On Jan 29, 2005, at 1:29 PM, Kirk Durston wrote:
It will be obvious that I am a newbie here when you see my question, but here goes. I am writing my first program, and have encountered a problem which I’ve been able to reproduce in the short program below. Basically, I select a row (B) from a matrix and I want to keep B constant. Then I create a new matrix by copying the old, and mutate the new matrix. I then want to substitute the unmutated row B into the mutated new matrix.
Problem: B changes, as you can see when you run the short program below. So does the original matrix, although I don’t care about that.
Question: Why does the list B change when I don’t work on it? I want to understand this.
Question #2: A solution would be to convert the list B into a tuple and then reconvert the tuple back into a list after the new matrix has been mutated and insert it, but I still want to understand why a list would change when I’m haven’t done anything to it.
What you're missing here is that everything in Python is an object, and variables are just *references* to objects. When you say ``a = b[0]``, ``a`` is a reference to ``b[0]``. If ``b[0]`` is some mutable object, like a list or a dict, you're simply referencing it. In this case, changing ``a`` would "also change" ``b[0]`` because they ``a`` and ``b[0]`` are references to the *same object*.
You should read this: http://starship.python.net/crew/mwh/hacks/objectthink.html
There is no need to convert a list to a tuple and back again. Two simple ways to make a shallow copy of a list are ``list(anotherList)`` or ``anotherList[:]``. You should also read the documentation for the ``copy`` module, whether or not you end up using it in your implementation.
-bob
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