beginner, idomatic python 2

[Bruno Desthuilliers]

bambam a écrit :

<OT>Steve, could you please stop top-posting ?-) TIA </OT>

> "Dan Bishop" <danb_83 at yahoo.com> wrote in message 
> news:1187927388.173203.298660 at e9g2000prf.googlegroups.com...
>> On Aug 23, 10:21 pm, "bambam" <da... at asdf.asdf> wrote:
>>> Would someone like to suggest a replacement for this? This is a
>>> function that returns different kinds of similar objects, depending
>>> on what is asked for. PSP and PWR are classes.  I don't really
>>> want to re-write the calling code very much: I'm just wondering
>>> if the function can be replaced with some kind of OOP pattern.
>>>
>>> def Device(DeviceType):
>>>     if DeviceType=='PSP':
>>>         return PSP()
>>>     elif DeviceType=="Power Supply"
>>>         return PWR()
>>>     etc...
>>>
>>> Thanks!
>> Typically, you'd use a dictionary:
>>
>> DEVICE_DICT = {
>>    'PSP': PSP.
>>    'Power Supply': PWR,
>>    # etc.
>> }
>>
>> and your function would simply return DEVICE_DICT[device_type]()
>>
> Thank you. I didn't reply earlier because I was trying to get my
> head around what you wrote, which was strange and foreign
> to me.
> 
> It seems to me that the dictionary object you suggested is a
> direct replacement for the function code,

Almost, yes !-)

> only more efficient
> because the case table is internalised with a hash table, and
> the original if/elif/else case table was unlikely to be implemented
> as a hash table.
> 
> And presumably, it is idiomatic

It is. Dicts are probably the central data structure in Python, and are 
highly optimised. You'll find quite a lot of dict-based dispatch in 
Python code.

> because Python programmers
> expect to use dictionaries for their lookup tables.

Indeed - that's what dictionnaries are for.

> You have answered a question I didn't know enough to ask :~)
> --which is why I started with the general question, so I don't
> feel too stupid about that --.
> 
> And now I wonder about the 'other' question. Should I consider
> dynamically overriding the methods in my 'Device' class, instead
> of creating separate classes for the Psp and Pwr devices?
> I could create an object of the base Device class, and at init
> I could make sure the methods were connected for a Psp or
> a Pwr device. When (if ever) is that a good idea?

While it's technically possible, I wouldn't advise such a design (which 
looks pretty close to the Prototype pattern) unless you need to have 
something *highly* extensible and customisable (and even then, there are 
other - possibly better - ways, depending on the context). If the Device 
taxonomy is stable and well defined, you're certainly 
DoingTheSimplestThing (here, a dict-based dispatch encapsulated in a 
factory function...).

As a side note, in Python, inheritance is mostly an implementation 
detail, and should usually not be used for typing.

My 2 cents...