append to non-existing list

[Yves Glodt]

bruno at modulix wrote:
> Yves Glodt wrote:
>> Hello,
>>
>> if I do this:
>>
>> for row in sqlsth:
>> ________pkcolumns.append(row[0].strip())
>> ________etc
>>
>>
>> without a prior:
>>
>> pkcolumns = [];
>>
>>
>> I get this error on first iteration:
>> UnboundLocalError: local variable 'pkcolums' referenced before assignment
>>
>>
>> I guess that's normal as it's the way python works...?!?
> 
> yes sir.
> 
>> My question is: Is there no way to append to a non existing list?
> 
> No. Definitively. And that's a Good Thing(tm).
> 
> How would you use something that doesn't exist ???
> 
>> I am lazy for declaring it first,
> 
> s/declaring/instantiating/
> 
> If you were to use your own class Toto, would you ever hope that the
> following code would work :
> 
> for v in some_seq:
>   toto.dothis(v)
> 
> without instantiating Toto and binding it to the name 'toto' before ?
> Well, in Python, a list is an instance of class list. There are
> syntactic sugar to instanciate a list (or a tuple or a string or a dict
> etc), but this:
> 
> my_list = []
> 
> is strictly equivalent to this:
> 
> my_list = list()
> 
> Now let's try something else:
> 
> class Machin(object):
>   def append(self, value): pass
> 
> class Bidule(object):
>   def append(self, value): pass
> 
> for i in range(10):
>   m.append(i)
> 
> 
> How should Python interpret this ? Should it create a list, or a Machin,
>  or a Bidule, or an instance of whatever imported class having a
> append() method ?

ok I see your point, and python's...

(just FYI, and not to start a flamewar ;-):
In php, the [] means "append to an array object".

If the array does not exist yet, it's created. [] *is* explicit for 
arrays, thus for php it's clear what you want.)

>> IMHO it bloats the code,
> 
> run your python interpreter and type:
> import this
> 
> Then read carefully.
> 
> Now if being explicit still hurts your personal convictions, there's
> this other language with a name starting with p... !-)

no thanks, this is the 21st century ;-)

>> and (don't
>> know if it's good to say that here) where I come from (php) I was used
>> to not-needing it...
> 
> Not creating an Array before using it is Bad Style in PHP (and generate
> a Warning BTW).

an "undefined" notice, yes, not a warning... ;-)

> There are warts in Python (as in any other languages), and there are
> things that sometimes bore me but are not really warts. But having to
> explicitely instanciate objects can't be seen as a wart in any language
> IMHO !-)

Ok... I thank you for all the explanations.

It helps me to see more far. I (and will continue to) use php for web, 
and wanna standardize on python for all non-web stuff we are doing, so I 
might be a frequent guest on this list...

have a nice day,
Yves