Python-ideas March 2016

python-ideas@python.org

85 participants
40 discussions

Specify number of items to allocate for array.array() constructor

by Sven Rahmann

At the moment, the array module of the standard library allows to create arrays of different numeric types and to initialize them from an iterable (eg, another array). What's missing is the possiblity to specify the final size of the array (number of items), especially for large arrays. I'm thinking of suffix arrays (a text indexing data structure) for large texts, eg the human genome and its reverse complement (about 6 billion characters from the alphabet ACGT). The suffix array is a long int array of the same size (8 bytes per number, so it occupies about 48 GB memory). At the moment I am extending an array in chunks of several million items at a time at a time, which is slow and not elegant. The function below also initializes each item in the array to a given value (0 by default). Is there a reason why there the array.array constructor does not allow to simply specify the number of items that should be allocated? (I do not really care about the contents.) Would this be a worthwhile addition to / modification of the array module? My suggestions is to modify array generation in such a way that you could pass an iterator (as now) as second argument, but if you pass a single integer value, it should be treated as the number of items to allocate. Here is my current workaround (which is slow): def filled_array(typecode, n, value=0, bsize=(1<<22)): """returns a new array with given typecode (eg, "l" for long int, as in the array module) with n entries, initialized to the given value (default 0) """ a = array.array(typecode, [value]*bsize) x = array.array(typecode) r = n while r >= bsize: x.extend(a) r -= bsize x.extend([value]*r) return x

7 months

Implicit string literal concatenation considered harmful?

by Guido van Rossum

I just spent a few minutes staring at a bug caused by a missing comma -- I got a mysterious argument count error because instead of foo('a', 'b') I had written foo('a' 'b'). This is a fairly common mistake, and IIRC at Google we even had a lint rule against this (there was also a Python dialect used for some specific purpose where this was explicitly forbidden). Now, with modern compiler technology, we can (and in fact do) evaluate compile-time string literal concatenation with the '+' operator, so there's really no reason to support 'a' 'b' any more. (The reason was always rather flimsy; I copied it from C but the reason why it's needed there doesn't really apply to Python, as it is mostly useful inside macros.) Would it be reasonable to start deprecating this and eventually remove it from the language? -- --Guido van Rossum (python.org/~guido)

2 years, 6 months

https://docs.python.org/fr/ ?

by Julien Palard

o/ The french translation of the Python Documentation [1][2] has translated 20% of the pageviews of docs.python.org. I think it's the right moment to push it do docs.python.org. So there's some questions ! And I'd like feedback. TL;DR (with my personal choices): - URL may be "http://docs.python.org/fr/" - For localized variations of languages we should use dash and lowercase like "docs.python.org/pt-br/" - po files may be hosted on the python's github - existing script to build doc may be patched to build translations - each translations may crosslink to others - untranslated strings may be visually marked as so I also opened: http://bugs.python.org/issue26546. # Chronology, dependencies The only blocking decision here is the URL, (also reviewing my patch ...), with those two, translated docs can be pushed to production, and the other steps can be discussed and applied one by one. # The URL ## CCTLD vs path vs subdomain I think we should use a variation of "docs.python.org/fr/" for simplicity and clarity. I think we should avoid using CCTLDs as they're sometime hard or near impossible to obtain (may cost a lot of time), also some are expensive, so it's time and money we clearly don't need to loose. Last possibility I see is to use a subdomain, like fr.docs.python.org or docs.fr.python.org but I don't think it's the role / responsibility of the sub-domain to do it. So I'm for docs.python.org/LANGUAGE_TAG/ (without moving current documentation inside a /en/). ## Language tag in path ### Dropping the default locale of a language I personally think we should not show the region in case it's redundant: so to use "fr" instead of "fr-FR", "de" instead of "de-DE", but keeping the possibility to use a locale code when it's not redundant like for "pt-br" or "de-AT" (German ('de') as used in Austria ('AT')). I think so because I don't think we'll have a lot of locale variations (like de-AT, fr-CH, fr-CA, ...) so it will be most of the time redundant (visually heavy, longer to type, longer to read) but we'll still need some locale (pt-BR typically). ### gettext VS IETF language tag format gettext goes by using an underscore between language and locale [3] and IETF goes by using a dash [4][5]. As sphinx is using gettext, and gettext uses underscore we may choose underscore too. But URLs are not here to leak the underlying implementation, and the IETF looks like to be the standard way to represent language tags. Also I visually prefer the dash over the underscore, so I'm for the dash here. ### Lower case vs upper case local tag RFC 5646 section-2.1 tells us language tags are not case sensitive, yet ISO3166-1 recommends that country codes (part of the language tag) be capitalized. I personally prefer the all-lowercase one as paths in URLs typically are lowercase. I searched for `inurl:"pt-br"` to see if I'm not too far away from the usage here and usage seems to agree with me, although there's some "pt-BR" in urls. # Where to host the translated files Currently we're hosting the *po* files in the afpy's (Francophone association for python) [6] github [1] but it may make sense to use (in the generation scripts) a more controlled / restricted clone in the python github, at least to have a better view of who can push on the documentation. We may want to choose between aggregating all translations under the same git repository but I don't feel it's useful. # How to Currently, a python script [7] is used to generate `docs.python.org`, I proposed a patch in [8] to make this script clone and build the french translation too, it's a simple and effective way, I don't think we need more ? Any idea welcome. In our side, we have a Makefile [12] to build the translated doc which is only a thin layer on top of the Sphinx Makefile. So my proposed patch to build scripts "just" delegate the build to our Makefile which itself delegate the hard work to the Sphinx Makefile. # Next ? ## Document how to translate Python I think I can (should) write a documentation on "how to start a Python doc translation project" and "how to migrate existing [9][10][11] python doc translation projects to docs.python.org" if french does goes docs.python.org because it may hopefully motivate people to do the same, and I think our structure is a nice way to do it (A Makefile to generate the doc, all versions translated, people mainly working on latest version, scripts to propagating translations to older version, etc...). ## Crosslinking between existing translations Once the translations are on `docs.python.org`, crosslinks may be established so people on a version can be aware of other version, and easily switch to them. I'm not a UI/UX man but I think we may have a select box right before the existing select box about version, on the top-left corner. Right before because it'll reflect the path: /fr/3.5/ -> [select box fr][select box 3.5]. ## Marking as "untranslated, you can help" the untranslated paragraphs The translations will always need work to follow upstream modifications: marking untranslated paragraphs as so may transform the "Oh they suck, this paragraph is not even translated :-(" to "Hey, nice I can help translating that !". There's an opened sphinx-doc ticket to do so [13] but I have not worked on it yet. As previously said I'm real bad at designing user interfaces, so I don't even visualize how I'd like it to be. [1] http://www.afpy.org/doc/python/3.5/ [2] https://github.com/afpy/python_doc_fr [3] https://www.gnu.org/software/gettext/manual/html_node/Locale-Names.html [4] http://tools.ietf.org/html/rfc5646 [5] https://en.wikipedia.org/wiki/IETF_language_tag [6] http://www.afpy.org/ [7] https://github.com/python/docsbuild-scripts/ [8] http://bugs.python.org/issue26546 [9] http://docs.python.jp/3/ [10] https://github.com/python-doc-ja/python-doc-ja [11] http://docs.python.org.ar/tutorial/3/index.html [12] https://github.com/AFPy/python_doc_fr/blob/master/Makefile [13] https://github.com/sphinx-doc/sphinx/issues/1246 -- Julien Palard

