List weirdness - what the heck is going on here?

[Owen Jacobson]

On 2010-01-27 21:06:28 -0500, Rotwang <sg552 at hotmail.co.uk> said:

> Hi all, I've been trying to make a class with which to manipulate sound 
> data, and have run into some behaviour I don't understand which I hope 
> somebody here can explain. The class has an attribute called data, 
> which is a list with two elements, one for each audio channel, each of 
> which is a list containing the audio data for that channel. It also has 
> various methods to write data such as sine waves and so on, and a 
> method to insert data from one sound at the start of data from another. 
> Schematically, the relevant bits look like this:
> 
> class sound:
>      def f(self):
>          self.data = [[0]]*2

Consider that this is equivalent to

def f(self):
    x = [0]
    self.data = [x, x]

self.data is now a list containing two references to the list 
referenced by x -- so changes via either of the elements of self.data 
will affect both elements. Your comprehension version creates a list 
containing two distinct list objects, so this doesn't happen.

> Can anybody tell me what's going on?

-o