Newbie Question regarding __init__()

Dave Angel davea at ieee.org
Sun Aug 2 11:51:45 CEST 2009


Simon wrote:
> Okay I will fix my code and include "self" and see what happens.  I
> know I tried that before and got another error which I suspect was
> another newbie error.
>
> The idea behind the init_Pre is that I can put custom code here to
> customize the __init__ instead of creating a new subclass.  This kind
> of hook pattern allows you to flatten your inheritance hierarchy.  I
> can choose in the init_Pre method to execute code before the init_exec
> (which contains the default __init__ code) and still execute the
> init_Exec method or I can completely customize the entire __init__ by
> returning False from init_Pre and prevent the init_Exec from being
> called.  I use this type of pattern with almost all my methods.  In
> this way I can create a less complicated inheritance chain but still
> have have custom code when needed without sub-classing.
>
> I am use to Visual FoxPro which where you can do
>
> =is.init_Pre().And.This.init_Exec() and the result is discarded so
> that is why it looks the way it does.  In this form init_Exec has to
> return a value.  However, If self.init_Pre(): self.init_Exec() would
> work the same and then I could avoid returning a value.
>
> Thanks,
> Simon
>
>
> On Aug 1, 5:52 am, Dave Angel <da... at ieee.org> wrote:
>   
>> Nat Williams wrote:
>>     
>>> As MRAB described, ALL instance methods need to accept 'self' as a first
>>> parameter, as that will be passed to them implicitly when they are called.
>>> This includes __init__.  The name 'self' is just a commonly accepted
>>> convention for the name of the instance object passed to methods.  You don't
>>> have to call it that, but you really should.
>>>       
>>> Take a look athttp://docs.python.org/tutorial/classes.html#class-objects
>>> It might help shed some light on how methods and instances work.
>>>       
>>> One other thing.  I'm a little confused by the first line of
>>> dcObject.__init__:
>>>       
>>> self.init_Pre() and self.init_Exec()
>>>       
>>> I suspect this does not do what you think it does.  init_Pre and init_Exec
>>> will both be called by this expression (unless init_Pre throws an exception,
>>> of course).  You're not getting anything here that you wouldn't by just
>>> calling each method on a separate line, except just making it harder to
>>> read.
>>>       
>> Read the doc-string for init_Pre() and for init_Exec().  The final
>> version of init_Pre() will return False in some circumstances, and in
>> those circumstances Simon doesn't want init_Exec() to be called.  He's
>> deliberately using the short-circuit evaluation of  'and'  to accomplish
>> that.
>>
>>
>>
>>
>>
>>
>>
>>     
>>> On Fri, Jul 31, 2009 at 8:53 PM, Simon <dciphercomput... at gmail.com> wrote:
>>>       
>>>> Hi
>>>>         
>>>> So should the dcObject class include the "self" as well since I have
>>>> not defined an __init__ method in dcCursor?
>>>>         
>>>> Simon
>>>>         
>>>> --
>>>> http://mail.python.org/mailman/listinfo/python-list
>>>>         
>> Every one of those methods in both of those classes need a "self" first
>> argument.  As others have said, all instance methods need a 'self.'
>>     
>
>   
(Please don't top-post.  You should put new responses at the end of 
quoted text, or sometimes inline if that's clearer.)

I don't understand your comparison to Foxpro.  read on.

As your code was last posted, you don't need a return value from 
init_Exec()  Every function that doesn't have an explicit return will 
return None.  And None is interpreted as False in an "and" expression.  
If you had an "if" around the whole thing, then you'd care.

DaveA



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