[Tutor] design?--having subclassed methods add logic in the middel of class methods

[Brian van den Broek]

Kent Johnson said unto the world upon 2005-02-21 16:40:
> Brian van den Broek wrote:
<SNIP>
>>
>> I am exploring ways of having the methods of a sub-class insert 
>> additional logic into their version of a class's methods, while still 
>> running the original class's method logic.
>>
>> All code below is pseudo-code.
>>
>> My Node class has a parse method which in part looks something like:
>>
>> def _parse(self, list_of_lines):
>>
>>     # some parsing based on whole list_of_lines
>>
>>     for line in list_of_lines:
>>         # Do line parsing stuff here
>>
>> In a subclass of Node class, I am wanting Subclassed_Node._parse() to 
>> behave somewhat differently than does Node._parse(). I need the 
>> subclass method to insert new logic into the for line in list_of_lines 
>> part of _parse method.
>>
>> Is this an acceptable design for doing this?
> 
> 
> Yes. This is an example of the Template Method pattern. It is a very 
> useful technique for allowing subclasses to specialize an operation.
> http://home.earthlink.net/~huston2/dp/templateMethod.html

Hi all,

thanks for the link, Kent. I came up with something that has a 
non-pejorative name? I must be getting warmer! ;-)

<SNIP my code sketch of how to do this `Template Method Pattern'>

>> Does this scream `Danger -- spaghetti code will result down the road'? 
>> The method name containing `preparse' indicates I might also feel a 
>> need for a `postparse' structure, too.
> 
> 
> No, IMO it's a reasonable design.
> 
>>
>> The only viable alternative I see (copy and paste is clearly worse ;-) 
>> is to break my Node._parse up into a part that works on the whole 
>> list_of_lines and a part that works on each line as in
>>

<SNIP example code sketch>

>>
>> Are there general reasons to prefer one approach over the other? 
>> Alternative ways I've overlooked?
> 
> 
> They are both template methods. I would choose between them based on 
> whether there is common processing between the subclasses or not. In 
> other words, in the version with _subclass_preparse() is there actually 
> some work for the base class to do after _subclass_preparse() is called? 
> If so, _subclass_preparse() might be a better design; if not, you might 
> as well just use _parse_line().

Thanks for the advice; useful as ever. :-)

Best to all,

Brian vdB