Mailman 3 November 2020 - Python-ideas

Inline dict manipulations
by Tomasz Hławiczka Nov. 17, 2020

Nov. 17, 2020

Hello, This is my first mail here, so greetings for everyone :) I would like to introduce an idea of the new operators for dict objects. Any thoughts are welcome. Since Python3.9 there is a great feature for merging dicts using | (OR) operator: {1: "a"} | {2: "b"} == {1: "a", 2: "b"} Thus, this basic operation can be done smoothly (also inline). PEP: https://www.python.org/dev/peps/pep-0584/ Another common operation on dicts is a substitution - building a new dict with only a part of the data. Usually the part of the data is described as a list of keys to keep or a list of keys to skip. Therefore it would be very handy to have a build-in option to filter out a dict with a similar fashion, for example using & (AND) operator against a list/tuple/set/frozenset of keys that should be kept in the result): {1: "a", 2: "b", 3: "c"} & [1, 3, 4] == {1: "a", 3: "c"} {1: "a", 2: "b", 3: "c"} & {1, 3, 4} == {1: "a", 3: "c"} Using the & operator: dict_object & list_of_keys would be equal to the following expression: {key: value for key, value in dict_object .items() if key in list_of_keys} Additionally, a similar option for omitting specified keys could be done with another - (minus) operator against a list/tuple/set/frozenset of keys that should be suppressed): {1: "a", 2: "b", 3: "c"} - [3, 4] == {1: "a", 2: "b"} {1: "a", 2: "b", 3: "c"} - {3, 4} == {1: "a", 2: "b"} Using the - operator: dict_object - list_of_keys would be equal to following expression: {key: value for key, value in dict_object .items() if key not in list_of_keys} Best, Tomasz

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Re: PEP 634-636: Mapping patterns and extra keys
by Valentin Berlier Nov. 17, 2020

Nov. 17, 2020

I'm in favor of keeping the PEP as it currently is. Mappings are naturally structural subtypes of one another, therefore mapping patterns should be consistent with class patterns. car = Car(...) match car: case Vehicle(): pass case Car(): # will never match pass This example is analogous to the first one in the discussion. If Car is a subclass of Vehicle, then Vehicle() is a more general pattern than Car() and will always match despite the instance not being exactly of type Vehicle. With mapping patterns it's exactly the same thing. You need to match the more specific patterns first. Matching x and y is more general than matching x, y and z. One more thing. If we compare the proposed behavior to other languages the most relevant example would be object destructuring in javascript: const { 'x': x, 'y': y } = { 'x': 1, 'y': 2, 'z': 3 }; const { x, y } = { x: 1, y: 2, z: 3 }; // more common short form Object destructuring only matches the specified fields. You can also match the remaining fields but it's always explicit: const { x, y, ...rest } = { x: 1, y: 2, z: 3 }; console.log(rest); // { z: 3 } The pattern is widely adopted and the behavior generally lines up with people's expectations.

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Use __bytes__ to access buffer-protocol from "user-land"
by phk＠freebsd.dk Nov. 16, 2020

Nov. 16, 2020

I am working on a toolbox for computer-archaeology where old data media are "excavated" and presented on a web-page. (https://github.com/Datamuseum-DK/AutoArchaeologist for anybody who cares). Since these data-media can easily sum tens of gigabytes, mmap and virtual memory is my weapons of choice and that has brought me into an obscure corner of python where few people seem to venture: I want to access the buffer-protocol from "userland". The fundamental problem is that if I have a image of a disk and it has 2 partitions, I end up with the "mmap.mmap" object that mapped the raw disk image, and two "bytes" or "bytearray" objects, each containing one partition, for a total memory footprint of twice the size of the disk. As the tool dives into the filesystems in the partitions and creates objects for the individual files in the filesystem, that grows to three times the size of the disk etc. To avoid this, I am writing a "bytes-like" scatter-gather class (not yet committed), and that is fine as far as it goes. If I want to write one of my scatter-gather objects to disk, I have to: fd.write(bytes(myobj)) As a preliminary point, I think that is just wrong: A class with a __bytes__ method should satisfy any needs the buffer-protocol might have, so this should work: fd.write(myobj) But taking this a little bit further, I think __bytes__ should be allowed to be an iterator, provided the object also offers __len__, so that this would work: class bar(): def __len__(self): return 3 def __bytes__(self): yield b'0' yield b'1' yield b'2' open("/tmp/_", "wb").write(foo()) This example is of course trivial, but hav the yield statements hand out hundreds of megabytes, and the savings in time and malloc-space becomes very tangible. Poul-Henning

