[Tutor] method, type?

[Alan Gauld]

On 06/01/16 23:00, Cameron Simpson wrote:

>>> ... ensures that if you subclass the class, you automatically
>>> get a valid constructor as well.
>>
>> Wouldn't it return an instance of the superclass rather than
>> the sub class? You'd need to override it wouldn't you?
> 
> No, when you call:
> 
>   SubClass.from_blah(...)
> 
> where from_blah is defined in the superclass, the leading "cls" parameter is 
> the subclass because you invoked it through the subclass, 

Yes, but my point is that constructors by their nature tend
to be very  specific.

For example a constructor that fetches data from a database
will embed a SQL query that is table specific and so may be
completely wrong for a sub class (unless it somehow shares
the superclass's table). Similarly reading from a network it
will only pull the fields necessary for the superclass from
the message. Indeed the trigger message it sends to the
server to initiate the transfer may well have the class
details included. So although you might wind up with an
object whose type is subclass, its details may be totally
wrong or even fail to operate (if the table has no
corresponding ID for example).

It would certainly work for a few cases but I suspect
most real-world constructors will fail unless overridden.
Other class methods are much more likely to work OK. (For
example those maintaining a cache of instances or simply
keeping a count) I'm just dubious about constructors
because they are so specific about where/how they obtain
their data. (Unless you get very clever about how you
use cls to access meta-data, or maintain some kind of
config mapping (possibly as class attributes) to tell
the constructor what to do.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos