Mailman 3 March 2020 - Python-Dev

Mixed Python/C debugging
by Skip Montanaro 26 Apr '23

26 Apr '23

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

5 6

PEP 448 review
by Guido van Rossum 30 Mar '23

30 Mar '23

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)

7 17

PEP 1, PEP Purpose and Guidelines
by barry＠zope.com 18 May '21

18 May '21

It has been a while since I posted a copy of PEP 1 to the mailing lists and newsgroups. I've recently done some updating of a few sections, so in the interest of gaining wider community participation in the Python development process, I'm posting the latest revision of PEP 1 here. A version of the PEP is always available on-line at http://www.python.org/peps/pep-0001.html Enjoy, -Barry -------------------- snip snip -------------------- PEP: 1 Title: PEP Purpose and Guidelines Version: $Revision: 1.36 $ Last-Modified: $Date: 2002/07/29 18:34:59 $ Author: Barry A. Warsaw, Jeremy Hylton Status: Active Type: Informational Created: 13-Jun-2000 Post-History: 21-Mar-2001, 29-Jul-2002 What is a PEP? PEP stands for Python Enhancement Proposal. A PEP is a design document providing information to the Python community, or describing a new feature for Python. The PEP should provide a concise technical specification of the feature and a rationale for the feature. We intend PEPs to be the primary mechanisms for proposing new features, for collecting community input on an issue, and for documenting the design decisions that have gone into Python. The PEP author is responsible for building consensus within the community and documenting dissenting opinions. Because the PEPs are maintained as plain text files under CVS control, their revision history is the historical record of the feature proposal[1]. Kinds of PEPs There are two kinds of PEPs. A standards track PEP describes a new feature or implementation for Python. An informational PEP describes a Python design issue, or provides general guidelines or information to the Python community, but does not propose a new feature. Informational PEPs do not necessarily represent a Python community consensus or recommendation, so users and implementors are free to ignore informational PEPs or follow their advice. PEP Work Flow The PEP editor, Barry Warsaw <peps(a)python.org>, assigns numbers for each PEP and changes its status. The PEP process begins with a new idea for Python. It is highly recommended that a single PEP contain a single key proposal or new idea. The more focussed the PEP, the more successfully it tends to be. The PEP editor reserves the right to reject PEP proposals if they appear too unfocussed or too broad. If in doubt, split your PEP into several well-focussed ones. Each PEP must have a champion -- someone who writes the PEP using the style and format described below, shepherds the discussions in the appropriate forums, and attempts to build community consensus around the idea. The PEP champion (a.k.a. Author) should first attempt to ascertain whether the idea is PEP-able. Small enhancements or patches often don't need a PEP and can be injected into the Python development work flow with a patch submission to the SourceForge patch manager[2] or feature request tracker[3]. The PEP champion then emails the PEP editor <peps(a)python.org> with a proposed title and a rough, but fleshed out, draft of the PEP. This draft must be written in PEP style as described below. If the PEP editor approves, he will assign the PEP a number, label it as standards track or informational, give it status 'draft', and create and check-in the initial draft of the PEP. The PEP editor will not unreasonably deny a PEP. Reasons for denying PEP status include duplication of effort, being technically unsound, not providing proper motivation or addressing backwards compatibility, or not in keeping with the Python philosophy. The BDFL (Benevolent Dictator for Life, Guido van Rossum) can be consulted during the approval phase, and is the final arbitrator of the draft's PEP-ability. If a pre-PEP is rejected, the author may elect to take the pre-PEP to the comp.lang.python newsgroup (a.k.a. python-list(a)python.org mailing list) to help flesh it out, gain feedback and consensus from the community at large, and improve the PEP for re-submission. The author of the PEP is then responsible for posting the PEP to the community forums, and marshaling community support for it. As updates are necessary, the PEP author can check in new versions if they have CVS commit permissions, or can email new PEP versions to the PEP editor for committing. Standards track PEPs consists of two parts, a design document and a reference implementation. The PEP should be reviewed and accepted before a reference implementation is begun, unless a reference implementation will aid people in studying the PEP. Standards Track PEPs must include an implementation - in the form of code, patch, or URL to same - before it can be considered Final. PEP authors are responsible for collecting community feedback on a PEP before submitting it for review. A PEP that has not been discussed on python-list(a)python.org and/or python-dev(a)python.org will not be accepted. However, wherever possible, long open-ended discussions on public mailing lists should be avoided. Strategies to keep the discussions efficient include, setting up a separate SIG mailing list for the topic, having the PEP author accept private comments in the early design phases, etc. PEP authors should use their discretion here. Once the authors have completed a PEP, they must inform the PEP editor that it is ready for review. PEPs are reviewed