[Python-ideas] Positional-only parameters
bussonniermatthias 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-Only
+1, it's likely possible to backport it to previous version using a
decorator and faking __signature__.
On 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.
> * 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
> * 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.
> * 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.
> On 2017-02-28 4:17 PM, Victor Stinner wrote:
>> 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)
>> replace(self, old, new, count=-1, /) # <== notice "/" at the end
>>>>> "a".replace("x", "y") # ok
>>>>> "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.
>> I just noticed a module on PyPI to implement this behaviour on Python
>> 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
>> Python 3.5 help (docstring) uses "S.replace(old, new[, count])".
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