3 years, 7 months

proposal: "python -m foo" should bind sys.modules['foo']

by Cameron Simpson

Hello all, This is a writeup of a proposal I floated here: https://mail.python.org/pipermail/python-list/2015-August/694905.html last Sunday. If the response is positive I wish to write a PEP. Briefly, it is a natural expectation in users that the command: python -m module_name ... used to invoke modules in "main program" mode on the command line imported the module as "module_name". It does not, it imports it as "__main__". An import within the program of "module_name" makes a new instance of the module, which causes cognitive dissonance and has the side effect that now the program has two instances of the module. What I propose is that the above command line _should_ bind sys.modules['module_name'] as well as binding '__main__' as it does currently. I'm proposing that the python -m option have this effect (python pseudocode): % python -m module.name ... runs: # pseudocode, with values hardwired for clarity import sys M = new_empty_module(name='__main__', qualname='module.name') sys.modules['__main__'] = M sys.modules['module.name'] = M # load the module code from wherever (not necessarily a file - CPython # already must do this phase) M.execfile('/path/to/module/name.py') Specificly, this would have the following two changes to current practice: 1) the module is imported _once_, and bound to both its canonical name and also to __main__. 2) imported modules acquire a new attribute __qualname__ (analogous to the recent __qualname__ on functions). This is always the conanoical name of the module as resolved by the importer. For most modules __name__ will be the same as __qualname__, but for the "main" module __name__ will be '__main__'. This change has the following advantages: The current standard boilerplate: if __name__ == '__main__': ... invoke "main program" here ... continues to work unchanged. Importantly, if the program then issues "import module_name", it is already there and the existing instance is found and used. The thread referenced above outlines my most recent encounter with this and the trouble it caused me. Followup messages include some support for this proposed change, and some criticism. The critiquing article included some workarounds for this multiple module situation, but they were (1) somewhat dependent on modules coming from a file pathname and (2) cumbersome and require every end user to adopt these changes if affected by the situation. I'd like to avoid that. Cheers, Cameron Simpson <cs(a)zip.com.au> The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man. - George Bernard Shaw

3 years, 8 months

Importing public symbols and simultainiously privatizing them, is too noisy

by Rick Johnson

Has anyone else found this to be too syntactically noisy? from module import Foo as _Foo, bar as _bar That is horrifically noisy IMO. The problem is, how do we remove the noise without sacrificing intuitiveness? My first idea was to do this: from module import_private Foo, bar And while it's self explanatory, it's also too long. So i thought... from module _import Foo, bar I'm leaning more towards the latter, but i'm not loving it either. Any ideas?

3 years, 9 months

Re: [Python-ideas] Exposing regular expression bytecode

by Jonathan Goble

OK, thanks for the comments, everyone. I'm glad to hear that people generally think this is a useful idea. Some specific replies: On Tue, Feb 16, 2016 at 4:22 AM, Chris Angelico <rosuav(a)gmail.com> wrote: > For what it's worth, I read your post with interest, but didn't have > anything substantive to reply - mainly because I don't use regexes > much. But it would be rather cool to be able to decompile a regex. > Imagine a regex pretty-printer: compile an arbitrary string, and if > it's a valid regex, decompile it to a valid source code form, using > re.VERBOSE. That could help _hugely_ with debugging, if the trick can > be pulled off. > > ChrisA That's exactly the type of tools I envision being made available by third parties. Depending on how much I get invested into this project, I may even write such a tool myself (though that's not guaranteed). On Tue, Feb 16, 2016 at 4:55 AM, Paul Moore <p.f.moore(a)gmail.com> wrote: > Sorry. I don't personally have any issue with the proposal, and it > sounds like a reasonable idea. I don't think it's likely to be > *hugely* controversial - although it will likely need a little care in > documenting the feature to ensure that we are clear that there's no > guarantees of backward compatibility that we don't want to commit to > on the newly - exposed data. And we should also ensure that by > exposing this information, we don't preclude changes such as the > incorporation of the regex module (I don't know if the regex module > has a bytecode implementation like the re module does). The regex implementation is indeed something I would need to investigate here, and will do so before I go too far. > The next step is probably simply to raise a tracker issue for this. I > know you said you have little C experience, but the big problem is > that it's unlikely that any of the core devs with C experience will > have the time or motivation to code up your idea. So without a working > patch, and someone willing and able to respond to comments on the > patch, it's not likely to progress. Tracker issue is already filed: http://bugs.python.org/issue26336 I actually filed the issue before I realized that the mailing lists were a better place to discuss it. > But if you are willing to dig into Python's C API yourself (and it > sounds like you are) there are definitely people who will help you. > You might want to join the core mentorship list (see > http://pythonmentors.com/) where you should get plenty of assistance. > This proposal sounds like a great "beginner" task, as well - so even > if you don't want to implement it yourself, still put it on the > tracker, and mark it as an "easy" change, and maybe some other > newcomer who wants a task to help them learn the C API will pick it > up. I'll look into the mentorship list; thanks for the link. As for marking it "easy", I don't seem to have the necessary permissions to change the Keywords field; perhaps you or someone else can set that flag for me? If so, I'd appreciate it. :-) > Hope that helps - thanks for the suggestion and sorry if it seems like > no-one was interested at first. It's an unfortunate fact of life > around here that things *do* take time to get people's interest. You > mention patience in one of your messages - that's definitely something > you'll need to cultivate, I'm afraid... :-) Patience is something I've been working on since I was a little kid. I'm 29 years old now, and it still eludes me from time to time. But yes, it's something I'll have to work on. :-P Also, I received a small patch off-list from Petr Viktorin implementing a getter for the code list (thanks, Petr). I'll need to test it, but from the little I know of the C API it looks like it will get me started in the right direction. Assuming that works, what's left is a public constructor for the regex type (to enable optimizers), a dis-like module, and docs and tests. I don't think this would be major enough to require a PEP, but of course being new here, I'm open to being told I'm wrong. :-)

