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copied!<p>Well, the first problem is that your function just returns <code>None</code>, just throwing away the two lists, so there's no way to even see whether it's doing the right thing.</p> <p>If you fix that, you'll see that it <em>is</em> doing the right thing.</p> <pre><code>def balance_equation(species,coeff): data=zip(coeff,species) positive=[] negative=[] for (mul,el) in data: if mul<0: negative.append((el,mul)) if mul>0: positive.append((el,mul)) return negative, positive >>> n, p = balance_equation(balance_equation('abcdef', range(-3,3)) >>> n [('a', -3), ('b', -2), ('c', -1)] >>> p [('e', 1), ('f', 2)] </code></pre> <p>So, there are two possibilities:</p> <ol> <li>Since the code you pasted is clearly not the actual code you're running, maybe you fixed the bug while rewriting it to post here.</li> <li>You're not calling it with sensible inputs. For example, if you pass the parameters backward, since <code>species</code> is presumably a collection of strings, they'll all end up positive. Or, likewise, if you pass the coeffs as string representations of integers.</li> </ol> <p>If it's the last problem—you're passing, say, <code>'abcdef', ['-3', '-2', '-1', '0', '1', '2', '3']</code>, and you want to deal with that within balance_equation instead of in the calling code, that's easy. Just add this line before the <code>zip</code>:</p> <pre><code>coeff = [int(x) for x in coeff] </code></pre> <p>Or change your <code>zip</code> to:</p> <pre><code>data = zip((int(x) for x in coeff), species) </code></pre> <p>By the way, I'm assuming you're on CPython 2. In Python 3, trying to compare a string to 0 will raise a <code>TypeError</code> instead of always returning <code>True</code>, while in other Python 2 implementations it might always return <code>False</code> instead of <code>True</code>…</p>

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