default value in __init__

Bruno Desthuilliers bdesth.quelquechose at
Tue Oct 14 20:50:13 CEST 2008

David C. Ullrich a écrit :
> In article <48ef37fe$0$22798$426a34cc at>,
>  Bruno Desthuilliers <bruno.42.desthuilliers at websiteburo.invalid> 
>  wrote:
>> David C. Ullrich a écrit :
>>> 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

Hum... Who are "they", exactly ?

>>> 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

As far as I'm concerned, I started with the FineManual(tm)'s tutorial.


>> Well... How to say.. Is there any chance these people will read anything 
>> *at all* ?
> No. That's exactly the point! 

Yeps. But I don't think we derive the same conclusions from that point.

> Basic Python is so transparent that
> you can start using it without reading anything, just looking at
> a few examples.

And immediatly fall into a well-known (and well-documented) "gotcha" - 
like forgetting to apply the call operator to function, using mutable 
default arguments, using mutable class attributes instead of instance 
attributes, etc, etc, etc...

While it has been heavily publicized that Python is easy to get started 
with - which, AFAICT, seems to still be true for any programmer already 
having some experience *and* serious enough to actually _read_ that 
fucking manual -, it doesn't mean that, by the virtue of some 
extraterrestrial magic, one could master the language just reading a 
couple dummy examples. Learning a language requires some home work, 

> _Because_ of that it's their

who are "they" in this "their" ?

> responsibility to
> ensure that if you look at a few examples you then have a complete
> understanding of the language.

I'm afraid I have to say this is *totally* utopic - except perhaps for 
the most braindead and limited language one could imagine. Heck, even 
Java - which has explicitely been designed for morons^Mdummies - would 
fail this test. How could "a few examples" provide complete 
understanding of attribute lookup rules, the descriptor protocol, 
metaclasses, iterators, and generator expression - just to name a few ?

Granted, Python is easy to get started with. This doesn't mean anyone 
should expect it to work like any language XXX they already know. Every 
language has it's own pecularities, every language requires a serious 
learning effort, and Python is no exception here.

FWIW, on most programming language-related newgroups, the only answer to 
such FAQs would be (at best) a raw RTFM. Not that I'd advertise such a 
behaviour here - this newsgroup is mostly newbies-friendly and I 
*really* appreciate it to be that way (and happily answer the same 101 
questions year after year FWIW), but I really fail to see how anything 
more could be done for points that are 1/ clearly documented and 2/ 
reexplained here month after month.

> In particular default parameters should work the way the user
> expects! The fact that different users will expect different
> things here is no excuse...

If different users expect different - mostly incompatible - things, how 
would it be possible to have it working "the way the user expect" ? 
Should Python grow some telepathic features to guess the user's 
expectations and modifies itself to meet these expectations ?-)

More information about the Python-list mailing list