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constants in Python: Starting simple and gradually adding more features, was: Re: Pattern Matching controversy
by Paul Sokolovsky Nov. 16, 2020

Nov. 16, 2020

Hello, On Mon, 16 Nov 2020 08:39:30 +1100 Steven D'Aprano <steve(a)pearwood.info> wrote: [] > > The baseline of my version is much simpler: > > > > # This makes "const" a kind of hard keyword for this module > > from __future__ import const > > > > FOO: const = 1 # obviously, this is constant > Oh, well, To start with, in the original thread I wanted to concentrate on issues of the PEP634/PEP635, and whether these issues are prominent enough to hope for them being addressed. So, I changed the subject (and we'd soon be excused to python-ideas, indeed, I cc: there). > if all it takes is to add a new keyword, then constants are > easy! A new annotation. And the new subject is "constants in Python: Starting simple and gradually adding more", which hopefully should set the theme. Please remember how we arrived here: it's from the fact that PEP634 doesn't allow for the following trivial code: MY_CONST = 1 match foo: case MY_CONST: We're considering (another alternative) how to address that. Please don't make a leap towards supporting (at once) const-ness equivalent to statically typed languages. > No need to worry about how constantness affects name resolution, > or how the syntax interacts with type-hints: "const" is *the* type hint. > spam: const: Widget = build_widget(*args) That syntax is clearly invalid. And composability of type annotations (aka type hints) is a known, looming issue. We now have https://www.python.org/dev/peps/pep-0593/ , but in all fairness, it seems like a stopgap measure rather than an elegant solution. (In full fairness, entire "typing" module's annotations aren't very elegant, but as we know, it's not a part of language core, but part of CPython's stdlib. That means it's only *one* of possible annotation schemes for *Python*). So, under PEP593, the above would be written spam: Annotated[Widget, const] = build_widget(*args) If you want a glimpse of what alternatives might look, then: given that "|" is going to be used for union types, why not try "&" for composition? spam: Widget & const = build_widget(*args) But again, for "initial support of constants in Python, prompted by the introduction of pattern matching facilities", we don't need to worry about all that right away. > # Maybe this is better? > # let spam: Widget = build_widget(*args) As you remember, at the beginning of my proposal, I wrote "The baseline of my version ...". Baseline == level 0. For "level 2", I considered const spam: Widget = ... I don't think that "let" should be used. We're putting emphasis on *constantness* (not lexical scoping of variables [immutable by functional tradition, though that's "minor"]). JavaScript (now) has both "const" and "let", and I think that's pretty reasonable approach. So, let's save "let" for possible later uses. So, what's "level 1"? As I mentioned, under "level 0", "from __future__ import const" has a magic meaning (akin to other "from __future__"'s). Under "level 1", "const" is just a normal annotation symbol imported from a module. So, following would be possible: ========= from __future__ import const FOO: const = 1 def fun(): const = 1 # Just a variable in function scope def sub(): # Gimme powerz back from __future__ import const BAR: const = 2 # Back to annotation in global scope BAZ: const = 3 ========= "level 0" should be implementable in CPython in ~1hr. "level 1" is realistically what we should shoot for. "level 2" (the dedicated keyword), I'm still not sure about. "const" is very baseline annotation, and deserves a dedicated keyword. But the underlying problem is composability of (arbitrary) annotations. I'd rather keep searching for graal in that regard, before giving up and introduce a dedicated thing just for "const". > or how "constant" interacts with mutability: > > spam: const = [] > spam.append(99) # Allowed? > spam.extend(items) > spam.sort() "const" is an annotation just like any other. And it affects *runtime* in the same as any other annotation in CPython affects it: in no way. "const" is however introduced as a hint for *compile-time*, so compiler could make some simple inferences and maybe even optimizations based on it. > or for that matter, whether or not constants are actually constant. > > spam: const = 1 > spam = 2 Compiler can, and thus would, catch that as an error (warning for a beta version?). > If constants aren't actually constant, but just a tag on symbols, > then you would be right, it probably would be trivially easy to add > "constants" to Python. Right. > But I think most people agree making them behave as constants is a > pretty important feature. As mentioned, "const" is just an annotation like any other, except compiler has some insight into it. Dealing with runtime is distant goal for CPython. (Which is for now offloaded to static type checkers and libraries/alternative Python implementations.) > *The* most critical feature of all, really. > > Given a declared constant: > > # Module a.py > x: const = None > > how do you prevent not just code in module `a` from rebinding the > value: Outside of the current scope of the discussion. However, if you're interested in that topic, then I've implemented it in my Python dialect, Pycopy (https://github.com/pfalcon/pycopy). It's on my TODO to post an RFC to python-ideas for beating. [] -- Best regards, Paul mailto:pmiscml@gmail.com

