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POIronpython: Function works in CPython, mysterious null pointer exception in IronPython
 

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  1. POIronpython: Function works in CPython, mysterious null pointer exception in IronPython
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    <p>I'm trying to do something that seems very simple, and falls within the range of standard python. The following function takes a collection of sets, and returns all of the items that are contained in two or more sets.</p> <p>To do this, while the collection of sets is not empty, it simply pops one set out of the collection, intersects it with the remaining sets, and updates a set of items that fall in one of these intersections.</p> <pre><code>def cross_intersections(sets): in_two = set() sets_copy = copy(sets) while sets_copy: comp = sets_copy.pop() for each in sets_copy: new = comp &amp; each print new, # Print statements to show that these references exist print in_two in_two |= new #This is where the error occurs in IronPython return in_two </code></pre> <p>Above is the function I'm using. To test it, in CPython, the following works:</p> <pre><code>&gt;&gt;&gt; a = set([1,2,3,4]) &gt;&gt;&gt; b = set([3,4,5,6]) &gt;&gt;&gt; c = set([2,4,6,8]) &gt;&gt;&gt; cross = cross_intersections([a,b,c]) set([2, 4]) set([]) set([4, 6]) set([2, 4]) set([3, 4]) set([2, 4, 6]) &gt;&gt;&gt; cross set([2, 3, 4, 6]) </code></pre> <p>However, when I try to use IronPython:</p> <pre><code>&gt;&gt;&gt; b = cross_intersections([a,b,c]) set([2, 4]) set([]) set([4, 6]) set([2, 4]) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; File "c:/path/to/code.py", line 10, in cross_intersections SystemError: Object reference not set to an instance of an object. </code></pre> <p>In the title I said this was a mysterious null pointer exception. I probably have no idea how .NET handles null pointers (I've never worked with a C-like language, and have only been using IronPython for a month or so), but if my understanding is correct, it occurs when you attempt to access some property of an object that points to <code>null</code>.</p> <p>In this case, the error occurs at line 10 of my function: <code>in_two |= new</code>. However, I've put <code>print</code> statements right before this line that (at least to me) indicate that neither of these objects point to <code>null</code>.</p> <p>Where am I going wrong?</p>
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    1. COIt seems to be a bug: http://ironpython.codeplex.com/workitem/30386. Should be alredy fixed in trunk.
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    2. COLooks like it. I was wondering if this could be the case, but I never like to blame the environment. Put that as an answer and I'll accept it.
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