUnicode_AsASCIIString()`` \(1) ``PyUnicode_EncodeLatin1()`` ``PyUnicode_AsLatin1String()`` \(1) ``PyUnicode_EncodeUTF7()`` \(2) ``PyUnicode_EncodeUTF8()`` ``PyUnicode_AsUTF8String()`` \(1) ``PyUnicode_EncodeUTF16()`` ``PyUnicode_AsUTF16String()`` \(3) ``PyUnicode_EncodeUTF32()`` ``PyUnicode_AsUTF32String()`` \(3) ``PyUnicode_EncodeUnicodeEscape()`` ``PyUnicode_AsUnicodeEscapeString()`` ``PyUnicode_EncodeRawUnicodeEscape()`` ``PyUnicode_AsRawUnicodeEscapeString()`` ``PyUnicode_EncodeCharmap()`` ``PyUnicode_AsCharmapString()`` \(1) ``PyUnicode_TranslateCharmap()`` ``PyUnicode_Translate()`` ``PyUnicode_EncodeDecimal()`` \(4) ``PyUnicode_TransformDecimalToASCII()`` \(4) ========================================= ========================================== Notes: (1) ``const char *errors`` parameter is missing. (2) There is no public alternative API. But user can use generic ``PyUnicode_AsEncodedString()`` instead. (3) ``const char *errors, int byteorder`` parameters are missing. (4) There is no direct replacement. But ``Py_UNICODE_TODECIMAL`` can be used instead. CPython uses ``_PyUnicode_TransformDecimalAndSpaceToASCII`` for converting from Unicode to numbers instead. Plan ==== Python 3.9 ---------- Add ``Py_DEPRECATED(3.3)`` to following APIs. This change is committed already [3]_. All other APIs have been marked ``Py_DEPRECATED(3.3)`` already. * ``PyUnicode_EncodeDecimal()`` * ``PyUnicode_TransformDecimalToASCII()``. Document all APIs as "will be removed in version 3.11". Python 3.11 ----------- These APIs are removed. * ``PyUnicode_Encode()`` * ``PyUnicode_EncodeASCII()`` * ``PyUnicode_EncodeLatin1()`` * ``PyUnicode_EncodeUTF7()`` * ``PyUnicode_EncodeUTF8()`` * ``PyUnicode_EncodeUTF16()`` * ``PyUnicode_EncodeUTF32()`` * ``PyUnicode_EncodeUnicodeEscape()`` * ``PyUnicode_EncodeRawUnicodeEscape()`` * ``PyUnicode_EncodeCharmap()`` * ``PyUnicode_TranslateCharmap()`` * ``PyUnicode_EncodeDecimal()`` * ``PyUnicode_TransformDecimalToASCII()`` Alternative ideas ================= Instead of just removing deprecated APIs, we may be able to use thier names with different signature. Make some private APIs public ------------------------------ ``PyUnicode_EncodeUTF7()`` doesn't have public alternative APIs. Some APIs have alternative public APIs. But they are missing ``const char *errors`` or ``int byteorder`` parameters. We can rename some private APIs and make them public to cover missing APIs and parameters. ============================= ================================ Rename to Rename from ============================= ================================ ``PyUnicode_EncodeASCII()`` ``_PyUnicode_AsASCIIString()`` ``PyUnicode_EncodeLatin1()`` ``_PyUnicode_AsLatin1String()`` ``PyUnicode_EncodeUTF7()`` ``_PyUnicode_EncodeUTF7()`` ``PyUnicode_EncodeUTF8()`` ``_PyUnicode_AsUTF8String()`` ``PyUnicode_EncodeUTF16()`` ``_PyUnicode_EncodeUTF16()`` ``PyUnicode_EncodeUTF32()`` ``_PyUnicode_EncodeUTF32()`` ============================= ================================ Pros: * We have more consistent API set. Cons: * We have more public APIs to maintain. * Existing public APIs are enough for most use cases, and ``PyUnicode_AsEncodedString()`` can be used in other cases. Replace ``Py_UNICODE*`` with ``Py_UCS4*`` ----------------------------------------- We can replace ``Py_UNICODE`` (typedef of ``wchar_t``) with ``Py_UCS4``. Since builtin codecs support UCS-4, we don't need to convert ``Py_UCS4*`` string to Unicode object. Pros: * We have more consistent API set. * User can encode UCS-4 string in C without creating Unicode object. Cons: * We have more public APIs to maintain. * Applications which uses UTF-8 or UTF-32 can not use these APIs anyway. * Other Python implementations may not have builtin codec for UCS-4. * If we change the Unicode internal representation to UTF-8, we need to keep UCS-4 support only for these APIs. Replace ``Py_UNICODE*`` with ``wchar_t*`` ----------------------------------------- We can replace ``Py_UNICODE`` to ``wchar_t``. Pros: * We have more consistent API set. * Backward compatible. Cons: * We have more public APIs to maintain. * They are inefficient on platforms ``wchar_t*`` is UTF-16. It is because built-in codecs supports only UCS-1, UCS-2, and UCS-4 input. Rejected ideas ============== Using runtime warning --------------------- These APIs doesn't release GIL for now. Emitting a warning from such APIs is not safe. See this example. .. code-block:: PyObject *u = PyList_GET_ITEM(list, i); // u is borrowed reference. PyObject *b = PyUnicode_EncodeUTF8(PyUnicode_AS_UNICODE(u), PyUnicode_GET_SIZE(u), NULL); // Assumes u is still living reference. PyObject *t = PyTuple_Pack(2, u, b); Py_DECREF(b); return t; If we emit Python warning from ``PyUnicode_EncodeUTF8()``, warning filters and other threads may change the ``list`` and ``u`` can be a dangling reference after ``PyUnicode_EncodeUTF8()`` returned. Additionally, since we are not changing behavior but removing C APIs, runtime ``DeprecationWarning`` might not helpful for Python developers. We should warn to extension developers instead. Discussions =========== * `Plan to remove Py_UNICODE APis except PEP 623 <https://mail.python.org/archives/list/python-dev@python.org/thread/S7KW2U6I…>`_ * `bpo-41123: Remove Py_UNICODE APIs except PEP 623: <https://bugs.python.org/issue41123>`_ References ========== .. [1] Source package list chosen from top 4000 PyPI packages. (https://github.com/methane/notes/blob/master/2020/wchar-cache/package_list.…) .. [2] pyodbc -- Don't use PyUnicode_Encode API #792 (https://github.com/mkleehammer/pyodbc/pull/792) .. [3] Uncomment Py_DEPRECATED for Py_UNICODE APIs (GH-21318) (https://github.com/python/cpython/commit/9c3840870814493fed62e140cfa43c2883…) Copyright ========= This document has been placed in the public domain. -- Inada Naoki <songofacandy(a)gmail.com>

