[newbie] Looking for a good introduction to object oriented programming with Python

[lipska the kat]

On 07/08/12 06:19, Steven D'Aprano wrote:
> On Mon, 06 Aug 2012 09:55:24 +0100, lipska the kat wrote:
>
>> On 06/08/12 01:22, Steven D'Aprano wrote:
>>> On Sun, 05 Aug 2012 20:46:23 +0100, lipska the kat wrote:
>>>

[snip]

>>
>> The clue is in the name 'Object Oriented' ... anything else is (or
>> should be) implementation detail.
>
> So your argument is that any programming which is "oriented" (meaning
> what?) towards "objects" (which are...?) is OOP, and everything else is
> "implementation detail".
>
> Well now I'm enlightened. That certainly clears that up for me.

Glad I could help :-)

[snip]

>
> As you go on to explain:
>
>> A class variable is static and can be
>> accessed without instantiation an instance variable is accessed via an
>> instance
>
> which are semantic differences, differences in meaning and function.

Yes but when we TALK about these things a String variable is a String 
variable is a String variable. The words 'Class variable' and 'instance 
variable' are 'abstractions'. It saves us saying,

'this is a variable that can only be accessed from an instance and may 
hold values of the type Integer or String or Weak Reference ... (and so 
on ad nauseum) ... but only of one type unless you count runtime 
polymorphism in which case the runtime type may be different from the 
compile time type ... etc etc'

or 'this is a variable that can be accessed without instantiating a 
class (see above)'

If you don't like the term abstraction then perhaps we can agree on 
something else.

>>> Since a "string variable" is a variable holding a string, and a "float
>>> variable" is a variable holding a float, an instance variable should be
>>> a variable holding an instance, and a class variable should be a
>>> variable holding a class.

See above

>> Class variable and instance variable are higher level abstractions.
>
> Of what? Of variables?

Exactly, now you're getting the hang of it

[snip]

> Simply put, the choice of terminology is crap --

possibly but it's the best we've got.

> But what *really* gets me is not the existence of poor terminology. I
> couldn't care less what terminology Java programmers use among
> themselves.

I'd be most grateful if you could park the whole 'This person is a 'Java 
developer so must be a moron thing' it's getting a bit wearing.
As I said in a previous post, Java has indeed been good to me but my 
brain IS capable of dealing with more than one language.

> What gets me is that the ubiquity of them means that
> substandard terminology spreads into other languages, like dryrot.

Yea well welcome to the world of spin, if you can't fight it then learn 
to roll with it.

>>>> The ONLY concept that you should never try to encapsulate is/are human
>>>> beings or their aliases.
>>
>> snip
>>
>>> Is this some sort of mystical "humans aren't objects, they're
>>> SPECIAL!!!" rubbish? Because it sure sounds like it.

[snip]

Well now this is a personal thing born of bitter experience. In my 
experience, when you have an entity called 'Person' or some such in your 
Class model it soon becomes what we 'in the trade' call a 'God Object' 
The name should be self explanatory but hold tight, here comes some more 
jargon.

Objects can have a 'has a' relationship with other Objects or an 'is a' 
relationship with other objects

The 'has a' relationship means that an Object 'owns' another object, the 
'is a' relationship means that an Object 'is of a particular type'
Of course in an 'Object Oriented' language such as Java an Object 
reference can have a different type at runtime than it does at compile 
time. In Python too.

Anyway, this Person thing soon ends up with a 'has a' relationship with 
everything in sight. a Person 'has a[n]' Address, a Person 'has a[n]' 
account, a Person 'has a' Doughnut etc etc etc

Also, inevitably a Person 'is a' Customer, a Person 'is a' Contact, a 
Person 'is a' security risk, well you get the idea.

Of course this is a problem with the actual design process itself and 
yes, the identification of the 'nouns in the language of the domain' is 
an important part of the process. Sorry, but it just works when it's 
done properly, I know it works as I used this technique to turn around 
this 'Person as God' design from a failing money pit into a working 
system that delivered (almost) on time. The very first thing I did was 
to exorcise Person from the design.

>>>> I've already managed to write meaningful code but I haven't invented a
>>>> single class yet.
>>>
>>> And why do you think this is a problem?
>>
>> A problem? I wasn't ware that I'd stated it was a problem it just
>
> You said "BUT [emphasis added] I haven't invented a single class yet",
> which implies that this is a bad thing

[snip]

No it implies that I noticed it was possible to do meaningful work in 
Python without writing a class.


Well this HAS been fun, I look forward to your reply but at the moment I 
have to go and pick some runner beans as it's been a fantastic year for 
beans and the freezer awaits.

lipska

-- 
Lipska the Kat: Troll hunter, sandbox destroyer
and farscape dreamer of Aeryn Sun