Class extension confusion :(

[r0g]

On 11/11/10 09:34, Peter Otten wrote:
> r0g wrote:
>
>> Question B) The only reason I can think of so far is that I don't have a
>> clear picture of how those names came to end up in that scope, it seems
>> very convenient but I'm worried it's black magic of some sort! Could
>> anyone explain or point me to the right docs please?
>
> Python looks for attributes in the instance first, and then in the class as
> a fallback. You only need to put them in the instance if you expect that you
> want a different value for every instance.
>
> Peter
>


Thanks Peter, I think I understand now, I'll quickly explain my picture 
of things though so you or some other denizens of the mighty 
comp.lang.python can correct me if I'm wrong!...

I can access the parameters I pass to __init__ within the classes' 
methods without using self. as these methods are run within the 
constructor itself and are therefore within it's local scope.

That also explains why I had to call the constructor _after_ creating 
the new data attributes to have them be included. My initial belief that 
one has to call the parent constructor as the first action in the 
extended constructor is not technically valid, in most cases it can be 
called at any point but in situations like the above it can mess things 
up so I ought to put it at the end of my constructors for it to always 
work. Are there any drawbacks to calling it last or is that how it is 
supposed to work?

The BaseHTTPRequestHandler seemed curiously constructed to me at first 
too but I think I can see why now... as it's a handler it's only ever 
meant to be a transient thing and it requires no external parameters 
(unless you're me!) as it's always called by HTTPServer which has all 
the info necessary  so there's be no sense requiring users to create an 
instance then call a method to get the servers response, you may as well 
just bundle it all into one. Also as it might be called hundreds or 
thousands of times in quick succession it would be important to have it 
finish and get garbage collected.

I also see that I ought to at least assign these extended params to data 
attributes and access them via self. like I would normally do as I can't 
guarantee that implementation of BaseHTTPREquestHandler will remain 
constant over time. If I understand correctly it may also be possible 
(and more efficient) to use setattr() to inject the parameters I want 
into the class as class attributes before use, rather than assigning 
them to data attributes every time I instantiate an new instance.

Actually looking at the code from BaseRequestHandler it seems I may have 
overlooked the proper way of associating a callback function anyway. It 
looks like I should just define self.handle() in my subclass so that's 
one less parameter to worry about, although I think I may still need to 
use the factory function method you showed me to get the "paths" 
dictionary in there.

Thanks so much for all your help, I really appreciate it and, assuming 
I'm not totally wrong about all of the above, I find it reassuring that 
I'm not going mad!

Cheers,

Roger.