3 months

Improve CPython tracing performance

by Fabio Zadrozny

Hi all, Right now, when a debugger is active, the number of local variables can affect the tracing speed quite a lot. For instance, having tracing setup in a program such as the one below takes 4.64 seconds to run, yet, changing all the variables to have the same name -- i.e.: change all assignments to `a = 1` (such that there's only a single variable in the namespace), it takes 1.47 seconds (in my machine)... the higher the number of variables, the slower the tracing becomes. ``` import time t = time.time() def call(): a = 1 b = 1 c = 1 d = 1 e = 1 f = 1 def noop(frame, event, arg): return noop import sys sys.settrace(noop) for i in range(1_000_000): call() print('%.2fs' % (time.time() - t,)) ``` This happens because `PyFrame_FastToLocalsWithError` and `PyFrame_LocalsToFast` are called inside the `call_trampoline` ( https://github.com/python/cpython/blob/master/Python/sysmodule.c#L946). So, I'd like to simply remove those calls. Debuggers can call `PyFrame_LocalsToFast` when needed -- otherwise mutating non-current frames doesn't work anyways. As a note, pydevd already has such a call: https://github.com/fabioz/PyDev.Debugger/blob/0d4d210f01a1c0a8647178b2e665b… and PyPy also has a counterpart. As for `PyFrame_FastToLocalsWithError`, I don't really see any reason to call it at all. i.e.: something as the code below prints the `a` variable from the `main()` frame regardless of that and I checked all pydevd tests and nothing seems to be affected (it seems that accessing f_locals already does this: https://github.com/python/cpython/blob/cb9879b948a19c9434316f8ab6aba9c4601a…, so, I don't see much reason to call it at all). ``` def call(): import sys frame = sys._getframe() print(frame.f_back.f_locals) def main(): a = 1 call() if __name__ == '__main__': main() ``` Does anyone see any issue with this? If it's non controversial, is a PEP needed or just an issue to track it would be enough to remove those 2 lines? Thanks, Fabio

3 months, 1 week

pathlib.Path: inconsistent symlink_to() and link_to()

by Barney Gale

Hi, Pathlib's symlink_to() and link_to() methods have different argument orders, so: a.symlink_to(b) # Creates a symlink from A to B a.link_to(b) # Creates a hard link from B to A I don't think link_to() was intended to be implemented this way, as the docs say "Create a hard link pointing to a path named target.". It's also inconsistent with everything else in pathlib, most obviously symlink_to(). Bug report here: https://bugs.python.org/issue39291 This /really/ irks me. Apparently it's too late to fix link_to(), so I'd like to suggest we add a new hardlink_to() method that matches the symlink_to() argument order. link_to() then becomes deprecated/undocumented. Any thoughts? Barney

