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    <p>This is because the <code>stars</code> list is mutable - it is a changeable variable. When you call <code>stars.append</code>, it does not create a new list - it simple edits the existing list. And when you call <code>history.append(x)</code>, python does not create a fresh, clean copy of <code>x</code> - it assumes you want the "real" <code>x</code> to be placed in the <code>history</code> array.</p> <p>If you'd like to copy the state of the list at each iteration, there are several options (see <a href="https://stackoverflow.com/questions/2612802/how-to-clone-a-list-in-python">How to clone a list in python?</a>). </p> <p>The <a href="http://docs.python.org/2/library/copy.html" rel="nofollow noreferrer"><code>copy</code> module</a> will do the trick. It offers both 'shallow' and 'deep' copies - roughly speaking, deep copies attempt to also copy any other variables they find inside an object (for example, lists containing other lists), while shallow ones just go one layer down:</p> <p>Here's shallow copies with <code>copy</code>:</p> <pre><code>import copy history=[] for n in range(100): history.append(copy.copy(gal.time_stepper(.1))) </code></pre> <p>And deep copies:</p> <pre><code>import copy history=[] for n in range(100): history.append(copy.deepcopy(gal.time_stepper(.1))) </code></pre> <p>Slicing the array will work, since it always shallow copies as an intermediate step:</p> <pre><code>history = [] for n in range(100): history.append(gal.time_stepper(.1)[:]) </code></pre> <p>You can also call <code>list</code> on the existing list - this typecast-esque operation always returns a new list object (again, shallowly):</p> <pre><code>history = [] for n in range(100): history.append(list(gal.time_stepper(.1))) </code></pre>
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    1. POpython over-writing my list elements
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    1. COWhy use `copy.copy` instead of just `[:]`?
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    2. CO@abarnert Readability. `[:]` falls into the 'neat python tricks' category, to me.
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    3. COIt's a standard idiom, and it's used in both the tutorial section and the official FAQ answer that talk about this issue, so avoiding it is being unpythonic.
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