by the BDFL and his chosen consultants, who may accept or reject a PEP or send it back to the author(s) for revision. Once a PEP has been accepted, the reference implementation must be completed. When the reference implementation is complete and accepted by the BDFL, the status will be changed to `Final.' A PEP can also be assigned status `Deferred.' The PEP author or editor can assign the PEP this status when no progress is being made on the PEP. Once a PEP is deferred, the PEP editor can re-assign it to draft status. A PEP can also be `Rejected'. Perhaps after all is said and done it was not a good idea. It is still important to have a record of this fact. PEPs can also be replaced by a different PEP, rendering the original obsolete. This is intended for Informational PEPs, where version 2 of an API can replace version 1. PEP work flow is as follows: Draft -> Accepted -> Final -> Replaced ^ +----> Rejected v Deferred Some informational PEPs may also have a status of `Active' if they are never meant to be completed. E.g. PEP 1. What belongs in a successful PEP? Each PEP should have the following parts: 1. Preamble -- RFC822 style headers containing meta-data about the PEP, including the PEP number, a short descriptive title (limited to a maximum of 44 characters), the names, and optionally the contact info for each author, etc. 2. Abstract -- a short (~200 word) description of the technical issue being addressed. 3. Copyright/public domain -- Each PEP must either be explicitly labelled as placed in the public domain (see this PEP as an example) or licensed under the Open Publication License[4]. 4. Specification -- The technical specification should describe the syntax and semantics of any new language feature. The specification should be detailed enough to allow competing, interoperable implementations for any of the current Python platforms (CPython, JPython, Python .NET). 5. Motivation -- The motivation is critical for PEPs that want to change the Python language. It should clearly explain why the existing language specification is inadequate to address the problem that the PEP solves. PEP submissions without sufficient motivation may be rejected outright. 6. Rationale -- The rationale fleshes out the specification by describing what motivated the design and why particular design decisions were made. It should describe alternate designs that were considered and related work, e.g. how the feature is supported in other languages. The rationale should provide evidence of consensus within the community and discuss important objections or concerns raised during discussion. 7. Backwards Compatibility -- All PEPs that introduce backwards incompatibilities must include a section describing these incompatibilities and their severity. The PEP must explain how the author proposes to deal with these incompatibilities. PEP submissions without a sufficient backwards compatibility treatise may be rejected outright. 8. Reference Implementation -- The reference implementation must be completed before any PEP is given status 'Final,' but it need not be completed before the PEP is accepted. It is better to finish the specification and rationale first and reach consensus on it before writing code. The final implementation must include test code and documentation appropriate for either the Python language reference or the standard library reference. PEP Template PEPs are written in plain ASCII text, and should adhere to a rigid style. There is a Python script that parses this style and converts the plain text PEP to HTML for viewing on the web[5]. PEP 9 contains a boilerplate[7] template you can use to get started writing your PEP. Each PEP must begin with an RFC822 style header preamble. The headers must appear in the following order. Headers marked with `*' are optional and are described below. All other headers are required. PEP: <pep number> Title: <pep title> Version: <cvs version string> Last-Modified: <cvs date string> Author: <list of authors' real names and optionally, email addrs> * Discussions-To: <email address> Status: <Draft | Active | Accepted | Deferred | Final | Replaced> Type: <Informational | Standards Track> * Requires: <pep numbers> Created: <date created on, in dd-mmm-yyyy format> * Python-Version: <version number> Post-History: <dates of postings to python-list and python-dev> * Replaces: <pep number> * Replaced-By: <pep number> The Author: header lists the names and optionally, the email addresses of all the authors/owners of the PEP. The format of the author entry should be address(a)dom.ain (Random J. User) if the email address is included, and just Random J. User if the address is not given. If there are multiple authors, each should be on a separate line following RFC 822 continuation line conventions. Note that personal email addresses in PEPs will be obscured as a defense against spam harvesters. Standards track PEPs must have a Python-Version: header which indicates the version of Python that the feature will be released with. Informational PEPs do not need a Python-Version: header. While a PEP is in private discussions (usually during the initial Draft phase), a Discussions-To: header will indicate the mailing list or URL where the PEP is being discussed. No Discussions-To: header is necessary if the PEP is being discussed privately with the author, or on the python-list or python-dev email mailing lists. Note that email addresses in the Discussions-To: header will not be obscured. Created: records the date that the PEP was assigned a number, while Post-History: is used to record the dates of when new versions