Mailman 3 February 2013 - NumPy-Discussion

flatnonzero / nonzero returned indices order
by Gabor Kovacs 27 Feb '13

27 Feb '13

Hello, for 1-D arrays, can one assume that the indices returned by flatnonzero is ordered? It looks so, but no such statement anywhere in the docs. (In general, order of indices in case of higher dimensions?) Thanks Gabor

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1.7.1
by Charles R Harris 27 Feb '13

27 Feb '13

When should we put out 1.7.1? Discuss ;) Chuck

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numpy.scipy.org giving 404 error
by Peter Cock 26 Feb '13

26 Feb '13

Hello all, http://numpy.scipy.org is giving a GitHub 404 error. As this used to be a widely used URL for the project, and likely appears in many printed references, could it be fixed to point to or redirect to the (relatively new) http://www.numpy.org site please? Thanks, Peter

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What should np.ndarray.__contains__ do
by Sebastian Berg 26 Feb '13

26 Feb '13

Hello all, currently the `__contains__` method or the `in` operator on arrays, does not return what the user would expect when in the operation `a in b` the `a` is not a single element (see "In [3]-[4]" below). The first solution coming to mind might be checking `all()` for all dimensions given in argument `a` (see line "In [5]" for a simplistic example). This does not play too well with broadcasting however, but one could maybe simply *not* broadcast at all (i.e. a.shape == b.shape[b.ndim-a.ndim:]) and raise an error/return False otherwise. On the other hand one could say broadcasting of `a` onto `b` should be "any" along that dimension (see "In [8]"). The other way should maybe raise an error though (see "In [9]" to understand what I mean). I think using broadcasting dimensions where `a` is repeated over `b` as the dimensions to use "any" logic on is the most general way for numpy to handle this consistently, while the other way around could be handled with an `all` but to me makes so little sense that I think it should be an error. Of course this is different to a list of lists, which gives False in these cases, but arrays are not list of lists... As a side note, since for loop, etc. use "for item in array", I do not think that vectorizing along `a` as np.in1d does is reasonable. `in` should return a single boolean. I have opened an issue for it: https://github.com/numpy/numpy/issues/3016#issuecomment-14045545 Regards, Sebastian In [1]: a = np.array([0, 2]) In [2]: b = np.arange(10).reshape(5,2) In [3]: b Out[3]: array([[0, 1], [2, 3], [4, 5], [6, 7], [8, 9]]) In [4]: a in b Out[4]: True In [5]: (b == a).any() Out[5]: True In [6]: (b == a).all(0).any() # the 0 could be multiple axes Out[6]: False In [7]: a_2d = a[None,:] In [8]: a_2d in b # broadcast dimension means "any" -> True Out[8]: True In [9]: [0, 1] in b[:,:1] # should not work (or be False, not True) Out[9]: True

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another discussion on numpy correlate (and convolution)
by Pierre Haessig 26 Feb '13

