In article <[EMAIL PROTECTED]>,
Bruno Desthuilliers <[EMAIL PROTECTED]>
wrote:
> David C. Ullrich a écrit :
> > In article
> > <[EMAIL PROTECTED]>,
> > kenneth <[EMAIL PROTECTED]> wrote:
> >
> >> On Oct 9, 10:14 am, Christian Heimes <[EMAIL PROTECTED]> wrote:
> >>> kenneth wrote:
> >>>> the 'd' variable already contains the 'self.d' value of the first
> >>>> instance and not the default argument {}.
> >>>> Am I doing some stupid error, or this is a problem ?
> >>> No, it always contains the default argument because default values are
> >>> created just ONE
> >>> TIME.http://effbot.org/pyfaq/why-are-default-values-shared-between-objects
> >>> ..
> >>> .
> >>
> >> Wow, it's a very "dangerous" behavior ...
> >>
> >> Just to know, is this written somewhere in the python documentation or
> >> one has to discover it when his programs fails to work ;-) ?
> >
> > At least once a week someone discovers this "problem", makes a
> > post about it here, and then someone points to the spot in the
> > documentation where it's explained.
> >
> > Seems to me that people often site the "important warning" in
> > the tutorial. Of course there's no reason anyone would bother
> > going through the tutorial
>
> Indeed. No reason at all.
>
> > - just for fun I looked in the
> > official Python Reference Manual to see whether they're explicit
> > about this or require the reader to figure it out from something
> > else they say.
> >
> > There's a section titled "7.6 Function definitions". About halfway
> > through that section there's a _bold face_ statement
> > "Default parameter values are evaluated when the function definition is
> > executed.", followed by an explanation of how that can lead to
> > the sort of problem above.
>
> But there's no reason to read the reference manual neither.
>
> > So I guess it _is_ awfully dangerous. They should really explain
> > this aspect of the language's behavior to people who don't read
> > the formal definition and also don't work through the tutorial.
>
> You mean : "to people that don't bother reading the FineManual *nor*
> searching the newsgroup / ML archives ?"
Yes. Also add "don't read any books". I think I started with some
book - the book pointed out the "surprise" you get when you say
L = [[0]] * 10
L[0][0] = 1
or something similar. I was probably surprised once by the
mutable default parameter thing once but having understood
that other example it wasn't hard to see what was happening.
> 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.
In particular default parameters should work the way the user
expects! The fact that different users will expect different
things here is no excuse...
--
David C. Ullrich
--
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