List weirdness - what the heck is going on here?

Rotwang sg552 at
Thu Jan 28 13:11:05 CET 2010

Owen Jacobson wrote:
> On 2010-01-27 21:06:28 -0500, Rotwang <sg552 at> 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):
>> = [[0]]*2
> Consider that this is equivalent to
> def f(self):
>    x = [0]
> = [x, x]
> is now a list containing two references to the list referenced 
> by x -- so changes via either of the elements of 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.

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