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Fwd: Re: Global flag for whether a module is __main__
by André Roberge Nov. 15, 2020

Nov. 15, 2020

Sorry, forgot to use "reply to all" ---------- Forwarded message --------- From: André Roberge <andre.roberge(a)gmail.com> Date: Sat, Nov 14, 2020 at 11:06 AM Subject: Re: [Python-ideas] Re: Global flag for whether a module is __main__ To: Steven D'Aprano <steve(a)pearwood.info> On Sat, Nov 14, 2020 at 10:45 AM Steven D'Aprano <steve(a)pearwood.info> wrote: > On Sat, Nov 14, 2020 at 08:10:44AM -0400, André Roberge wrote: > > > > What if you import the `__main__` module? What does `__imported__` say > > > now, and how do you check for "running as a script" if `__main__` has > > > imported itself -- or some other module has imported it? > > > > > > > Running a module (no matter what its name is) from a command line would > set > > __imported__ to False for that module. > > Using import some_module (or __import__("some_module")) would set > > some_module.__imported__ to True. > > Do you understand that a module can be both run and imported at the > same time? > > > # example.py > import __main__ > print(__main__.__file__) > > As others have mentioned, many beginners are thoroughly confused by the meaning of the idiom if __name__ == "__main__": ... The idea behind the name __imported__ (and, I gather, somewhat similar to the original suggestion of __main__ that started this thread) is to reduce such confusion. For what I had in mind, the semantic would be the same as though the following was inserted at the top of the module: __imported__ = True if __name__ == "__main__": __imported__ = False André > If you save that snippet as "example.py", and then run it: > > python3 example.py > > you have an example of a module that is being run and imported > simultaneously. > > > -- > Steve > _______________________________________________ > Python-ideas mailing list -- python-ideas(a)python.org > To unsubscribe send an email to python-ideas-leave(a)python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/MK4ZO… > Code of Conduct: http://python.org/psf/codeofconduct/ >

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Global flag for whether a module is __main__
by Matt Wozniski Nov. 15, 2020

Nov. 15, 2020

Currently, the simplest and most idiomatic way to check whether a module was run as a script rather than imported is: if __name__ == "__main__": People generally learn this by rote memorization, because users often want the ability to add testing code or command line interfaces to their modules before they understand enough about Python's data model to have any idea why this works. Understanding what's actually happening requires you to know that: 1. the script you ask Python to run is technically a module, 2. every module has a unique name assigned to it, 3. a module's `__name__` global stores this unique import name, 4. and "__main__" is a magic name for the initial script's module. A new (writable) global attribute called `__main__` would simplify this case, allowing users to simply test if __main__: It would behave as though __main__ = (__name__ == "__main__") is executed in each module's namespace before executing it. Because this would be writable, I don't see any backwards compatibility issues. It wouldn't negatively impact any modules that might already be defining `__main__` (for example, by doing `import __main__`). They'd simply redefine it and go on using the `__main__` module as they always have. And a package with a `__main__.py` does not have a `__main__` attribute. It would be easier to teach, easier to learn, and easier to memorize, and a nice simplification for users at the cost of only very slightly more complexity in the data model.

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unix filter, one liners, awk
by Hans Ginzel Nov. 14, 2020

Nov. 14, 2020

Is there a reason, why not to make python useful for practical one liners to replace perl and awk? There is page Powerful Python One-Liners, https://wiki.python.org/moin/Powerful%20Python%20One-Liners. But almost none of them handles files, behaves like unix filter or is ugly – must import modules, etc. It seems that the command line options like -a, -n, -p, -F are still free. :-) https://docs.python.org/3/using/cmdline.html Why not use them almost the same way as in Perl? https://perldoc.perl.org/perlrun#-n https://perldoc.perl.org/perlrun#-p https://perldoc.perl.org/perlrun#-a https://perldoc.perl.org/perlrun#-Fpattern E.g. -n would be almost equivalent to import sys,os,re from fileinput import * for line in input(): <-c code comes here> close() -p will print(line) as last command of the for cycle. Why to learn Perl/awk/datamash/mlr/…, for “one line like” tasks? Thank you in advance Hans PS: https://blogs.oracle.com/linux/the-top-10-tricks-of-perl-one-liners-v2 https://gist.github.com/joyrexus/7328094 https://stackoverflow.com/questions/1589994/how-do-i-write-a-unix-filter-in… Why not to have an elegant (one line) answer in python for questions like this? https://stackoverflow.com/questions/40708370/drop-duplicates-and-keep-first…

