List weirdness - what the heck is going on here?

[Rotwang]

Owen Jacobson wrote:
> 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.

Thanks, and likewise to everyone else who replied.