Mailman 3 October 2016 - Python-ideas

@classproperty, @abc.abstractclasspropery, etc.
by K. Richard Pixley 17 Dec '20

17 Dec '20

There's a whole matrix of these and I'm wondering why the matrix is currently sparse rather than implementing them all. Or rather, why we can't stack them as: class foo(object): @classmethod @property def bar(cls, ...): ... Essentially the permutation are, I think: {'unadorned'|abc.abstract}{'normal'|static|class}{method|property|non-callable attribute}. concreteness implicit first arg type name comments {unadorned} {unadorned} method def foo(): exists now {unadorned} {unadorned} property @property exists now {unadorned} {unadorned} non-callable attribute x = 2 exists now {unadorned} static method @staticmethod exists now {unadorned} static property @staticproperty proposing {unadorned} static non-callable attribute {degenerate case - variables don't have arguments} unnecessary {unadorned} class method @classmethod exists now {unadorned} class property @classproperty or @classmethod;@property proposing {unadorned} class non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract {unadorned} method @abc.abstractmethod exists now abc.abstract {unadorned} property @abc.abstractproperty exists now abc.abstract {unadorned} non-callable attribute @abc.abstractattribute or @abc.abstract;@attribute proposing abc.abstract static method @abc.abstractstaticmethod exists now abc.abstract static property @abc.staticproperty proposing abc.abstract static non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract class method @abc.abstractclassmethod exists now abc.abstract class property @abc.abstractclassproperty proposing abc.abstract class non-callable attribute {degenerate case - variables don't have arguments} unnecessary I think the meanings of the new ones are pretty straightforward, but in case they are not... @staticproperty - like @property only without an implicit first argument. Allows the property to be called directly from the class without requiring a throw-away instance. @classproperty - like @property, only the implicit first argument to the method is the class. Allows the property to be called directly from the class without requiring a throw-away instance. @abc.abstractattribute - a simple, non-callable variable that must be overridden in subclasses @abc.abstractstaticproperty - like @abc.abstractproperty only for @staticproperty @abc.abstractclassproperty - like @abc.abstractproperty only for @classproperty --rich

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Specify number of items to allocate for array.array() constructor
by Sven Rahmann 22 Feb '20

22 Feb '20

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

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Implicit string literal concatenation considered harmful?
by Guido van Rossum 15 Mar '18

15 Mar '18

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)

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https://docs.python.org/fr/ ?
by Julien Palard 02 Feb '17

02 Feb '17

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

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proposal: "python -m foo" should bind sys.modules['foo']
by Cameron Simpson 18 Jan '17

18 Jan '17

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

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Importing public symbols and simultainiously privatizing them, is too noisy
by Rick Johnson 29 Dec '16

29 Dec '16

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?

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singledispatch for instance methods
by Tim Mitchell 26 Dec '16

26 Dec '16

Hi All, We have a modified version of singledispatch at work which works for methods as well as functions. We have open-sourced it as methoddispatch (pypi: https://pypi.python.org/pypi/methoddispatch). IMHO I thought it would make a nice addition to python stdlib. What does everyone else think?

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Distribution agnostic Python project packaging
by James Pic 26 Nov '16

26 Nov '16

Hi all ! Ive heard some people saying it was rude to post on a mailing list without introducing yourself so here goes something: my name is James Pic and I've been developing and deploying a wide variety fof Python projects Python for the last 8 years, I love to learn and share and writing documentation amongst other things such as selling liquor. The way I've been deploying Python projects so far is probably similar to what a lot of people do and it almost always includes building runtime dependencies on the production server. So, nobody is going to congratulate me for that for sure but I think a lot of us have been doing so. Now I'm fully aware of distribution specific packaging solutions like dh-virtualenv shared by Spotify but here's my mental problem: I love to learn and to hack. I'm always trying now distributions and I rarely run the one that's in production in my company and when I'm deploying personal projects I like funny distributions like arch, Alpine Linux, or interesting paas solutions such as cloudfoundry, openshift, rancher and many others. And I'm always facing the same problem: I have to either build runtime dependencies on the server, either package my thing in the platform specific way. I feel like I've spent a really huge amount of time doing this king of thing. But the java people, they have jars, and they have smooth deployments no matter where they deploy it. So that's the idea I'm trying to share: I'd like to b able to build a file with my dependencies and my project in it. I'm not sure packaging only Python bytecode would work here because of c modules. Also, I'm always developing against a different Python version because I'm using different distributions because it's part of my passions in life, as ridiculous as it could sound to most people, I'm expecting at least some understanding from this list :) So I wonder, do you think the best solution for me would be to build an elf binary with my Python and dependencies that I could just run on any distribution given its on the right architecture ? Note that I like to use Arm too, so I know I'd need to be able to cross compile too. Thanks a lot for reading and if you can to take some time to share your thoughts and even better : point me in a direction, if that idea is the right solution and I'm going to be the only one interested I don't care if it's going to take years for me to achieve this. Thanks a heap ! Beat regards PS: I'm currently at the openstack summit in Barcelona if anybody there would like to talk about it in person, in which case I'll buy you the drinks ;)

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str(slice(10)) should return "slice(10)"
by Neil Girdhar 14 Nov '16

14 Nov '16

Currently str(slice(10)) returns "slice(None, 10, None)" If the start and step are None, consider not emitting them. Similarly slice(None) is rendered slice(None, None, None). When you're printing a lot of slices, it's a lot of extra noise.

