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    <p><strong>EDIT:</strong></p> <p>Indeed there was a <a href="http://bugs.python.org/msg58786" rel="noreferrer">patch</a> which included <code>sign()</code> in <a href="http://docs.python.org/library/math.html#math.copysign" rel="noreferrer">math</a>, but it wasn't accepted, because they didn't agree on <a href="http://bugs.python.org/msg59137" rel="noreferrer">what it should return in all the edge cases</a> (+/-0, +/-nan, etc)</p> <p>So they decided to implement only copysign, which (although more verbose) can be <a href="http://bugs.python.org/msg59154" rel="noreferrer">used to delegate to the end user the desired behavior for edge cases</a> - which <a href="http://bugs.python.org/msg59152" rel="noreferrer">sometimes might require the call to <code>cmp(x,0)</code></a>.</p> <hr> <p>I don't know why it's not a built-in, but I have some thoughts.</p> <pre><code>copysign(x,y): Return x with the sign of y. </code></pre> <p>Most importantly, <code>copysign</code> is a superset of <code>sign</code>! Calling <code>copysign</code> with x=1 is the same as a <code>sign</code> function. So you could just use <code>copysign</code> and <em>forget about it</em>.</p> <pre><code>&gt;&gt;&gt; math.copysign(1, -4) -1.0 &gt;&gt;&gt; math.copysign(1, 3) 1.0 </code></pre> <p>If you get sick of passing two whole arguments, you can implement <code>sign</code> this way, and it will still be compatible with the IEEE stuff mentioned by others:</p> <pre><code>&gt;&gt;&gt; sign = functools.partial(math.copysign, 1) # either of these &gt;&gt;&gt; sign = lambda x: math.copysign(1, x) # two will work &gt;&gt;&gt; sign(-4) -1.0 &gt;&gt;&gt; sign(3) 1.0 &gt;&gt;&gt; sign(0) 1.0 &gt;&gt;&gt; sign(-0.0) -1.0 &gt;&gt;&gt; sign(float('nan')) -1.0 </code></pre> <p>Secondly, usually when you want the sign of something, you just end up multiplying it with another value. And of course that's basically what <code>copysign</code> does.</p> <p>So, instead of:</p> <pre><code>s = sign(a) b = b * s </code></pre> <p>You can just do:</p> <pre><code>b = copysign(b, a) </code></pre> <p>And yes, I'm surprised you've been using Python for 7 years and think <code>cmp</code> could be so easily removed and replaced by <code>sign</code>! Have you never implemented a class with a <code>__cmp__</code> method? Have you never called <code>cmp</code> and specified a custom comparator function?</p> <p>In summary, I've found myself wanting a <code>sign</code> function too, but <code>copysign</code> with the first argument being 1 will work just fine. I disagree that <code>sign</code> would be more useful than <code>copysign</code>, as I've shown that it's merely a subset of the same functionality.</p>
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    1. POWhy doesn't Python have a sign function?
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    1. COUsing `[int(copysign(1, zero)) for zero in (0, 0.0, -0.0)]` gives `[1, 1, -1]`. That should have been `[0, 0, 0]` according to http://en.wikipedia.org/wiki/Sign_function
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    2. COYou have the order wrong. [int(copysign(zero, 1)) for zero in (0, 0.0, -0.0)] gives the [0,0,0] you're looking for.
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    3. CO@Andrew - @user238424's calling order is correct. `copysign(a,b)` returns a with the sign of b - b is the varying input, a is the value to normalize to with b's sign. In this case, the commenter is illustrating that copysign(1,x) as a replacement for sign(x) fails, since it returns 1 for x=0, whereas sign(0) would evaluate to 0.
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