4 years, 3 months

solving multi-core Python

by Eric Snow

tl;dr Let's exploit multiple cores by fixing up subinterpreters, exposing them in Python, and adding a mechanism to safely share objects between them. This proposal is meant to be a shot over the bow, so to speak. I plan on putting together a more complete PEP some time in the future, with content that is more refined along with references to the appropriate online resources. Feedback appreciated! Offers to help even more so! :) -eric -------- Python's multi-core story is murky at best. Not only can we be more clear on the matter, we can improve Python's support. The result of any effort must make multi-core (i.e. parallelism) support in Python obvious, unmistakable, and undeniable (and keep it Pythonic). Currently we have several concurrency models represented via threading, multiprocessing, asyncio, concurrent.futures (plus others in the cheeseshop). However, in CPython the GIL means that we don't have parallelism, except through multiprocessing which requires trade-offs. (See Dave Beazley's talk at PyCon US 2015.) This is a situation I'd like us to solve once and for all for a couple of reasons. Firstly, it is a technical roadblock for some Python developers, though I don't see that as a huge factor. Regardless, secondly, it is especially a turnoff to folks looking into Python and ultimately a PR issue. The solution boils down to natively supporting multiple cores in Python code. This is not a new topic. For a long time many have clamored for death to the GIL. Several attempts have been made over the years and failed to do it without sacrificing single-threaded performance. Furthermore, removing the GIL is perhaps an obvious solution but not the only one. Others include Trent Nelson's PyParallels, STM, and other Python implementations.. Proposal ======= In some personal correspondence Nick Coghlan, he summarized my preferred approach as "the data storage separation of multiprocessing, with the low message passing overhead of threading". For Python 3.6: * expose subinterpreters to Python in a new stdlib module: "subinterpreters" * add a new SubinterpreterExecutor to concurrent.futures * add a queue.Queue-like type that will be used to explicitly share objects between subinterpreters This is less simple than it might sound, but presents what I consider the best option for getting a meaningful improvement into Python 3.6. Also, I'm not convinced that the word "subinterpreter" properly conveys the intent, for which subinterpreters is only part of the picture. So I'm open to a better name. Influences ======== Note that I'm drawing quite a bit of inspiration from elsewhere. The idea of using subinterpreters to get this (more) efficient isolated execution is not my own (I heard it from Nick). I have also spent quite a bit of time and effort researching for this proposal. As part of that, a number of people have provided invaluable insight and encouragement as I've prepared, including Guido, Nick, Brett Cannon, Barry Warsaw, and Larry Hastings. Additionally, Hoare's "Communicating Sequential Processes" (CSP) has been a big influence on this proposal. FYI, CSP is also the inspiration for Go's concurrency model (e.g. goroutines, channels, select). Dr. Sarah Mount, who has expertise in this area, has been kind enough to agree to collaborate and even co-author the PEP that I hope comes out of this proposal. My interest in this improvement has been building for several years. Recent events, including this year's language summit, have driven me to push for something concrete in Python 3.6. The subinterpreter Module ===================== The subinterpreters module would look something like this (a la threading/multiprocessing): settrace() setprofile() stack_size() active_count() enumerate() get_ident() current_subinterpreter() Subinterpreter(...) id is_alive() running() -> Task or None run(...) -> Task # wrapper around PyRun_*, auto-calls Task.start() destroy() Task(...) # analogous to a CSP process id exception() # other stuff? # for compatibility with threading.Thread: name ident is_alive() start() run() join() Channel(...) # shared by passing as an arg to the subinterpreter-running func # this