26 Feb '13

Hi everybody, (just coming from a discussion on the performance of Matplotlib's (x)corr function which uses np.correlate) There have been already many discussions on how to compute (cross-)correlations of time-series in Python (like http://stackoverflow.com/questions/6991471/computing-cross-correlation-func…). The discussion is spread between the various stakeholders (just to name some I've in mind : scipy, statsmodel, matplotlib, ...). There are basically 2 implementations : time-domain and frequency-domain (using fft + multiplication). My discussion is only on time-domain implementation. The key usecase which I feel is not well adressed today is when computing the (cross)correlation of two long timeseries on only a few lagpoints. For time-domain, one can either write its own implementation or rely on numpy.correlate. The latter rely on the fast C implementation _pyarray_correlate() in multiarraymodule.c (https://github.com/numpy/numpy/blob/master/numpy/core/src/multiarray/multia…) Now, the signature of this function is meant to accept one of three *computation modes* ('valid', 'same', 'full'). Thoses modes make a lot of sense when using this function in the context of convoluting two signals (say an "input array" and a "impulse response array"). In the context of computing the (cross)correlation of two long timeseries on only a few lagpoints, those computation modes are unadapted and potentially leads to huge computational and memory wastes (for example : https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/axes.py…). For some time, I was thinking the solution was to write a dedicated function in one of the stakeholder modules (but which ? statsmodel ?) but now I came to think numpy is the best place to put a change and this would quickly benefit to all stackeholders downstream. Indeed, I looked more carefully at the C _pyarray_correlate() function, and I've come to the conclusion that these three computation modes are an unnecessary restriction because the actual computation relies on a triple for loop (https://github.com/numpy/numpy/blob/master/numpy/core/src/multiarray/multia…) which boundaries are governed by two integers `n_left` and `n_right` instead of the three modes. Maybe I've misunderstood the inner-working of this function, but I've the feeling that the ability to set these two integers directly instead of just the mode would open up the possibility to compute the correlation on only a few lagpoints. I'm fully aware that changing the signature of such a core numpy is out-of-question but I've got the feeling that a reasonably small some code refactor might lift the longgoing problem of (cross-)correlation computation. The Python np.correlate would require two additional keywords -`n_left` and `n_right`)which would override the `mode` keyword. Only the C function would need some more care to keep good backward compatibility in case it's used externally. What do other people think ? Best, Pierre

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Leaking memory problem
by Jaakko Luttinen 26 Feb '13

26 Feb '13

Hi! I was wondering if anyone could help me in finding a memory leak problem with NumPy. My project is quite massive and I haven't been able to construct a simple example which would reproduce the problem.. I have an iterative algorithm which should not increase the memory usage as the iteration progresses. However, after the first iteration, 1GB of memory is used and it steadily increases until at about 100-200 iterations 8GB is used and the program exits with MemoryError. I have a collection of objects which contain large arrays. In each iteration, the objects are updated in turns by re-computing the arrays they contain. The number of arrays and their sizes are constant (do not change during the iteration). So the memory usage should not increase, and I'm a bit confused, how can the program run out of memory if it can easily compute at least a few iterations.. I've tried to use Pympler, but I've understood that it doesn't show the memory usage of NumPy arrays.. ? I also tried gc.set_debug(gc.DEBUG_UNCOLLECTABLE) and then printing gc.garbage at each iteration, but that doesn't show anything. Does anyone have any ideas how to debug this kind of memory leak bug? And how to find out whether the bug is in my code, NumPy or elsewhere? Thanks for any help! Jaakko

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Calling scipy blas from cython is extremely slow
by Sergio Callegari 24 Feb '13

24 Feb '13

Hi, following the excellent advice of V. Armando Sole, I have finally succeeded in calling the blas routines shipped with scipy from cython. I am doing this to avoid shipping an extra blas library for some project of mine that uses scipy but has some things coded in cython for extra speed. So far I managed getting things working on Linux. Here is what I do: The following code snippet gives me the dgemv pointer (which is a pointer to a fortran function, even if it comes from scipy.linalg.blas.cblas, weird). from cpython cimport PyCObject_AsVoidPtr import scipy as sp __import__('scipy.linalg.blas') ctypedef void (*dgemv_ptr) (char *trans, int *m, int *n,\ double *alpha, double *a, int *lda, double *x,\ int *incx,\ double *beta, double *y, int *incy) cdef dgemv_ptr dgemv=<dgemv_ptr>PyCObject_AsVoidPtr(\ sp.linalg.blas.cblas.dgemv._cpointer) Then, in a tight loop, I can call dgemv by first defining the constants and then calling dgemv inside the loop cdef int one=1 cdef double onedot = 1.0 cdef double zerodot = 0.0 cdef char trans = 'N' for i in xrange(N): dgemv(&trans, &nq, &order,\ &onedot, <double *>np.PyArray_DATA(C), &order, \ <double*>np.PyArray_DATA(c_x0), &one, \ &zerodot, <double*>np.PyArray_DATA(y0), &one) It works, but it is many many times slower than linking to the cblas that is available on the same system. Specifically, I have about 8 calls to blas in my tight loop, 4 of them are to dgemv and the others are to dcopy. Changing a single dgemv call from the system cblas to the blas function returned by scipy.linalg.blas.cblas.dgemv._cpointer makes the execution time of a test case jump from about 0.7 s to 1.25 on my system. Any clue about why is this happening? In the end, on linux, scipy dynamically link to atlas exactly as I link to atlas when I use the cblas functions.

