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copied!<p>You should not remove items from a list you are looping over. Create a copy instead:</p> <pre><code>for a in aliens[:]: </code></pre> <p>and</p> <pre><code>for b in bolts[:]: </code></pre> <p>Modifying a list while looping over it, affects the loop:</p> <pre><code>>>> lst = [1, 2, 3] >>> for i in lst: ... print i ... lst.remove(i) ... 1 3 >>> lst [2] </code></pre> <p>Removing items from a list you are looping over twice makes things a little more complicated still, resulting in a ValueError:</p> <pre><code>>>> lst = [1, 2, 3] >>> for i in lst: ... for a in lst: ... print i, a, lst ... lst.remove(i) ... 1 1 [1, 2, 3] 1 3 [2, 3] Traceback (most recent call last): File "<stdin>", line 4, in <module> ValueError: list.remove(x): x not in list </code></pre> <p>When creating a copy of the lists you are modifying at <em>each</em> level of your loops, you avoid the problem:</p> <pre><code>>>> lst = [1, 2, 3] >>> for i in lst[:]: ... for i in lst[:]: ... print i, lst ... lst.remove(i) ... 1 [1, 2, 3] 2 [2, 3] 3 [3] </code></pre> <p>When you have a collision, you only need to remove the <code>b</code> bolt <em>once</em>, not in the loop where you hurt the aliens. Clean out the aliens separately later:</p> <pre><code>def manage_collide(bolts, aliens): for b in bolts[:]: for a in aliens: if b['rect'].colliderect(a['rect']) and a['health'] > 0: bolts.remove(b) for a in aliens: a['health'] -= 1 for a in aliens[:]: if a['health'] <= 0: aliens.remove(a) return bolts, aliens </code></pre>

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