On Sun, Oct 24, 2021 at 9:58 AM Chris Angelico <[email protected]> wrote:
> > > def puzzle(*, a=>b+1, b=>a+1):
> > > return a, b
> > >
> > > Aside: In a functional programming language
> > > a = b + 1
> > > b = a + 1
> > > would be a syntax (or at least compile time) error.
>
but it's a NameError in Python, yes? or maybe an UnboundLocalError,
depending on context.
> > There are two possibilities: either it's a SyntaxError, or it's a
> > run-time UnboundLocalError if you omit both of them (in which case it
> > would be perfectly legal and sensible if you specify one of them).
>
This, indeed, would be similar to the behaviour of the current idiom we're
trying to replace:
In [42]: def fun(a=None, b=None):
...: a = a if a is not None else b + 1
...: b = b if b is not None else a + 1
...: print(f'{a=} {b=}')
similar in the sense that whether it works or not depends on how the
function is called.
In [45]: fun()
---------------------------------------------------------------------------
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
In [46]: fun(3)
a=3 b=4
> > I'm currently inclined towards SyntaxError,
Except that it's not invalid syntax -- as far as syntax is concerned, you
can put any valid expression in there -- it's only "bad" if it happens to
use a name that is also being late bound. Is there any other place in
Python where a syntax error depends on what names are used (other than
keyworks, of course)?
I'll leave it to folks that understand the implementation a lot better than
I, but how hard would it be to make that trigger a SyntaxError?
So I vote for UnboundLocalError
> since permitting it would
> > open up some hard-to-track-down bugs, but am open to suggestions about
> > how it would be of value to permit this.
>
Even if not, maybe it should raise UnboundLocalError, rather than
SyntaxError
I like how it closely mirrors the current idiom, and I don't think saying
that it's evaluated left to right is all that complex.
But no, I can't think of a use case :-)
-CHB
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