default value in __init__

[Chris Rebert]

On Sun, Oct 19, 2008 at 12:56 AM, Paul McGuire <ptmcg at austin.rr.com> wrote:
> On Oct 14, 1:36 pm, "David C. Ullrich" <dullr... at sprynet.com> wrote:
>> > Well... How to say.. Is there any chance these people will read anything
>> > *at all* ?
>>
>> No. That's exactly the point! Basic Python is so transparent that
>> you can start using it without reading anything, just looking at
>> a few examples. _Because_ of that it's their responsibility to
>> ensure that if you look at a few examples you then have a complete
>> understanding of the language.
>>
> I agree, Python really does strive to be intuitive and easy-to-learn.
> So the oddity of the behavior of "optional_list_arg=[]" is a recurring
> surprise to those who jump first and read documentation later.
> Besides the tutorials, reference docs, and FAQs, there are also some
> web pages with titles like "Python Gotchas" and "Common Mistakes in
> Python" that usually tread this ground too.

Specifically:
http://www.onlamp.com/pub/a/python/2004/02/05/learn_python.html?page=2
http://www.ferg.org/projects/python_gotchas.html#contents_item_6
http://zephyrfalcon.org/labs/python_pitfalls.html

Cheers,
Chris
-- 
Follow the path of the Iguana...
http://rebertia.com

>
>> In particular default parameters should work the way the user
>> expects! The fact that different users will expect different
>> things here is no excuse...
>>
> Are you being sarcastic?  Short of "import mindreading", I don't know
> how Python would know which behavior a given user would expect.  Maybe
> instead of a "code smell", this particular Python wart is a "design
> smell".
>
> What is surprising is that Python cannot discriminate between this:
> y = 100
> def f(a,x=y):
>  print a+x
>>>> f(1)
> 101
>>>> y=200
>>>> f(1)
> 101
>
> and this:
>
> def f(a,x=[]):
>  print a+len(x)
>  x.append(a)
>>>> f(1)
> 1
>>>> f(1)
> 2
>>>> f(1,[1,2,3])
> 4
>>>>
>
> Is x supposed to be a default arg or a "static" arg (in the sense of a
> static var within a function as one finds in C)?
>
> -- Paul
> --
> http://mail.python.org/mailman/listinfo/python-list
>