[Python-ideas] Positional-only parameters

Victor Stinner victor.stinner at gmail.com
Tue Feb 28 16:17:31 EST 2017


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".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 functions:


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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