Andrew Dalke wrote:
> On Feb 19, 2008 1:38 PM, Andrew Dalke <[EMAIL PROTECTED]> wrote:
>> def spam((a) = c):
>> print a
>
> On Feb 20, 2008 12:29 AM, Brett Cannon <[EMAIL PROTECTED]> wrote:
[..]
>> Are you asking why the decision was made to make the expression
>> illegal, or why the grammar is flagging it is wrong?
>
> Why it's illegal. Supporting a comma doesn't seem to make anything
> ambiguous, but PLY's LALR(1) might handle things that Python itself
> doesn't and I don't have the experience to tell.
>
It's illegal, in short, because formal parameters can be names or
tuples, and a parenthesized expression (albeit one containing only a
single term) is neither.
You could argue that parenthesizing a name shouldn't turn it into a
value, but to me there does seem to be a significant difference between
a and (a) - in Algol 68 terms, the former can be dereferenced
automatically while the latter cannot.
It's a pretty fine hair to split, though.
> There are other places where the grammar definition allows syntax that
> is rejected later on. Those I can justify by saying it's easier to
> define the grammar that way (hence f(1+1=2) is legal grammar but
> illegal Python), or to produce better error messages (like "from .a
> import *").
>
> But this one prohibiting [x for x in 1,] I can't figure out.
>
The one that surprised me was the legality of
def eggs((a, )=c):
pass
That just seems like unpacking-abuse to me.
You might argue that c might be a singleton tuple, though that is known
when the definition is processed, but there's no way you can say the
same of a float.
Python 2.5.1 (r251:54863, May 18 2007, 16:56:43)
[GCC 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def eggs((a, )=2.1):
... pass
...
>>>
Oops. 'def eggs((a, )=(2.1, )):' I'd have gone for, but your example
just seems broken. You really *have* been poking around in little-known
crevices, haven't you?
regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
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