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Reduce/fold and scan with generator expressions and comprehensions
by Danilo J. S. Bellini 14 Nov '16

14 Nov '16

The idea is to let generator expressions and list/set comprehensions have a clean syntax to access its last output. That would allow them to be an alternative syntax to the scan higher-order function [1] (today implemented in the itertools.accumulate function), which leads to an alternative way to write a fold/reduce. It would be nice to have something like: >>> last(abs(prev - x) for x in [3, 4, 5] from prev = 2) 2 instead of a reduce: >>> from functools import reduce >>> reduce(lambda prev, x: abs(prev - x), [3, 4, 5], 2) 2 or an imperative approach: >>> prev = 2 >>> for x in [3, 4, 5]: ... prev = abs(prev - x) >>> prev 2 or getting the last from accumulate: >>> from itertools import accumulate >>> list(accumulate([2, 3, 4, 5], lambda prev, x: abs(prev - x)))[-1] 2 or... >>> [prev for prev in [2] ... for x in [3, 4, 5] ... for prev in [abs(prev - x)] ... ][-1] 2 Actually, I already wrote a solution for something similar to that: PyScanPrev [2]. I'm using bytecode manipulation to modify the generator expression and set/list comprehensions semantics to create a "scan", but it has the limitation of using only code with a valid syntax as the input, so I can't use "from" inside a generator expression / list comprehension. The solution was to put the first output into the iterable and define the "prev" name elsewhere: >>> last(abs(prev - x) for x in [2, 3, 4, 5]) 2 That line works with PyScanPrev (on Python 3.4 and 3.5) when defined in a function with a @enable_scan("prev") decorator. That was enough to create a "test suite" of doctest-based examples that shows several scan use cases [2]. This discussion started in a Brazilian list when someone asked how she could solve a simple uppercase/lowercase problem [3]. The goal was to alternate the upper/lower case of a string while neglecting the chars that doesn't apply (i.e., to "keep the state" when the char isn't a letter). After the discussion, I wrote the PyScanPrev package, and recently I've added this historical "alternate" function as the "conditional toggling" example [4]. Then I ask, can Python include that "scan" access to the last output in its list/set/dict comprehension and generator expression syntax? There are several possible applications for the scan itself as well as for the fold/reduce (signal processing, control theory, physics, economics, etc.), some of them I included as PyScanPrev examples. Some friends (people who like control engineering and/or signal processing) liked the "State-space model" example, where I included a "leaking bucket-spring-damper" simulation using the scan-enabled generator expressions [5]. About the syntax, there are several ideas on how that can be written. Given a "prev" identifier, a "target" identifier, an input "iterable" and an optional "start" value (and perhaps an optional "echo_start", which I assume True by default), some of them are: [func(prev, target) for target in iterable from prev = start] [func(prev, target) for target in iterable] -> prev = start [func(prev, target) for target in iterable] -> prev as start [func(prev, target) for target in iterable] from prev = start [func(prev, target) for target in iterable] from prev as start [func(prev, target) for target in iterable] with prev as start prev = start -> [func(prev, target) for target in iterable] prev(start) -> [func(prev, target) for target in iterable] [func(prev, target) for prev -> target in start -> iterable] [prev = start -> func(prev, target) for target in iterable] # With ``start`` being the first value of the iterable, i.e., # iterable = prepend(start, data) [func(prev, target) for target in iterable from prev] [func(prev, target) for target in iterable] -> prev [func(prev, target) for target in iterable] from prev prev -> [func(prev, target) for target in iterable] Before writing PyScanPrev, in [6] (Brazilian Portuguese) I used stackfull [7] to implement that idea, an accumulator example using that library is: >>> from stackfull import push, pop, stack >>> [push(pop() + el if stack() else el) for el in range(5)] [0, 1, 3, 6, 10] >>> list(itertools.accumulate(range(5))) [0, 1, 3, 6, 10] There are more I can say (e.g. the pyscanprev.scan function has a "start" value and an "echo_start" keyword argument, resources I missed in itertools.accumulate) but the links below already have a lot of information. [1] https://en.wikipedia.org/wiki/Prefix_sum#Scan_higher_order_function [2] https://pypi.python.org/pypi/pyscanprev [3] https://groups.google.com/forum/#!topic/grupy-sp/wTIj6G5_5S0 [4] https://github.com/danilobellini/pyscanprev/blob/v0.1.0/examples/conditiona… [5] https://github.com/danilobellini/pyscanprev/blob/v0.1.0/examples/state-spac… [6] https://groups.google.com/forum/#!topic/grupy-sp/UZp-lVSWK1s [7] https://pypi.python.org/pypi/stackfull -- Danilo J. S. Bellini --------------- "*It is not our business to set up prohibitions, but to arrive at conventions.*" (R. Carnap)

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Python-ideas October 2016