[Python-ideas] Positional-only parameters

[Victor Stinner]

I am thinking at writing a PEP, yes. I need time to think about it, find
all corner cases. Maybe also include something for "optional parameter
without default value".

Don't expect it soon, I have some pending work to finish before :-)

Victor

Le 2 mars 2017 7:16 PM, "Brett Cannon" <brett at python.org> a écrit :

> It seems all the core devs who have commented on this are in the positive
> (Victor, Yury, Ethan, Yury, Guido, Terry, and Steven; MAL didn't explicitly
> vote). So to me that suggests there's enough support to warrant writing a
> PEP. Are you up for writing it, Victor, or is someone else going to write
> it?
>
> On Tue, 28 Feb 2017 at 13:18 Victor Stinner <victor.stinner at gmail.com>
> 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
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>
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