API is a bit uncooked still... pop() push() poison() # maybe select() # maybe Note that Channel objects will necessarily be shared in common between subinterpreters (where bound). This sharing will happen when the one or more of the parameters to the function passed to Task() is a Channel. Thus the channel would be open to the (sub)interpreter calling Task() (or Subinterpreter.run()) and to the new subinterpreter. Also, other channels could be fed into such a shared channel, whereby those channels would then likewise be shared between the interpreters. I don't know yet if this module should include *all* the essential pieces to implement a complete CSP library. Given the inspiration that CSP is providing, it may make sense to support it fully. It would be interesting then if the implementation here allowed the (complete?) formalisms provided by CSP (thus, e.g. rigorous proofs of concurrent system models). I expect there will also be a _subinterpreters module with low-level implementation-specific details. Related Ideas and Details Under Consideration ==================================== Some of these are details that need to be sorted out. Some are secondary ideas that may be appropriate to address in this proposal or may need to be tabled. I have some others but these should be sufficient to demonstrate the range of points to consider. * further coalesce the (concurrency/parallelism) abstractions between threading, multiprocessing, asyncio, and this proposal * only allow one running Task at a time per subinterpreter * disallow threading within subinterpreters (with legacy support in C) + ignore/remove the GIL within subinterpreters (since they would be single-threaded) * use the GIL only in the main interpreter and for interaction between subinterpreters (and a "Local Interpreter Lock" for within a subinterpreter) * disallow forking within subinterpreters * only allow passing plain functions to Task() and Subinterpreter.run() (exclude closures, other callables) * object ownership model + read-only in all but 1 subinterpreter + RW in all subinterpreters + only allow 1 subinterpreter to have any refcounts to an object (except for channels) * only allow immutable objects to be shared between subinterpreters * for better immutability, move object ref counts into a separate table * freeze (new machinery or memcopy or something) objects to make them (at least temporarily) immutable * expose a more complete CSP implementation in the stdlib (or make the subinterpreters module more compliant) * treat the main interpreter differently than subinterpreters (or treat it exactly the same) * add subinterpreter support to asyncio (the interplay between them could be interesting) Key Dependencies ================ There are a few related tasks/projects that will likely need to be resolved before subinterpreters in CPython can be used in the proposed manner. The proposal could implemented either way, but it will help the multi-core effort if these are addressed first. * fixes to subinterpreter support (there are a couple individuals who should be able to provide the necessary insight) * PEP 432 (will simplify several key implementation details) * improvements to isolation between subinterpreters (file descriptors, env vars, others) Beyond those, the scale and technical scope of this project means that I am unlikely to be able to do all the work myself to land this in Python 3.6 (though I'd still give it my best shot). That will require the involvement of various experts. I expect that the project is divisible into multiple mostly independent pieces, so that will help. Python Implementations =================== They can correct me if I'm wrong, but from what I understand both Jython and IronPython already have subinterpreter support. I'll be soliciting feedback from the different Python implementors about subinterpreter support. C Extension Modules ================= Subinterpreters already isolate extension modules (and built-in modules, including sys). PEP 384 provides some help too. However, global state in C can easily leak data between subinterpreters, breaking the desired data isolation. This is something that will need to be addressed as part of the effort.

