Python-ideas March 2021

python-ideas@python.org

78 participants
44 discussions

Bringing the print statement back
by Guido van Rossum 02 Mar '22

02 Mar '22

In Python 3.10 we will no longer be burdened by the old parser (though 3rd party tooling needs to catch up). One thing that the PEG parser makes possible in about 20 lines of code is something not entirely different from the old print statement. I have a prototype: Python 3.10.0a0 (heads/print-statement-dirty:5ed19fcc1a, Jun 9 2020, 16:31:17) [Clang 11.0.0 (clang-1100.0.33.8)] on darwin Type "help", "copyright", "credits" or "license" for more information. Cannot read termcap database; using dumb terminal settings. >>> print 2+2 4 >>> print "hello world" hello world >>> print "hello", input("Name:") Name:Guido hello Guido >>> print 1, 2, 3, sep=", " 1, 2, 3 >>> But wait, there's more! The same syntax will make it possible to call *any* function: >>> len "abc" 3 >>> Or any method: >>> import sys >>> sys.getrefcount "abc" 24 >>> Really, *any* method: >>> class C: ... def foo(self, arg): print arg ... >>> C().foo 2+2 4 >>> There are downsides too, though. For example, you can't call a method without arguments: >>> print <built-in function print> >>> Worse, the first argument cannot start with a parenthesis or bracket: >>> print (1, 2, 3) 1 2 3 >>> C().foo (1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: C.foo() takes 2 positional arguments but 4 were given >>> print (2+2), 42 4 (None, 42) >>> C().foo [0] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'method' object is not subscriptable >>> No, it's not April 1st. I am seriously proposing this (but I'll withdraw it if the response is a resounding "boo, hiss"). After all, we currently have a bunch of complexity in the parser just to give a helpful error message to people used to Python 2's print statement: >>> print 1, 2, 3 File "<stdin>", line 1 print 1, 2, 3 ^ SyntaxError: Missing parentheses in call to 'print'. Did you mean print(1, 2, 3)? >>> And IIRC there have been a number of aborted attempts at syntactic hacks to allow people to call functions (like print) without parentheses, although (I think) none of them made it into a PEP. The PEG parser makes this much simpler, because it can simply backtrack -- by placing the grammar rule for this syntax (tentatively called "call statement") last in the list of alternatives for "small statement" we ensure that everything that's a valid expression statement (including print() calls) is still an expression statement with exactly the same meaning, while still allowing parameter-less function calls, without lexical hacks. (There is no code in my prototype that checks for a space after 'print' -- it just checks that there's a name, number or string following a name, which is never legal syntax.) One possible extension I didn't pursue (yet -- dare me!) is to allow parameter-less calls inside other expressions. For example, my prototype does not support things like this: >>> a = (len "abc") File "<stdin>", line 1 a = (len "abc") ^ SyntaxError: invalid syntax >>> I think that strikes a reasonable balance between usability and reduced detection of common errors. I could also dial it back a bit, e.g. maybe it's too much to allow 'C().foo x' and we should only allow dotted names (sufficient to access functions in imported modules and method calls on variables). Or maybe we should only allow simple names (allowing 'len x' but disallowing 'sys.getrefcount x'. Or maybe we should really only bring back print statements. I believe there are some other languages that support a similar grammar (Ruby? R? Raku?) but I haven't investigated. Thoughts? -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-…>

42 66

Adding a thin wrapper class around the functions in stdlib.heapq
by bunslow 17 Feb '22

17 Feb '22

Nothing so bombastic this time. The heapq functions are basically all named "heapsomething", and basically all take a "heap" for their first argument, with supplementary args coming after. It's a textbook example of the (hypothetical) Object Oriented Manifesto™ where defining a class increases type safety and programmers' conceptual clarity. There're practically no drawbacks, and the code to be added would be very simple. Updating the tests and docs would probably be harder. In pure Python, such a class would look like this: class Heap(list): def __init__(self, iterable=None): if iterable: super().__init__(iterable) else: super().__init__() self.heapify() push = heapq.heappush pop = heapq.heappop pushpop = heapq.heappushpop replace = heapq.heapreplace heapify = heapq.heapify # This could be a simple wrapper as well, but I had the following thoughts anyways, so here they are def nsmallest(self, n, key=None): # heapq.nsmallest makes a *max* heap of the first n elements, # while we know that self is already a min heap, so we can # make the max heap construction faster self[:n] = reversed(self[:n]) return heapq.nsmallest(n, self, key) # do we define nlargest on a minheap?? Wrapping around the C builtin functions (which aren't descriptors) would be a bit harder, but not much so: from functools import partial class Heap(list): def __init__(self, iterable=None): if iterable: super().__init__(iterable) else: super().__init__() self.heapify = partial(heapq.heapify, self) self.push = partial(heapq.heappush, self) ... self.heapify() Thoughts?

17 32

Allow syntax "func(arg=x if condition)"
by Peter O'Connor 19 Jan '22

19 Jan '22

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)

6 9

Barrier Object in asyncio lib
by Yves Duprat 10 May '21

10 May '21

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

6 13

allow initial comma
by roland.puntaier＠chello.at 16 Apr '21

16 Apr '21

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.

19 27

Top Level Await in Python like in Deno
by redradist＠gmail.com 15 Apr '21

15 Apr '21

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 ...

5 8

Add an export keyword to better manage __all__
by Theia Vogel 13 Apr '21

13 Apr '21

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.

15 28

Access to source code when using exec()
by Alex Hall 03 Apr '21

03 Apr '21

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-…

2 1

A "name" attribute for synchronization primitives
by Patrick Riehecky 26 Mar '21

26 Mar '21

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

2 1

Allow adding custom logging levels as attributes
by harry.lees＠gmail.com 26 Mar '21

26 Mar '21

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?

1 0

2022

2021

2020

2019

2018

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

Python-ideas March 2021