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POHow can I create my own datatype in python so that I could overwrite arithmetic operators?
 

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  1. POHow can I create my own datatype in python so that I could overwrite arithmetic operators?
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    <p>I am currently using Python/Numpy to deal with geographical/GPS data (loving it!), and I am facing the recurring task to calculate distances between geographical points defined by a coordinate pair <code>pn = [lon, lat]</code>.</p> <p>I have a function that I use like this: <code>dist = geodistance(p1, p2)</code> which is analog to euclidean distance in linear algebra (vector subtraction/difference), but occurs in geodesical (spherical) space instead of rectangular euclidean space.</p> <p>Programmatically, euclidean distance is given by</p> <pre><code>dist = ((p2[0] - p1[0])**2 + (p2[1] - p1[1])**2)**0.5 </code></pre> <p>Mathematically, this is equivalent to the "idiomatic" (for lack of a better word) sentence </p> <pre><code>dist = p1 - p1 # the "norm" of the vector difference, subtraction. </code></pre> <p>Currently, I get my distance like this:</p> <pre><code>p1 = [-51.598354,-29.953363] p2 = [-51.598701,-29.953045] dist = geodistance(p1, p2) print dist &gt;&gt; 44.3904032407 </code></pre> <p>I would like to do this:</p> <pre><code>print p2 - p1 # these points now are from some fancy datatype &gt;&gt; 44.3904032407 </code></pre> <p>And the final goal:</p> <pre><code>track = numpy.array([[-51.203018 -29.996149] [-51.203018 -29.99625 ] [-51.20266 -29.996229] [-51.20229 -29.996309] [-51.201519 -29.99416 ]], dtype=fancy) # (**) or something like print numpy.diff(track) &gt;&gt; ndarray([[ 0. ] [ 7.03531252] [ 39.82663316] [ 41.50958596] [ 172.49825765]]) </code></pre> <p>A similar thing is: if you take two <code>datetime</code> objects and subtract them, the operation returns a <code>timedelta</code> object. I want to subtract two coordinates and get a geodesic distance as the result.</p> <p>I wonder if a class would work, but dtype (a "subtype" of float32, for example) would help a lot upon array creation from lists (** which is how I read things from xml files).</p> <p>Thanks a lot!</p>
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    1. COLook into operator overloading.
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    2. COYou could subclass ndarray to overload the minus operator, but I would think twice before you do something potentially confusing. Could also add a new method to it, like `.dist()` of course. The numpy documentation has some simple examples. For just adding a method you don't really need to do much, otherwise it can be a bit tricky.
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    3. COA problem you will have with numpy.array(..., dtype=fancy) is that you will lose the speed of numpy, because it will be considering arrays of python objects instead of numbers. Maybe you want to instead have a Coord type that sometimes contains a pair of numbers and sometimes contains a pair of arrays..
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