4 years, 3 months

Exposing flat bytecode representation to optimizers

by Andrew Barnert

The biggest pain with dealing with the peephole optimizer is that it happens after all the complicated flattening and fixup[^1] the compiler does, which means you have to hack up the jump targets as you go along. The future bytecode optimizers that PEP 511 enables will have the same headache. But this isn't actually necessary. The optimizer could work on a flat array of instructions[^2] instead of an array of bytes, with relocatable jump targets instead of fixed byte offsets, and then the compiler could do the fixup _after_ the optimization.[^3] It would break the optimizer APIs, but `PyCode_Optimize` isn't public, and the API proposed by PEP 511 is public, but that PEP isn't even finalized, much less accepted yet. I don't think we need to expose the intermediate representation any farther along than the `PyCode_Optimize` step.[^4] Just moving the optimize one step earlier in the chain solves more than enough to be worth it. [^1]: If you think numbering the offsets and emitting the jump targets is easy: Every time you fix up a jump, that may require adding an `EXTENDED_ARG`, which means you have to redo any fixups that cross the the current instruction. The compiler does this by just looping until it hasn't added any more `EXTENDED_ARG`s. [^2]: In case anyone's thinking that wordcode would solve this problem, it doesn't. The `EXTENDED_ARG` jump targets are a much bigger hassle than the 1-or-3-byte-ops, and wordcode not only can't eliminate those, but makes `EXTENDED_ARG` more common. [^3]: The compiler doesn't actually have exactly what the optimizers would want, but it's pretty close: it has a linked list of block objects, each of which has an array of instruction objects, with jump targets being pointers to blocks. That's actually even better to work with, but too complicated to expose to optimizers. Flattening it would be trivial. Or, if that's too expensive, we could do something almost as simple and much cheaper: convert it in-line to a deque-like linked list of arrays, with jump targets being indices or pointers into that. Or we could just expose the list of blocks as-is, as an opaque thing with a mutable-deque-of-instructions API around it. [^4]: Later stages--import hooks, optimizing decorators, etc.--have the same pain as the peephole optimizer, but they also have code objects that contain other code objects, and they can be written in Python, and so on, so the same solution isn't feasible there. Of course we could add a function to convert bytecode back to a list of instructions, in some format that can be exposed to Python (in fact, `dis` already does 90% of that), and then another one to convert that back to bytecode and redo the fixup (which is basically the third-party `byteplay` module). But that's almost certainly overkill. (If we wanted that, I'd rather add `byteplay` to the stdlib, port it and `dis` to C, and expose a C API for them. And then we could use that for everything, including the peephole optimizer and PEP 511 optimizers. Which would all be really cool, but I think it's more work than we want to do, and I don't know if we'd actually want something like `byteplay` builtin even if it were easy...)

