Python-ideas March 2021

python-ideas@python.org

78 participants
42 discussions

by Peter O'Connor

I often find that python lacks a nice way to say "only pass an argument under this condition". (See previous python-list email in "Idea: Deferred Default Arguments?") Example 1: Defining a list with conditional elements include_bd = True current_way = ['a'] + (['b'] if include_bd else [])+['c']+(['d'] if include_bd else []) new_way = ['a', 'b' if include_bd, 'c', 'd' if include_bd] also_new_way = list('a', 'b' if include_bd, 'c', 'd' if include_bd) Example 2: Deferring to defaults of called functions def is_close(a, b, precicion=1e-9): return abs(a-b) < precision def approach(pose, target, step=0.1, precision=None): # Defers to default precision if not otherwise specified: velocity = step*(target-pose) \ if not is_close(pose, target, precision if precision is not None) \ else 0 return velocity Not sure if this has been discussed, but I cannot see any clear downside to adding this, and it has some clear benefits (duplicated default arguments and **kwargs are the scourge of many real world code-bases)

1 week

Barrier Object in asyncio lib

by Yves Duprat

Hi,the list, I'm wondering why Barrier object does not exist in the synchronization primitives of the asyncio lib while it is present in threading and multiprocessing libs ? This may not be the right place to ask this question, but I never found an answer on the web. Thanks for your help. Yves

8 months, 2 weeks

allow initial comma

by roland.puntaier＠chello.at

I had posted this as https://github.com/python/peps/issues/1867 The discussion so far is below. Please make some arguments. The major point to me is, that the symmetry is broken, which leads to extra editing actions, like removing the comma in the first line. I guess, this was the reason to allow the comma after the last line/entry: `[1,2,]`. ``[,1,2]`` should also be allowed, too. The best argument is one that says: less or more effort in this or that situation. For example, with `[1,2,]`, in line-wise formatting, one can do without special action at the last line (removing the comma there). All code from previous versions of Python would still work after a `[,1,2]` syntax allowance were introduced. ================================================================================= rpuntaie wrote: ================================================================================= Allow initial comma =================== Final comma works: t = ( 1, 2, ) x = [ 1, 2, ] y = { 1, 2, } z = { 1:11, 2:22, } def fun( a, b, ): pass Initial comma does not work: t = ( , 1 , 2 ) x = [ , 1 , 2 ] y = { , 1 , 2 } z = { , 1:11 , 2:22 } def fun( , a , b ): pass To make the syntax symmetric in this regard\ gives more freedom to format the code. I occasionally found the restriction an unnecessary nuisance. Before writing a PEP, I would like to discuss, - whether something like that has been proposed already? - what counter-arguments there could be? ================================================================================= pxeger wrote: ================================================================================= This is not the appropriate place to propose language changes. Try the [python-ideas](https://mail.python.org/mailman3/lists/python-ideas.python.or… mailing list. However, I don't think you'll get anyone to agree. What kind of code style are you using where you want to put commas at the start of the line? That is totally non-standard (see [PEP 8](https://www.python.org/dev/peps/pep-0008)), ugly, and confusing. Arbitrary symmetry is not a good reason for changing the language. We don't have a `tnirp` function just for the sake of symmetry with `print` because it would be pointless and add extra complication ================================================================================= rpuntaie wrote: ================================================================================= I surely agree, that not ignoring the sequence is essential. Else one would loose identifier space and thus information. I would never have the idea to make all permutations of `p.r.i.n.t` point to the same function. Therefore you just made a bad example. But the comma is just a separator. Why did they allow to have the comma before a closing bracket/parenthesis/brace? Because of symmetry between lines, is my guess. Occasionally one sees many spaces just the have the final comma aligned vertically. That could be avoided by placing the comma at the beginning. I personally also have a macro in the editor that evaluates a line in the parameter list, but drops an initial comma before doing that. Therefore this is my preferred formatting. I don't think that [PEP](https://www.python.org/dev/peps/pep-0008) is wrong. I just don't want to be restricted by unnecessary rules. Rules need to have a reason beyond someone dictating them. If that is the case, I follow them, because I see the reason, but not because someone dictates them. I'll go to [Python Ide as](https://mail.python.org/mailman3/lists/python-ideas.python.org/), then. Thanks.

9 months, 2 weeks

Top Level Await in Python like in Deno

by redradist＠gmail.com

Hi all, Deno is JavaScript runtime that has very nice feature like Top Level Await, I think it would be also nice to have such feature in Python, it will make using async/await more convenient What do you think ? Share your ideas lets discuss ...

