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POParallel mapping functions in IPython w/ multiple parameters
 

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  1. POParallel mapping functions in IPython w/ multiple parameters
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    <p>I'm trying to use IPython's parallel environment and so far, it's looking great but I'm running into a problem. Lets say that I have a function, defined in a library</p> <pre><code>def func(a,b): ... </code></pre> <p>that I use when I want to evaluate on one value of a and a bunch of values of b. </p> <pre><code>[func(myA, b) for b in myLongList] </code></pre> <p>Obviously, the real function is more complicated but the essence of the matter is that it takes multiple parameters and I'd like to map over only one of them. The problem is that map, @dview.parallel, etc. map over all the arguments. </p> <p>So lets say I want to get the answer to func(myA, myLongList). The obvious way to do this is to curry, either w/ functools.partial or just as </p> <pre><code>dview.map_sync(lambda b: func(myA, b), myLongList) </code></pre> <p>However, this does not work correctly on remote machines. The reason is that when the lambda expression is pickled, the value of myA is not included and instead, the value of myA from the local scope on the remote machine is used. When closures get pickled, the variables they close over don't. </p> <p>Two ways I can think of doing this that will actually work are to manually construct lists for every argument and have map work over all of the arguments,</p> <pre><code>dview.map_sync(func, [myA]*len(myLongList), myLongList) </code></pre> <p>or to horrifically use the data as default arguments to a function, forcing it to get pickled:</p> <pre><code># Can't use a lambda here b/c lambdas don't use default arguments :( def parallelFunc(b, myA = myA): return func(myA, b) dview.map_sync(parallelFunc, myLongList) </code></pre> <p>Really, this all seems horribly contorted when the real function takes a lot of parameters and is more complicated. Is there some idiomatic way of doing this? Something like</p> <pre><code>@parallel(mapOver='b') def bigLongFn(a, b): ... </code></pre> <p>but as far as I know, nothing like the 'mapOver' thing exists. I probably have an idea of how to implement it ... this just feels like a very basic operation that there should exist support for so I want to check if I'm missing something. </p>
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    1. COWhat do you mean "lambdas don't use default arguments"? Lambdas can have argument defaults like any other function. Also, I don't see anything "horrific" about passing the data as default arguments. That is the standard way to ensure an "early binding" of local variables at function def time. So I think you already found your answer.
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    2. COYou're right about default parameters; I was mistaken.
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    3. COpickled curry, I wonder what they taste like... anyway, in Python 3.1 and up, it is possible to pickle curried function, the bug has been fixed according to [#1398](http://bugs.python.org/issue1398).
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