of the PEP are posted to python-list and/or python-dev. Both headers should be in dd-mmm-yyyy format, e.g. 14-Aug-2001. PEPs may have a Requires: header, indicating the PEP numbers that this PEP depends on. PEPs may also have a Replaced-By: header indicating that a PEP has been rendered obsolete by a later document; the value is the number of the PEP that replaces the current document. The newer PEP must have a Replaces: header containing the number of the PEP that it rendered obsolete. PEP Formatting Requirements PEP headings must begin in column zero and the initial letter of each word must be capitalized as in book titles. Acronyms should be in all capitals. The body of each section must be indented 4 spaces. Code samples inside body sections should be indented a further 4 spaces, and other indentation can be used as required to make the text readable. You must use two blank lines between the last line of a section's body and the next section heading. You must adhere to the Emacs convention of adding two spaces at the end of every sentence. You should fill your paragraphs to column 70, but under no circumstances should your lines extend past column 79. If your code samples spill over column 79, you should rewrite them. Tab characters must never appear in the document at all. A PEP should include the standard Emacs stanza included by example at the bottom of this PEP. A PEP must contain a Copyright section, and it is strongly recommended to put the PEP in the public domain. When referencing an external web page in the body of a PEP, you should include the title of the page in the text, with a footnote reference to the URL. Do not include the URL in the body text of the PEP. E.g. Refer to the Python Language web site [1] for more details. ... [1] http://www.python.org When referring to another PEP, include the PEP number in the body text, such as "PEP 1". The title may optionally appear. Add a footnote reference that includes the PEP's title and author. It may optionally include the explicit URL on a separate line, but only in the References section. Note that the pep2html.py script will calculate URLs automatically, e.g.: ... Refer to PEP 1 [7] for more information about PEP style ... References [7] PEP 1, PEP Purpose and Guidelines, Warsaw, Hylton http://www.python.org/peps/pep-0001.html If you decide to provide an explicit URL for a PEP, please use this as the URL template: http://www.python.org/peps/pep-xxxx.html PEP numbers in URLs must be padded with zeros from the left, so as to be exactly 4 characters wide, however PEP numbers in text are never padded. Reporting PEP Bugs, or Submitting PEP Updates How you report a bug, or submit a PEP update depends on several factors, such as the maturity of the PEP, the preferences of the PEP author, and the nature of your comments. For the early draft stages of the PEP, it's probably best to send your comments and changes directly to the PEP author. For more mature, or finished PEPs you may want to submit corrections to the SourceForge bug manager[6] or better yet, the SourceForge patch manager[2] so that your changes don't get lost. If the PEP author is a SF developer, assign the bug/patch to him, otherwise assign it to the PEP editor. When in doubt about where to send your changes, please check first with the PEP author and/or PEP editor. PEP authors who are also SF committers, can update the PEPs themselves by using "cvs commit" to commit their changes. Remember to also push the formatted PEP text out to the web by doing the following: % python pep2html.py -i NUM where NUM is the number of the PEP you want to push out. See % python pep2html.py --help for details. Transferring PEP Ownership It occasionally becomes necessary to transfer ownership of PEPs to a new champion. In general, we'd like to retain the original author as a co-author of the transferred PEP, but that's really up to the original author. A good reason to transfer ownership is because the original author no longer has the time or interest in updating it or following through with the PEP process, or has fallen off the face of the 'net (i.e. is unreachable or not responding to email). A bad reason to transfer ownership is because you don't agree with the direction of the PEP. We try to build consensus around a PEP, but if that's not possible, you can always submit a competing PEP. If you are interested assuming ownership of a PEP, send a message asking to take over, addressed to both the original author and the PEP editor <peps(a)python.org>. If the original author doesn't respond to email in a timely manner, the PEP editor will make a unilateral decision (it's not like such decisions can be reversed. :). References and Footnotes [1] This historical record is available by the normal CVS commands for retrieving older revisions. For those without direct access to the CVS tree, you can browse the current and past PEP revisions via the SourceForge web site at http://cvs.sourceforge.net/cgi-bin/cvsweb.cgi/python/nondist/peps/?cvsroot=… [2] http://sourceforge.net/tracker/?group_id=5470&atid=305470 [3] http://sourceforge.net/tracker/?atid=355470&group_id=5470&func=browse [4] http://www.opencontent.org/openpub/ [5] The script referred to here is pep2html.py, which lives in the same directory in the CVS tree as the PEPs themselves. Try "pep2html.py --help" for details. The URL for viewing PEPs on the web is http://www.python.org/peps/ [6] http://sourceforge.net/tracker/?group_id=5470&atid=305470 [7] PEP 9, Sample PEP Template http://www.python.org/peps/pep-0009.html Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End:

