If you make an array and pass it to another class or function, by default they just get a reference to the same array.
so e.g.:
a = numpy.array ([1,2,3])
some_class.set_array(a)
a[1] = 10
Then both the local 'a' and the 'a' that some_class has are now [1,10,3].
If you don't want that sharing then you need to make an explicit copy of a by calling a.copy ().
Watch out for lists or dicts of arrays too. The python idom for copying a list: new_list = list_orig[:], won't copy the contents of elements that are array. If you want to be sure to make complete copies of complex data structures, there's the deepcopy method of the copy module. new_list = copy.deepcopy(list_orig).
I found a bunch of these sorts of bugs in some code I ported over from Matlab last week. Matlab uses copy semantics for everything, so if you pass a matrix A to a function in Matlab you can always treat it as a fresh local copy inside the function. Not so with Python. I found that locating and fixing those bugs was the most difficult thing about porting Matlab code to Numpy (that and the lack of some major toolkit or function you use in Matlab doesn't have an equivalent in Numpy... like eigs()).
--bb
On 8/26/06, Sven Schreiber <[EMAIL PROTECTED]> wrote:
Hi,
I experienced this strange bug which caused a totally unrelated variable
to be overwritten (no exception or error was raised, so it took me while
to rule out any errors of my own).
The context where this is in is a method of a class ( Vecm.getSW()), and
the instance of Vecm is created within a different class (GG.__init__).
Now, the affected variable is in the namespace of GG (it's GG.urate),
and so I would think that anything local in Vecm.getSW () should not
affect GG.urate, right?
Originally I did:
glx[lag:, :] -= temp
But that caused the described problem. Then I tried:
glx[lag:, :] = glx[lag:, :] - temp
But the same problem remains. Then I worked around the slice assignment
like this:
temp4 = r_[zeros([lag, n_y]), temp]
glx = glx - temp4
And everything is ok! However, when I alter the second line of this
workaround to:
glx -= temp4
The problem reappears! So I'm not even sure whether this is one or two
bugs...
This is with yesterday's numpy svn on windows, but the same thing
happens with an earlier svn (~b2) as well. If you need further info,
please tell me how to provide it.
Thanks,
Sven
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