Class Encapsulation Errors in Python 2.3.3

Jeff Shannon jeff at ccvcorp.com
Fri Nov 19 21:10:01 CET 2004


Tim Henderson wrote:

>Here is a partial implementation, i don't have the code with me but
>this is how the songs list is made
>
>code:
>-------------------------------------------
>class Album:
>    
>    name = ''
>    songs = []
>    artist = ''
>
>    def __init__(self, name, artist):
>        
>        self.name = name
>        self.artist = artist
>-------------------------------------------
>  
>

Yes, this gives you class-level attributes, shared by all instances of 
Album.  In the case of name and artist, you're then creating 
instance-level attributes ('self.name = name') which shadow the 
class-level attribute.  But you never bind 'songs' on the instance, so 
you're just modifying the class-level (shared) songs list.

>after reading the code i came up with a possible solution:
>
>possible code:
>-------------------------------------------
>class Album:
>    
>    name = ''
>    songs = False
>    artist = ''
>
>    def __init__(self, name, artist):
>        
>        self.name = name
>        self.artist = artist
>        if songs == False:
>            songs = []
>------------------------------------------
>  
>

That'll almost work -- within __init__(), you need to refer to songs as 
self.songs (both times).  But why bother with creating the class-level 
attributes?  If you just set them in __init__(), all will be fine.

class Album:

    def __init__(self, name, artist):

        self.name = name
        self.artist = artist
        self.songs = []

will have the same net effect as your code, for the vast majority of 
common purposes.  (There are corner cases where having class-level 
defaults is helpful, but I sincerely doubt that this is true for your 
program.)

Jeff Shannon
Technician/Programmer
Credit International





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