8 14

Boundaries between numbers and identifiers
by Serhiy Storchaka 15 Apr '21

15 Apr '21

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.

8 10

pathlib.Path: inconsistent symlink_to() and link_to()
by Barney Gale 22 Jan '21

22 Jan '21

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

4 5

The Python 2 death march
by Benjamin Peterson 16 Apr '20

16 Apr '20

Hi all, It's finally time to schedule the last releases in Python 2's life. There will be two more releases of Python 2.7: Python 2.7.17 and Python 2.7.18. Python 2.7.17 release candidate 1 will happen on October 5th followed by the final release on October 19th. I'm going to time Python 2.7.18 to coincide with PyCon 2020 in April, so attendees can enjoy some collective catharsis. We'll still say January 1st is the official EOL date. Thanks to Sumana Harihareswara, there's now a FAQ about the Python 2 sunset on the website: https://www.python.org/doc/sunset-python-2/ Regards, Benjamin

24 38

PEP 616 -- String methods to remove prefixes and suffixes
by Dennis Sweeney 03 Apr '20

03 Apr '20

Browser Link: https://www.python.org/dev/peps/pep-0616/ PEP: 616 Title: String methods to remove prefixes and suffixes Author: Dennis Sweeney <sweeney.dennis650(a)gmail.com> Sponsor: Eric V. Smith <eric(a)trueblade.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 19-Mar-2020 Python-Version: 3.9 Post-History: 30-Aug-2002 Abstract ======== This is a proposal to add two new methods, ``cutprefix`` and ``cutsuffix``, to the APIs of Python's various string objects. In particular, the methods would be added to Unicode ``str`` objects, binary ``bytes`` and ``bytearray`` objects, and ``collections.UserString``. If ``s`` is one these objects, and ``s`` has ``pre`` as a prefix, then ``s.cutprefix(pre)`` returns a copy of ``s`` in which that prefix has been removed. If ``s`` does not have ``pre`` as a prefix, an unchanged copy of ``s`` is returned. In summary, ``s.cutprefix(pre)`` is roughly equivalent to ``s[len(pre):] if s.startswith(pre) else s``. The behavior of ``cutsuffix`` is analogous: ``s.cutsuffix(suf)`` is roughly equivalent to ``s[:-len(suf)] if suf and s.endswith(suf) else s``. Rationale ========= There have been repeated issues [#confusion]_ on the Bug Tracker and StackOverflow related to user confusion about the existing ``str.lstrip`` and ``str.rstrip`` methods. These users are typically expecting the behavior of ``cutprefix`` and ``cutsuffix``, but they are surprised that the parameter for ``lstrip`` is interpreted as a set of characters, not a substring. This repeated issue is evidence that these methods are useful, and the new methods allow a cleaner redirection of users to the desired behavior. As another testimonial for the usefulness of these methods, several users on Python-Ideas [#pyid]_ reported frequently including similar functions in their own code for productivity. The implementation often contained subtle mistakes regarding the handling of the empty string (see `Specification`_). Specification ============= The builtin ``str`` class will gain two new methods with roughly the following behavior:: def cutprefix(self: str, pre: str, /) -> str: if self.startswith(pre): return self[len(pre):] return self[:] def cutsuffix(self: str, suf: str, /) -> str: if suf and self.endswith(suf): return self[:-len(suf)] return self[:] The only difference between the real implementation and the above is that, as with other string methods like ``replace``, the methods will raise a ``TypeError`` if any of ``self``, ``pre`` or ``suf`` is not an instace of ``str``, and will cast subclasses of ``str`` to builtin ``str`` objects. Note that without the check for the truthyness of ``suf``, ``s.cutsuffix('')`` would be mishandled and always return the empty string due to the unintended evaluation of ``self[:-0]``. Methods with the corresponding semantics will be added to the builtin ``bytes`` and ``bytearray`` objects. If ``b`` is either a ``bytes`` or ``bytearray`` object, then ``b.cutsuffix()`` and ``b.cutprefix()`` will accept any bytes-like object as an argument. Note that the ``bytearray`` methods return a copy of ``self``; they do not operate in place. The following behavior is considered a CPython implementation detail, but is not guaranteed by this specification:: >>> x = 'foobar' * 10**6 >>> x.cutprefix('baz') is x is x.cutsuffix('baz') True >>> x.cutprefix('') is x is