RFC deprecate positional arguments

I had a quick discussion with @NicolasHug following the discussion of keyword only arguments on the mailing list.

That should be possible to deprecate the use of positional arguments via a decorator, so that

def myfunction(a=1, b=2, c=3):
    return a + b + c

will become

def myfunction(*, a=1, b=2, c=3):
    return a + b + c

but will not break, but raise a deprecation warning if used with positional arguments.

Might look like:

from warnings import warn
from functools import wraps

def warn_args_wrapper(f):
    @wraps(f)
    def new_f(*args, **kwargs):
        if args:
            warn(DeprecationWarning("got args {}, you should use keyword args!".format(args)))
        return f(*args, **kwargs)
    return new_f

@warn_args_wrapper
def f(a=1, b=2, c=3):
    return a + b + c

We would need to put that decorator around all __init__ definitions (at least). It’s a big magic and I’m not sure how well it plays with sphinx.

This RFC is to float the idea and discuss whether it’s worth writing a SLEP / doing a prototype.

[thomasjpfan]

Here is a rough implementation:

import inspect

def warn_args_wrapper(f):
    argspec = inspect.getfullargspec(f)
    kwonlyargs = argspec.kwonlyargs
    orig_spec = argspec.args
    
    # For class methods
    has_self = 'self' in orig_spec
    
    @wraps(f)
    def inner_f(*args, **kwargs):
        extra_args = len(args) - len(orig_spec)
        if extra_args > 0:
            err_args = args[1:] if has_self else args
            warn("Got arguments, {}, should use keyword args for {}".format(
                 err_args, kwonlyargs[:extra_args]),
                 DeprecationWarning)
        kwargs.update({k: arg for k, arg in zip(orig_spec, args)})
        return f(**kwargs)
    return inner_f

For functions:

@warn_args_wrapper
def hello(a, b=3, *, h=4, j=6):
    return a + b + h + j

Calling hello(1, 3, 5) will result with a warning:

Got arguments, (1, 3, 5), should use keyword args for ['h']

For class methods:

class Hello:
    
    @warn_args_wrapper
    def __init__(self, penalty='l2', *, dual=False):

        self.penalty = penalty
        self.dual = dual

Running Hello('l2', True) will result in

DeprecationWarning: Got arguments, ('l2', True), should use keyword args for ['dual']

The warning can be adjusted.

[amueller]

closed with SLEP 009