Instance variables question

Thomas Jollans tjol at tjol.eu
Mon Apr 16 12:36:28 EDT 2018


On 2018-04-16 18:03, Irv Kalb wrote:
> He gives a demonstration using the following example:
> 
> class PartyAnimal():
>     x = 0
> 
>     def party(self):
>         self.x = self.x + 1
>         print('So far', self.x)
> 
> [snip]
> 
> But there is something there that seems odd.  My understanding is that the "x = 0" would be defining a class variable, that can be shared by all PartyAnimal objects.  But he explains that because x is defined between the class statement and the "party" method, that this defines an instance variable x.   That way, it can be used in the first line of the "party" method as self.x to increment itself.   
> 
> At the end of the video, he creates two objects from the same class, and each one gets its own self.x where each correctly starts at zero.  Again, I expected x to be a class variable (accessible through PartyAnimal.x).  
> 
> When I want to create an instance variable and to be used later in other methods, I do this:
> 
> class PartyAnimal():
>     def __init__(self):
>     	self.x = 0  
> 
>     def party(self):
>         self.x = self.x + 1
>         print('So far', self.x)
> 
> [snip]
> 
> That is, I would assign the instance variable in the __init__ method.  Both approaches give the same results.
> 
> I'm certainly not trying to argue with Dr. Chuck.  I am trying to understand his approach, but it's not clear to me why his code works.  Specifically, can anyone explain how his "x = 0" turns x into an instance variable - while also allowing the syntax for a class variable PartyAnimal.x to be used?
> 

"self.x = y", whatever self and y are, sets the attribute "x" of the
object "self". Whether "self.x" previously existed does not matter
(ignoring descriptors and the like).

If you access self.x, whether x exists obviously does matter, and
there's a fallback to looking in the class if the instance doesn't have it.

FWIW, I think this business of defining class attributes for things that
are instance-specific a bit daft. Your version strikes me as much more
pythonic.


-- Thomas


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