Okay, in light of Guido's comments, alternate idea:
We require all default values to be hash()-able, thus reasonably
ensuring their immutability. This doesn't deal with the 'x=None...'
dance, but at least it might stop dangerous code from being written. Or
if anyone else has ideas, that's great too. Anything to stop the abuses
of mutable default arguments.
- Chris Rebert
BJörn Lindqvist wrote:
> On 2/15/07, Jim Jewett <[EMAIL PROTECTED]> wrote:
>> On 2/13/07, Chris Rebert <[EMAIL PROTECTED]> wrote:
>> > There are currently few, if any, known good uses of the current
>> > behavior of mutable default arguments.
>>
>> Then are there *any* good use cases for the proposed semantics?
>
> Note that the PEP says _currently_, with the change in semantics the
> number of use cases increase drastically. See below.
>
>> Here are the use cases that I can remember seeing for mutable
>> default arguments.
>>
>> (1) Not really (treated as) mutable. ==> Doesn't care about the
>> mutability semantics.
>>
>> >>> def f(extra_settings={}) ...
>>
>> usually doesn't modify or even store extra_settings; it just wants an
>> empty (and perhaps iterable) mapping. (Sometimes, it doesn't even
>> need that, and is really just providing type information.)
>
> That is dangerous code. Sooner or later someone will modify the
> extra_settings dict. For me, that is the main attraction of the PEP,
> it removes that source of bugs (along with the annoying "if blaha is
> None:" thingy).
>
> class Vector:
> def __init__(self, x, y, z):
> self.x = x
> self.y = y
> self.z = z
>
> class Ray:
> def __init__(self, direction, origin = Vector(0, 0, 0)):
> self.direction = direction
> self.origin = origin
>
> ray1 = Ray(Vector(0, 0, -1))
> ray2 = Ray(Vector(0, 0, 1))
> ray3 = Ray(Vector(-1, 0, 0), Vector(2, 3, 4))
>
> The above code looks quite nice, but is wrong.
>
> Not that it matters much, Guido has already rejected the PEP. But the
> use cases does exist and there is a problem with how default argument
> values are evaluated. Hopefully someone can invent a fix even if this
> PEP wasn't it.
>
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