[Python-3000] pre-PEP: Default Argument Expressions

[Chris Rebert]

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 <jimjjewett at gmail.com> wrote:
>> On 2/13/07, Chris Rebert <cvrebert at gmail.com> 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.
>