9 months, 2 weeks

Add an export keyword to better manage __all__

by Theia Vogel

Hi, I was refactoring some code today and ran into an issue that always bugs me with Python modules. It bugged me enough this time that I spent an hour banging out this potential proposal to add a new contextual keyword. Let me know what you think! Theia -------------------------------------------------------------------------------- A typical pattern for a python module is to have an __init__.py that looks something like: from .foo import ( A, B, C, ) from .bar import ( D, E, ) def baz(): pass __all__ = [ "A", "B", "C", "D", "E", "baz", ] This is annoying for a few reasons: 1. It requires name duplication a. It's easy for the top-level imports to get out of sync with __all__, meaning that __all__, instead of being useful for documentation, is actively misleading b. This encourages people to do `from .bar import *`, which screws up many linting tools like flake8, since they can't introspect the names, and also potentially allows definitions that have been deleted to accidentally persist in __all__. 2. Many symbol-renaming tools won't pick up on the names in __all__, as they're strings. Prior art: ================================================================================ # Rust Rust distinguishes between "use", which is a private import, "pub use", which is a globally public import, and "pub(crate) use", which is a library-internal import ("crate" is Rust's word for library) # Javascript In Javascript modules, there's an "export" keyword: export function foo() { ... } And there's a pattern called the "barrel export" that looks similar to a Python import, but additionally exports the imported names: export * from "./foo"; // re-exports all of foo's definitions Additionally, a module can be gathered and exported by name, but not in one line: import * as foo from "./foo"; export { foo }; # Python decorators People have written utility Python decorators that allow exporting a single function, such as this SO answer: https://stackoverflow.com/a/35710527/1159735 import sys def export(fn): mod = sys.modules[fn.__module__] if hasattr(mod, '__all__'): mod.__all__.append(fn.__name__) else: mod.__all__ = [fn.__name__] return fn , which allows you to write: @export def foo(): pass # __all__ == ["foo"] , but this doesn't allow re-exporting imported values. # Python implicit behavior Python already has a rule that, if __all__ isn't declared, all non-underscore-prefixed names are automatically exported. This is /ok/, but it's not very explicit (Zen) -- it's easy to accidentally "import sys" instead of "import sys as _sys" -- it makes doing the wrong thing the default state. Proposal: ================================================================================ Add a contextual keyword "export" that has meaning in three places: 1. Preceding an "import" statement, which directs all names imported by that statement to be added to __all__: import sys export import .foo export import ( A, B, C, D ) from .bar # __all__ == ["foo", "A", "B", "C", "D"] 2. Preceding a "def", "async def", or "class" keyword, directing that function or class's name to be added to __all__: def private(): pass export def foo(): pass export async def async_foo(): pass export class Foo: pass # __all__ == ["foo", "async_foo", "Foo"] 3. Preceding a bare name at top-level, directing that name to be added to __all__: x = 1 y = 2 export y # __all__ == ["y"] # Big Caveat For this scheme to work, __all__ needs to not be auto-populated with names. While the behavior is possibly suprising, I think the best way to handle this is to have __all__ not auto-populate if an "export" keyword appears in the file. While this is somewhat-implicit behavior, it seems reasonable to me to expect that if a user uses "export", they are opting in to the new way of managing __all__. Likewise, I think manually assigning __all__ when using "export" should raise an error, as it would overwrite all previous exports and be very confusing.

9 months, 2 weeks

Access to source code when using exec()

by Alex Hall

When using `exec(string)`, the standard Python REPL, or `python -c` in a terminal, the executed source code is not stored anywhere and cannot be retrieved (except in the `-c` case by looking at `sys.argv`). This means that: - The source line doesn't show up in tracebacks, e.g: >>> exec("1/0") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<string>", line 1, in <module> ZeroDivisionError: division by zero - Users cannot retrieve the source code using `inspect`, although many want to: - https://stackoverflow.com/questions/29850508/python-how-to-inspect-code-of-… - https://stackoverflow.com/questions/12072252/cant-get-source-code-for-a-met… - https://stackoverflow.com/questions/62160529/how-to-get-source-inside-exec-… - https://stackoverflow.com/questions/59244323/how-can-you-get-the-source-cod… - https://stackoverflow.com/questions/50822656/source-code-for-an-evaled-dyna… - https://stackoverflow.com/questions/50515651/why-does-the-python-linecache-… - Libraries such as traceback formatters and debuggers which depend on source code can't work. This often manifests when people try out a library for the first time in the REPL and it doesn't seem to work. This leads to many similar looking bug reports which sometimes take a while to resolve. Probably many more people think these libraries don't work without ever opening an issue. - https://github.com/cknd/stackprinter/issues/34 - https://github.com/Qix-/better-exceptions/issues/17 - https://github.com/Qix-/better-exceptions/issues/68 - https://github.com/aroberge/friendly-traceback/issues/23 - https://github.com/gruns/icecream/issues/17 - https://github.com/cool-RR/PySnooper/issues/135 - https://github.com/cool-RR/PySnooper/issues/164 - https://github.com/alexmojaki/sorcery/issues/3 - https://github.com/alexmojaki/sorcery/issues/5 - https://github.com/alexmojaki/sorcery/issues/6 I suggest that: 1. `exec(string)` assigns a unique generated filename to the compiled code object and stores the source code in the `linecache` module under that filename, similar to what the IPython REPL does. I imagine there might be complaints about memory usage, security implications of revealing source code, compatibility, etc. If that's an issue this could be off by default with a flag to turn it on. 2. The REPL, `-c`, and other similar mechanisms (IDLE?) either follow the same strategy as `exec` or defer to it. 3. The standard exception hook falls back to `linecache` for filenames that don't exist. I don't know why it doesn't already. Here is a question on that topic: https://stackoverflow.com/questions/50515651/why-does-the-python-linecache-…