3 months, 2 weeks

Thoughts on PEP 634 (Structural Pattern Matching)

by Mark Shannon

Hi everyone, PEP 634/5/6 presents a possible implementation of pattern matching for Python. Much of the discussion around PEP 634, and PEP 622 before it, seems to imply that PEP 634 is synonymous with pattern matching; that if you reject PEP 634 then you are rejecting pattern matching. That simply isn't true. Can we discuss whether we want pattern matching in Python and the broader semantics first, before dealing with low level details? Do we want pattern matching in Python at all? --------------------------------------------- Pattern matching works really well in statically typed, functional languages. The lack of mutability, constrained scope and the ability of the compiler to distinguish let variables from constants means that pattern matching code has fewer errors, and can be compiled efficiently. Pattern matching works less well in dynamically-typed, functional languages and statically-typed, procedural languages. Nevertheless, it works well enough for it to be a popular feature in both erlang and rust. In dynamically-typed, procedural languages, however, it is not clear (at least not to me) that it works well enough to be worthwhile. That is not say that pattern matching could never be of value in Python, but PEP 635 fails to demonstrate that it can (although it does a better job than PEP 622). Should match be an expression, or a statement? ---------------------------------------------- Do we want a fancy switch statement, or a powerful expression? Expressions have the advantage of not leaking (like comprehensions in Python 3), but statements are easier to work with. Can pattern matching make it clear what is assigned? ---------------------------------------------------- Embedding the variables to be assigned into a pattern, makes the pattern concise, but requires discarding normal Python syntax and inventing a new sub-language. Could we make patterns fit Python better? Is it possible to make assignment to variables clear, and unambiguous, and allow the use of symbolic constants at the same time? I think it is, but PEP 634 fails to do this. How should pattern matching be integrated with the object model? ---------------------------------------------------------------- What special method(s) should be added? How and when should they be called? PEP 634 largely disregards the object model, meaning it has many special cases, and is inefficient. The semantics must be well defined. ----------------------------------- Language extensions PEPs should define the semantics of those extensions. For example, PEP 343 and PEP 380 both did. https://www.python.org/dev/peps/pep-0343/#specification-the-with-statement https://www.python.org/dev/peps/pep-0380/#formal-semantics PEP 634 just waves its hands and talks about undefined behavior, which horrifies me. In summary, I would ask anyone who wants pattern matching adding to Python, to not support PEP 634. PEP 634 just isn't a good fit for Python, and we deserve something better. Cheers, Mark.