x.cutsuffix('') True That is, for CPython's immutable ``str`` and ``bytes`` objects, the methods return the original object when the affix is not found or if the affix is empty. Because these types test for equality using shortcuts for identity and length, the following equivalent expressions are evaluated at approximately the same speed, for any ``str`` objects (or ``bytes`` objects) ``x`` and ``y``:: >>> (True, x[len(y):]) if x.startswith(y) else (False, x) >>> (True, z) if x != (z := x.cutprefix(y)) else (False, x) The two methods will also be added to ``collections.UserString``, where they rely on the implementation of the new ``str`` methods. Motivating examples from the Python standard library ==================================================== The examples below demonstrate how the proposed methods can make code one or more of the following: Less fragile: The code will not depend on the user to count the length of a literal. More performant: The code does not require a call to the Python built-in ``len`` function. More descriptive: The methods give a higher-level API for code readability, as opposed to the traditional method of string slicing. refactor.py ----------- - Current:: if fix_name.startswith(self.FILE_PREFIX): fix_name = fix_name[len(self.FILE_PREFIX):] - Improved:: fix_name = fix_name.cutprefix(self.FILE_PREFIX) c_annotations.py: ----------------- - Current:: if name.startswith("c."): name = name[2:] - Improved:: name = name.cutprefix("c.") find_recursionlimit.py ---------------------- - Current:: if test_func_name.startswith("test_"): print(test_func_name[5:]) else: print(test_func_name) - Improved:: print(test_finc_name.cutprefix("test_")) deccheck.py ----------- This is an interesting case because the author chose to use the ``str.replace`` method in a situation where only a prefix was intended to be removed. - Current:: if funcname.startswith("context."): self.funcname = funcname.replace("context.", "") self.contextfunc = True else: self.funcname = funcname self.contextfunc = False - Improved:: if funcname.startswith("context."): self.funcname = funcname.cutprefix("context.") self.contextfunc = True else: self.funcname = funcname self.contextfunc = False - Arguably further improved:: self.contextfunc = funcname.startswith("context.") self.funcname = funcname.cutprefix("context.") test_i18n.py ------------ - Current:: if test_func_name.startswith("test_"): print(test_func_name[5:]) else: print(test_func_name) - Improved:: print(test_finc_name.cutprefix("test_")) - Current:: if creationDate.endswith('\\n'): creationDate = creationDate[:-len('\\n')] - Improved:: creationDate = creationDate.cutsuffix('\\n') shared_memory.py ---------------- - Current:: reported_name = self._name if _USE_POSIX and self._prepend_leading_slash: if self._name.startswith("/"): reported_name = self._name[1:] return reported_name - Improved:: if _USE_POSIX and self._prepend_leading_slash: return self._name.cutprefix("/") return self._name build-installer.py ------------------ - Current:: if archiveName.endswith('.tar.gz'): retval = os.path.basename(archiveName[:-7]) if ((retval.startswith('tcl') or retval.startswith('tk')) and retval.endswith('-src')): retval = retval[:-4] - Improved:: if archiveName.endswith('.tar.gz'): retval = os.path.basename(archiveName[:-7]) if retval.startswith(('tcl', 'tk')): retval = retval.cutsuffix('-src') Depending on personal style, ``archiveName[:-7]`` could also be changed to ``archiveName.cutsuffix('.tar.gz')``. test_core.py ------------ - Current:: if output.endswith("\n"): output = output[:-1] - Improved:: output = output.cutsuffix("\n") cookiejar.py ------------ - Current:: def strip_quotes(text): if text.startswith('"'): text = text[1:] if text.endswith('"'): text = text[:-1] return text - Improved:: def strip_quotes(text): return text.cutprefix('"').cutsuffix('"') - Current:: if line.endswith("\n"): line = line[:-1] - Improved:: line = line.cutsuffix("\n") fixdiv.py --------- - Current:: def chop(line): if line.endswith("\n"): return line[:-1] else: return line - Improved:: def chop(line): return line.cutsuffix("\n") test_concurrent_futures.py -------------------------- In the following example, the meaning of the code changes slightly, but in context, it behaves the same. - Current:: if name.endswith(('Mixin', 'Tests')): return name[:-5] elif name.endswith('Test'): return name[:-4] else: return name - Improved:: return name.cutsuffix('Mixin').cutsuffix('Tests').cutsuffix('Test') msvc9compiler.py ---------------- - Current:: if value.endswith(os.pathsep): value = value[:-1] - Improved:: value = value.cutsuffix(os.pathsep) test_pathlib.py --------------- - Current:: self.assertTrue(r.startswith(clsname + '('), r) self.assertTrue(r.endswith(')'), r) inner = r[len(clsname) + 1 : -1] - Improved:: self.assertTrue(r.startswith(clsname + '('), r) self.assertTrue(r.endswith(')'), r) inner = r.cutprefix(clsname + '(').cutsuffix(')') Rejected Ideas ============== Expand the lstrip and rstrip APIs --------------------------------- Because ``lstrip`` takes a string as its argument, it could be viewed as taking an iterable of length-1 strings. The API could therefore be generalized to accept any iterable of strings, which would be successively removed as prefixes. While this behavior would be consistent, it would not be obvious for users to have to call ``'foobar'.cutprefix(('foo,))`` for the common use case of a single prefix. Allow multiple prefixes ----------------------- Some users discussed the desire to be able to remove multiple prefixes, calling, for example, ``s.cutprefix('From: ', 'CC: ')``. However, this adds ambiguity about the order in which the prefixes are removed, especially in cases like ``s.cutprefix('Foo', 'FooBar')``. After this proposal, this can be spelled explicitly as ``s.cutprefix('Foo').cutprefix('FooBar')``. Remove multiple copies of a prefix ---------------------------------- This is the behavior that would be consistent with the aforementioned expansion of the ``lstrip/rstrip`` API -- repeatedly applying the function until the argument is unchanged. This behavior is attainable from the proposed behavior via the following:: >>> s = 'foo' * 100 + 'bar' >>> while s != (s := s.cutprefix("foo")): pass >>> s 'bar' The above can be modififed by chaining multiple ``cutprefix`` calls together to achieve the full behavior of the ``lstrip``/``rstrip`` generalization, while being explicit in the order of removal. While the proposed API could later be extended to include some of these use cases, to do so before any observation of how these methods are used in practice would be premature and may lead to choosing the wrong behavior. Raising an exception when not found ----------------------------------- There was a suggestion that ``s.cutprefix(pre)`` should raise an exception if ``not s.startswith(pre)``. However, this does not match with the behavior and feel of other string methods. There could be ``required=False`` keyword added, but this violates the KISS principle. Alternative Method Names ------------------------ Several alternatives method names have been proposed. Some are listed below, along with commentary for why they should be rejected in favor of ``cutprefix`` (the same arguments hold for ``cutsuffix``) ``ltrim`` "Trim" does in other languages (e.g. JavaScript, Java, Go, PHP) what ``strip`` methods do in Python. ``lstrip(string=...)`` This would avoid adding a new method, but for different behavior, it's better to have two different methods than one method with a keyword argument that select the behavior. ``cut_prefix`` All of the other methods of the string API, e.g. ``str.startswith()``, use ``lowercase`` rather than ``lower_case_with_underscores``. ``cutleft``, ``leftcut``, or ``lcut`` The explicitness of "prefix" is preferred. ``removeprefix``, ``deleteprefix``, ``withoutprefix``, etc. All of these might have been acceptable, but they have more characters than ``cut``. Some suggested that the verb "cut" implies mutability, but the string API already contains verbs like "replace", "strip", "split", and "swapcase". ``stripprefix`` Users may benefit from the mnemonic that "strip" means working with sets of characters, while other methods work with substrings, so re-using "strip" here should be avoided. Reference Implementation ======================== See the pull request on GitHub [#pr]_. References ========== .. [#pr] GitHub pull request with implementation (https://github.com/python/cpython/pull/18939) .. [#pyid] Discussion on Python-Ideas (https://mail.python.org/archives/list/python-ideas@python.org/thread/RJARZS…) .. [#confusion] Comment listing Bug Tracker and StackOverflow issues (https://mail.python.org/archives/list/python-ideas@python.org/message/GRGAF…) Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

