List weirdness - what the heck is going on here?
angrybaldguy at gmail.com
Thu Jan 28 03:18:34 CET 2010
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 = []*2
Consider that this is equivalent to
x = 
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?
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