4 months, 1 week

Ideas for improving the contribution experience

by Tal Einat

(Context: Continuing to prepare for the core dev sprint next week. Since the sprint is near, *I'd greatly appreciate any quick comments, feedback and ideas!*) Following up my collection of past beginning contributor experiences, I've collected these experiences in a dedicated GitHub repo[1] and written a (subjective!) summary of main themes that I recognize in the stories, which I've also included in the repo[2]. A "TL;DR" bullet list of those main themes: * Slow/no responsiveness * Long, slow process * Hard to find where to contribute * Mentorship helps a lot, but is scarce * A lot to learn to get started * It's intimidating More specifically, something that has come up often is that maintaining momentum for new contributors is crucial for them to become long-term contributors. Most often, this comes up in relation to the first two points: Suggestions or PRs are completely receive no attention at all ("ignored") or stop receiving attention at some point ("lost to the void"). Unfortunately, the probability of this is pretty high for any issue/PR, so for a new contributor this is almost guaranteed to happen while working on one of their first few contributions. I've seen this happen many times, and have found that I have to personally follow promising contributors' work to ensure that this doesn't happen to them. I've also seen contributors learn to actively seek out core devs when these situations arise, which is often a successful tactic, but shouldn't be necessary so often. Now, this is in large part a result of the fact that us core devs are not a very large group, made up almost entirely of volunteers working on this in their spare time. Last I checked, the total amount of paid development time dedicated to developing Python is less than 3 full-time (i.e. ~100 hours a week). The situation being problematic is clear enough that the PSF had concrete plans to hire paid developers to review issues and PRs. However, those plans have been put on hold indefinitely, since the PSF's funding has shrunk dramatically since the COVID-19 outbreak (no PyCon!). So, what can be done? Besides raising more funds (see a note on this below), I think we can find ways to reduce how often issues/PRs become "stalled". Here are some ideas: 1. *Generate reminders for reviewers when an issue or PR becomes "stalled' due to them.* Personally, I've found that both b.p.o. and GitHub make it relatively hard to remember to follow up on all of the many issues/PRs you've taken part in reviewing. It takes considerable attention and discipline to do so consistently, and reminders like these would have helped me. Many (many!) times, all it took to get an issue/PR moving forward (or closed) was a simple "ping?" comment. 2. *Generate reminders for contributors when an issue or PR becomes "stalled" due to them.* Similar to the above, but I consider these separate. 3. *Advertise something like a "2-for-1" standing offer for reviews.* This would give contributors an "official", acceptable way to get attention for their issue/PR, other than "begging" for attention on a mailing list. There are good ways for new contributors to be of significant help despite being new to the project, such as checking whether old bugs are still relevant, searching for duplicate issues, or applying old patches to the current code and creating a PR. (This would be similar to Martin v. Löwis's 5-for-1 offer in 2012[3], which had little success but lead to some interesting followup discussion[4]). 4. *Encourage core devs to dedicate some of their time to working through issues/PRs which are "ignored" or "stalled".* This would require first generating reliable lists of issues/PRs in such states. This could be in various forms, such as predefined GitHub/b.p.o. queries, a dedicated web-page, a periodic message similar to b.p.o.'s "weekly summary" email, or dedicated tags/labels for issues/PRs. (Perhaps prioritize "stalled" over "ignored".) - Tal Einat [1]: https://github.com/taleinat/python-contribution-feedback [2]: https://github.com/taleinat/python-contribution-feedback/blob/master/Takeaw… [3]: https://mail.python.org/archives/list/python-dev@python.org/message/7DLUN4Y… [4]: https://mail.python.org/archives/list/python-dev@python.org/thread/N4MMHXXO…

5 months

PEP 642: Constraint Pattern Syntax for Structural Pattern Matching

by Nick Coghlan

Hi folks, This is a mailing list repost of the Discourse thread at https://discuss.python.org/t/pep-642-constraint-pattern-syntax-for-structur… The rendered version of the PEP can be found here: https://www.python.org/dev/peps/pep-0642/ The full text is also quoted in the Discourse thread. The remainder of this email is the same introduction that I posted on Discourse. I’m largely a fan of the Structural Pattern Matching proposal in PEP 634, but there’s one specific piece of the syntax proposal that I strongly dislike: the idea of basing the distinction between capture patterns and value patterns purely on whether they use a simple name or a dotted name. Thus PEP 642, which retains most of PEP 634 unchanged, but adjusts value checks to use an explicit prefix syntax (either `?EXPR` for equality constraints, or `?is EXPR` for identity constraints), rather than relying on users learning that literals and attribute lookups in a capture pattern mean a value lookup check, while simple names mean a capture pattern (unlike both normal expressions, where all three mean a value lookup, and assignment targets, where both simple and dotted names bind a new reference). The PEP itself has a lot of words explaining why I’ve made the design decisions I have, as well as the immediate and potential future benefits offered by using an explicit prefix syntax for value constraints, but the super short form goes like this: * if you don’t like match statements at all, or wish we were working on designing a C-style switch statement instead, then PEP 642 isn’t going to appeal to you any more than PEP 634 does * if, like me, you don’t like the idea of breaking the existing property of Python that caching the result of a value lookup subexpression in a local variable and then using that variable in place of the original subexpression should “just work”, then PEP 642’s explicit constraint prefix syntax may be more to your liking * however, if the idea of the `?` symbol becoming part of Python’s syntax doesn’t appeal to you, then you may consider any improved clarity of intent that PEP 642 might offer to not be worth that cost Cheers, Nick. -- Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia

5 months, 2 weeks

Steering Council Election Timeline for 2021 term

by Ewa Jodlowska

Hi! The timeline for this year's election will be the same as last year. * The nomination period will begin Nov 1, 2020 (do not post nominations until then) * Nomination period will end Nov 15, 2020 * Voting will begin Dec 1, 2020 * Voting will end Dec 15, 2020 Nominations will be collected via https://discuss.python.org/ (more details to follow on Nov 1). New for this year: Ernest W. Durbin III will be running the vote along with the assistance of Joe Carey, a PSF employee. They will be co-admins going forward. I have cc'ed them in on this thread as well in case there are any questions. Thanks, Ewa

5 months, 3 weeks

When to remove BytesWarning?

by Inada Naoki

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>

5 months, 3 weeks

Re: Drop Solaris, OpenSolaris, Illumos and OpenIndiana support in Python

by Garrett D'Amore

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

6 months

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Python-Dev October 2020