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Debug C++
by Leandro Müller 03 Apr '20

03 Apr '20

Hi Are there any away to debug C module during python debug? Att. Leandro Müller

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Moving threadstate to thread-local storage.
by Mark Shannon 02 Apr '20

02 Apr '20

Hi, As an experiment, I thought I would try moving the thread state (what you get from _PyThreadState_GET() ) to TLS. https://github.com/python/cpython/compare/master...markshannon:threadstate_… It works, passing all the tests, and seems sound. It is a small patch (< 50 lines) and doesn't increase the overall code size. My branch is GCC/Clang only, so will need a bit of extra code for Windows. It should only need a few more lines; I haven't done it as I don't have a Windows machine to test it on. This is a *much* cleaner approach to removing the global variable than adding lots of extra parameters all over the place. Cheers, Mark.

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PEP 585 "Type Hinting Generics In Standard Collections" accepted
by Victor Stinner 30 Mar '20

30 Mar '20

Hi, The Python Steering Council accepts PEP 585 "Type Hinting Generics In Standard Collections": https://www.python.org/dev/peps/pep-0585/ Congrats Łukasz Langa for your tenacity! (PEP written one year ago.) Thanks also to everyone who was involved in the discussion to help to get a better PEP ;-) It seems Guido van Rossum is already working on implementing the PEP: * https://github.com/python/cpython/pull/18239 * https://bugs.python.org/issue39481 Victor -- Night gathers, and now my watch begins. It shall not end until my death.

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