4 years, 4 months

The next major Python version will be Python 8

by Victor Stinner

Hi, Python 3 becomes more and more popular and is close to a dangerous point where it can become popular that Python 2. The PSF decided that it's time to elaborate a new secret plan to ensure that Python users suffer again with a new major release breaking all their legacy code. The PSF is happy to announce that the new Python release will be Python 8! Why the version 8? It's just to be greater than Perl 6 and PHP 7, but it's also a mnemonic for PEP 8. By the way, each minor release will now multiply the version by 2. With Python 8 released in 2016 and one release every two years, we will beat Firefox 44 in 2022 (Python 64) and Windows 2003 in 2032 (Python 2048). A major release requires a major change to justify a version bump: the new killer feature is that it's no longer possible to import a module which does not respect the PEP 8. It ensures that all your code is pure. Example: $ python8 -c 'import keyword' Lib/keyword.py:16:1: E122 continuation line missing indentation or outdented Lib/keyword.py:16:1: E265 block comment should start with '# ' Lib/keyword.py:50:1: E122 continuation line missing indentation or outdented (...) ImportError: no pep8, no glory Good news: since *no* module of the current standard library of Python 3 respect the PEP 8, the standard library will be simplified to one unique module, which is new in Python 8: pep8. The standard library will move to the Python Cheeseshop (PyPI), to reply to an old and popular request. DON'T PANIC! You are still able to import your legacy code into Python 8, you just have to rename all your modules to add a "_noqa" suffix to the filename. For example, rename utils.py to utils_noqa.py. A side effect is that you have to update all imports. For example, replace "import django" with "import django_noqa". After a study of the PSF, it's a best option to split again the Python community and make sure that all users are angry. The plan is that in 10 years, at least 50% of the 77,000 packages on the Python cheeseshop will be updated to get the "_noqa" tag. After 2020, the PSF will start to sponsor trolls to harass users of the legacy Python 3 to force them to migrate to Python 8. Python 8 is a work-in-progress (it's still an alpha version), the standard library was not removed yet. Hopefully, trying to import any module of the standard library fails. Don't hesitate to propose more ideas to make Python 8 more incompatible with Python 3! Note: The change is already effective in the default branch of Python: https://hg.python.org/cpython/rev/9aedec2dbc01 Have fun, Victor

4 years, 5 months

Control Flow - Never Executed Loop Body

by Sven R. Kunze

Hi Python-ideas, this is a follow up from an thread of Python-List: https://mail.python.org/pipermail/python-list/2016-March/705205.html What do you think about the following proposal? *Abstract* Control structures such as "for", "while" and "try" feature some various additional clauses being executed under certain conditions. This proposal adds another extra clause to "for" and "while" in order to provide an easy way to discover whether the body of a loop as never been executed at all. *Motivation** *Coming from the Web application development, we periodically can see a benefit of a hypothetical "empty" clause of Python's for loop like this: for item in my_iterator: # do per item empty: # do something else The same goes for "while" although there the choice of the name "empty" might be questionable. "empty" refers to the involved iterable which is not really existing in case of "while". However, there is not need of introducing inconsistencies among the loop constructs as both feature the "else" clause as well. Better suggestions are welcome; also see the keyword section. *Keyword* Keywords under consideration have been in (an attempt of dispassionate) order of preference: 1) empty -> most obvious, most people responded with a solution to solve the intended problem 2) else -> already taken but to some would be the preferred one 3) or -> as alternative if a new keyword is too much 4) except -> as alternative if a new keyword is too much 5) instead -> less obvious and new keyword *Use-Cases* 1) As humans mostly have a hard time to discover the difference between *nothing**as error* and *nothing**as really empty*, GUI/Web interfaces display an explicit notice in case of empty lists to signify the correct execution of a process/action/search or the other case. 2) Single way and no boilerplate of handling empty loops. As the discussion on Python-List showed are there quite some number of different ways of achieving this kind of functionality. 3) Less errors in development and maintenance. Some of the proposed solutions would not work in some circumstances (either during development or depending on the type of the items) which can hide errors. *Implementation* To be done. *Maintainer* Me. *Issues** *People I talked to suggested "else" as an alternative to "empty" because "empty is not quite clear". Unfortunately, "else" is already taken and is itself not quite clear. "then" has been proposed as an alternative for "else" in an attempt for proper understanding of "else". If that would be an accepted proposal, it would make room for "else" to be used for the usage of the "empty keyword proposed here. *Related Work** *This proposal is partly inspired by template engines like Django or jinja2 which both already provide such facility. Django: {% for item in my_iterator %} ... {% empty %} ... {% endfor %} jinja2: {% for item in my_iterator %} ... {% else %} ... {% endfor %} Best, Sven

4 years, 5 months

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Python-ideas March 2016