9 months, 4 weeks

A "name" attribute for synchronization primitives

by Patrick Riehecky

Hello, I've been chasing down various synchronization bugs in a large codebase I'm working on. In the process I began to realize how useful it would be to have some sort of descriptor (a name if you will) attached to some of my primitives. In this code base, I've a number of threading.Event objects that get passed around to check for conditions. In a perfect world, every developer would have used the same nomenclature and been consistent everywhere. Alas.... Currently there isn't a great way built into the language for me to say "which Event is this <threading.Event object at 0x7f66da73ed60> ?" unless I already have a mapping of the addresses to variable names in my logs. Would a `name=` keywordonly argument for things like Lock, Event, Condition, Barrier, Semaphore, etc be welcome (for threading, multiprocess, async, etc)? Inside the code base, such an attribute would let me do things like: mycondition.wait() print(f'Met condition {mycondition.name}') or print(f'Waiting on barrier {mybarrier.name}') mybarrier.wait() And similar. Simplifying deep introspection into which sync primitives are which feels like it would be a benefit as the async world grows within Python. Thoughts? Pat

10 months

Allow adding custom logging levels as attributes

by harry.lees＠gmail.com

I have just been writing some code in which I define a custom log level with the logging module. I wanted to call the new log level as an attribute but currently the logging library lacks a nice way to do this. I know that in the docs https://docs.python.org/3/howto/logging.html#custom-levels it discourages creating custom log levels so perhaps this was why there is no way to do this. This https://stackoverflow.com/questions/2183233/how-to-add-a-custom-loglevel-to… StackOverflow solution allows for the syntax I was after, but it is hard to understand for someone looking at the code. import logging CUSTOM_LEVEL = 31 CUSTOM_LEVEL_NAME = 'CUSTOM_WARNING' def custom_warning(self, message, *args, **kwargs): if logger.isEnabledFor(CUSTOM_LEVEL): logger._log(CUSTOM_LEVEL, message, args, **kwargs) logging.Logger.custom_warning = custom_warning logger = logging.getLogger(__name__) logger.warning('a warning here!') logger.custom_warning('a custom warning!') I would propose something like logger.addCustomLevel(CUSTOM_LEVEL, CUSTOM_LEVEL_NAME) There are some obvious limitations to this approach, all level names would also have to be valid function names otherwise something like logger.addCustomLevel(CUSTOM_LEVEL, "1 2 3") would be an issue what does everybody think?

10 months

Make fnmatch.filter accept a tuple of patterns

by Andre Delfino

Frequently, while globbing, one needs to work with multiple extensions. I’d like to propose for fnmatch.filter to handle a tuple of patterns (while preserving the single str argument functionality, alas str.endswith), as a first step for glob.i?glob to accept multiple patterns as well. Here is the implementation I came up with: https://github.com/python/cpython/compare/master...andresdelfino:fnmatch-mu… If this is deemed reasonable, I’ll write tests and documentation updates. Any opinion?

10 months

Improve the textual representation of IPv4-mapped IPv6 addresses

by Maxime Mouchet

Hi, Python supports IPv4-mapped IPv6 addresses as defined by RFC 4038: "the IPv6 address ::FFFF:x.y.z.w represents the IPv4 address x.y.z.w.” The current behavior is as follows: from ipaddress import ip_address addr = ip_address('::ffff:8.8.4.4') # IPv6Address('::ffff:808:404') addr.ipv4_mapped # IPv4Address('8.8.4.4') Note that the textual representation of the IPv6Address is *not* in IPv4-mapped format. It prints ::ffff:808:404 instead of ::ffff:8.8.4.4. This is technically correct, but it’s somewhat frustrating as it makes it harder to read IPv4s embedded in IPv6 addresses. My proposal would be to check, in __str__, if an IPv6 is an IPv4-mapped, and to return the appropriate representation : from ipaddress import ip_address addr = ip_address('::ffff:8.8.4.4') # Current behavior str(addr) # '::ffff:808:404' repr(addr) # IPv6Address('::ffff:808:404') # Proposed behavior str(addr) # '::ffff:8.8.4.4' repr(addr) # IPv6Address('::ffff:8.8.4.4') A few data points: - Julia prints ::ffff:808:404 (current behavior) - C (glibc) and ClickHouse prints ::ffff:8.8.4.4 (proposed behavior) Any thoughts? Maxime

10 months, 1 week

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Python-ideas March 2021