List weirdness - what the heck is going on here?
Rotwang
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
def insert(self, other):
for c in xrange(2):
self.data[c][0:0] = other.data[c]
However, the insert method doesn't work properly; x.insert(y) adds two
copies of y's data to the start of x's data, instead of one. From a
session in IDLE:
>>> x = sound()
>>> y = sound()
>>> x.f()
>>> y.f()
>>> x.data
[[0], [0]]
>>> x.insert(y)
>>> x.data
[[0, 0, 0], [0, 0, 0]]
But suppose I replace the line
self.data = [[0]]*2
with
self.data = [[0] for c in xrange(2)]
Then it works fine:
>>> x = sound()
>>> y = sound()
>>> x.f()
>>> y.f()
>>> x.data
[[0], [0]]
>>> x.insert(y)
>>> x.data
[[0, 0], [0, 0]]
Can anybody tell me what's going on?
Owen Jacobson
> 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
alex23
> Can anybody tell me what's going on?
Your problem is basically this:
>>> a = [1]
>>> b = [a] * 2
>>> b
[[1], [1]]
>>> a.append(2)
>>> b
[[1, 2], [1, 2]]
The expression '[a] * 2' doesn't make two copies of list of a list of
a, it makes two nested _references_ to it. When you modify the 'first'
list, you're seeing that changed reflected in the second reference.
The list comprehension, however, is returning a _new_ list each time.
Changing its contents doesn't affect any of the other independently
created lists.
Arnaud Delobelle
It's a FAQ!
http://www.python.org/doc/faq/programming/#how-do-i-create-a-multidimensional-list
--
Arnaud
Rotwang
Thanks, and likewise to everyone else who replied.