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[Python-ideas] Positional-only parameters
Matthias Bussonnierbussonniermatthias at gmail.com
Tue Feb 28 17:08:53 EST 2017
I think that was started as pep 457:https://www.python.org/dev/peps/pep-0457/ (Syntax For Positional-OnlyParameters)Still informal.+1, it's likely possible to backport it to previous version using adecorator and faking __signature__.-- MOn Tue, Feb 28, 2017 at 2:03 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:> I'm +0.5 to add positional-only parameters.>> Pros:>> * A lot of people don't know what '/' currently means in> functions signatures rendered by `help` and docs. Because> it's not a real syntax, it's really hard to find what it> means.>> * Some APIs do look better with positional-only parameters,> especially functions with one argument.>> * We already have this feature half-implemented: some> builtin methods have positional-only arguments,> inspect.signature API supports it already.>> Cons:>> * Function declarations will become a bit more complex,> making a bump in Python learning curve.>> * Performance? I'm not sure if adding another set of checks> will make a huge impact, but we'll need to benchmark this.>> Yury>>>> On 2017-02-28 4:17 PM, Victor Stinner wrote:>>>> Hi,>>>> For technical reasons, many functions of the Python standard libraries>> implemented in C have positional-only parameters. Example:>> ------->> $ ./python>> Python 3.7.0a0 (default, Feb 25 2017, 04:30:32)>>>>>>>>>> help(str.replace)>>>> replace(self, old, new, count=-1, /) # <== notice "/" at the end>> ...>>>>>>>>>> "a".replace("x", "y") # ok>>>> 'a'>>>>>>> "a".replace(old="x", new="y") # ERR!>>>> TypeError: replace() takes at least 2 arguments (0 given)>> ------->>>> When converting the methods of the builtin str type to the internal>> "Argument Clinic" tool (tool to generate the function signature,>> function docstring and the code to parse arguments in C), I asked if>> we should add support for keyword arguments in str.replace(). The>> answer was quick: no! It's a deliberate design choice.>>>> Quote of Yury Selivanov's message:>> """>> I think Guido explicitly stated that he doesn't like the idea to>> always allow keyword arguments for all methods. I.e. `str.find('aaa')`>> just reads better than `str.find(needle='aaa')`. Essentially, the idea>> is that for most of the builtins that accept one or two arguments,>> positional-only parameters are better.>> """>>http://bugs.python.org/issue29286#msg285578>>>> I just noticed a module on PyPI to implement this behaviour on Python>> functions:>>>>https://pypi.python.org/pypi/positional>>>> My question is: would it make sense to implement this feature in>> Python directly? If yes, what should be the syntax? Use "/" marker?>> Use the @positional() decorator?>>>> Do you see concrete cases where it's a deliberate choice to deny>> passing arguments as keywords?>>>> Don't you like writing int(x="123") instead of int("123")? :-) (I know>> that Serhiy Storshake hates the name of the "x" parameter of the int>> constructor ;-))>>>> By the way, I read that "/" marker is unknown by almost all Python>> developers, and [...] syntax should be preferred, but>> inspect.signature() doesn't support this syntax. Maybe we should fix>> signature() and use [...] format instead?>>>> Replace "replace(self, old, new, count=-1, /)" with "replace(self,>> old, new[, count=-1])" (or maybe even not document the default>> value?).>>>> Python 3.5 help (docstring) uses "S.replace(old, new[, count])".>>>> Victor>> _______________________________________________>> Python-ideas mailing list>>Python-ideas at python.org>>https://mail.python.org/mailman/listinfo/python-ideas>> Code of Conduct:http://python.org/psf/codeofconduct/>>> _______________________________________________> Python-ideas mailing list>Python-ideas at python.org>https://mail.python.org/mailman/listinfo/python-ideas> Code of Conduct:http://python.org/psf/codeofconduct/
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