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filterwarnings on a module in the current stack
by Bruno Beltran Nov. 7, 2020

Nov. 7, 2020

AFAICT, there is no way to filter warnings based on anything but their most immediate caller (please correct me if I'm wrong). This can lead to situations where it's impossible to write a warnings filter that's not brittle. For example: Say I want to filter a certain warnings `sphinx.ext.autodoc` during my build process (e.g. to ignore deprecation warnings when autodoc imports deprecated modules that I need to keep documented). I can filter: ```python warnings.filterwarnings('ignore', category=MatplotlibDeprecationWarning, message=r'(\n|.)*module was deprecated.*') ``` but then when sphinx is then building my gallery of examples, this hides the `DeprecationWarnings` that I do want to see! What I'd like to do is write ```python warnings.filterwarnings('ignore', category=MatplotlibDeprecationWarning, module='sphinx.ext.autodoc', message=r'(\n|.)*module was deprecated.*') ``` however the warning is not actually triggered by the `autodoc` module itself, but instead let's say hypothetically it's triggered by using `imp` internally to load the module. Then I would have to instead do something like the following: ```python warnings.filterwarnings('ignore', category=MatplotlibDeprecationWarning, module='imp', message=r'(\n|.)*module was deprecated.*') ``` but now let's say in the past couple of years, Sphinx, being the well-maintained library that it is, moves to using `importlib.import_module`. Now suddenly warnings are appearing and I have to go edit this guy to fix CI: ```python warnings.filterwarnings('ignore', category=MatplotlibDeprecationWarning, module='importlib', message=r'(\n|.)*module was deprecated.*') ``` This is (at worst) a minor inconvenience in this silly example case, but in general one could imagine that if you want to use an upstream library in your code internally, in a way that raises a warning (that you want to ignore), it's currently impossible to do that without relying on what *should be* implementation details of the library you're using. One obvious solution in my mind is to allow `filterwarnings` to inspect the current stack at the time that a warning is raised, since this contains all of the information needed in principle for the user to decide whether or not to ignore a warning. Has this kind of thing been discussed before? I tried my best to search but couldn't find anything, and while I'm a decades-long *user* of Python I don't tend to keep up with the development side of things.

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propose index heap
by master_lee Nov. 5, 2020

Nov. 5, 2020

Index heap is essential in efficient implementation of Dijkstra's algorithm, Prim's algorithm and other variants, I have implemented a concise version at https://github.com/yutao-li/libheap , I think it would be useful to have it in stdlib

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Enum and the Standard Library (and __str__ and __repr__)
by Ethan Furman Nov. 3, 2020

Nov. 3, 2020

TL;DR Changes may be coming to Enum str() and repr() -- your (informed) opinion requested. :-) Python-Dev thread [0], summary below: > As you may have noticed, Enums are starting to pop up all over the stdlib [1]. > > To facilitate transforming existing module constants to IntEnums there is > `IntEnum._convert_`. In Issue36548 [2] Serhiy modified the __repr__ of RegexFlag: > > >>> import re > >>> re.I > re.IGNORECASE > > I think for converted constants that that looks nice. For anyone that wants the > actual value, it is of course available as the `.value` attribute: > > >>> re.I.value > 2 > > I'm looking for arguments relating to: > > - should _convert_ make the default __repr__ be module_name.member_name? > > - should _convert_ make the default __str__ be the same, or be the > numeric value? After discussions with Guido I made a (largely done) PR [3] which: for stdlib global constants (such as RE) - repr() -> uses `module.member_name` - str() -> uses `member_name` for stdlib non-global constants, and enums in general - repr() -> uses `class.member_name` - str() -> uses `member_name` The questions I would most appreciate an answer to at this point: - do you think the change has merit? - why /shouldn't/ we make the change? As a reminder, the underlying issue is trying to keep at least the stdlib Enum representations the same for those that are replacing preexisting constants. -- ~Ethan~ [0] https://mail.python.org/archives/list/python-dev@python.org/message/CHQW6TH… [1] I'm working on making their creation faster. If anyone wanted to convert EnumMeta to C I would be grateful. [2] https://bugs.python.org/issue36548 [3] https://github.com/python/cpython/pull/22392

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