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
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