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Tabular package updated to v0.1
by Elaine Angelino 23 Feb '13

23 Feb '13

Hi there, We are writing to announce that we've updated "Tabular" to v0.1. Tabular is a package of Python modules for working with tabular data. Its main object is the tabarray class, a data structure for holding and manipulating tabular data. By putting data into a tabarray object, you’ll get a representation of the data that is more flexible and powerful than a native Python representation. More specifically, tabarray provides: -- ultra-fast filtering, selection, and numerical analysis methods, using convenient Matlab-style matrix operation syntax -- spreadsheet-style operations, including row & column operations, 'sort', 'replace', 'aggregate', 'pivot', and 'join' -- flexible load and save methods for a variety of file formats, including delimited text (CSV), binary, and HTML -- helpful inference algorithms for determining formatting parameters and data types of input files You can download Tabular from PyPI (http://pypi.python.org/pypi/tabular/) or alternatively clone our git repository from github (http://github.com/yamins81/tabular). We also have posted tutorial-style Sphinx documentation (http://web.mit.edu/yamins/www/tabular/). The tabarray object is based on the record array object from the Numerical Python package (NumPy), and Tabular is built to interface well with NumPy in general. Our intended audience is two-fold: (1) Python users who, though they may not be familiar with NumPy, are in need of a way to work with tabular data, and (2) NumPy users who would like to do spreadsheet-style operations on top of their more "numerical" work. We hope that some of you find Tabular useful! Best, Elaine and Dan

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Smart way to do this?
by santhu kumar 23 Feb '13

23 Feb '13

Hi all, I dont want to run a loop for this but it should be possible using numpy "smart" ways. a = np.ones(30) idx = np.array([2,3,2]) # there is a duplicate index of 2 a += 2 >>>a array([ 1., 1., 3., 3., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1.]) But if we do this : for i in range(idx.shape[0]): a[idx[i]] += 2 >>> a array([ 1., 1., 5., 3., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1.]) How to achieve the second result without looping?? Thanks Santhosh

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Windows, blas, atlas and dlls
by Sergio Callegari 22 Feb '13

22 Feb '13

Hi, I have a project that includes a cython script which in turn does some direct access to a couple of cblas functions. This is necessary, since some matrix multiplications need to be done inside a tight loop that gets called thousands of times. Speedup wrt calling scipy.linalg.blas.cblas routines is 10x to 20x. Now, all this is very nice on linux where the setup script can assure that the cython code gets linked with the atlas dynamic library, which is the same library that numpy and scipy link to on this platform. However, I now have trouble in providing easy ways to use my project in windows. All the free windows distros for scientific python that I have looked at (python(x,y) and winpython) seem to repackage the windows version of numpy/scipy as it is built in the numpy/scipy development sites. These appear to statically link atlas inside some pyd files. So I get no atlas to link against, and I have to ship an additional pre-built atlas with my project. All this seems somehow inconvenient. In the end, when my code runs, due to static linking I get 3 replicas of 2 slightly different atlas libs in memory. One coming with _dotblas.pyd in numpy, another one with cblas.pyd or fblas.pyd in scipy. And the last one as the one shipped in my code. Would it be possible to have a win distro of scipy which provides some pre built atlas dlls, and to have numpy and scipy dynamically link to them? This would save memory and also provide a decent blas to link to for things done in cython. But I believe there must be some problem since the scipy site says "IMPORTANT: NumPy and SciPy in Windows can currently only make use of CBLAS and LAPACK as static libraries - DLLs are not supported." Can someone please explain why or link to an explanation? Unfortunately, not having a good, prebuilt and cheap blas implementation in windows is really striking me as a severe limitation, since you loose the ability to prototype in python/scipy and then move to C or Cython the major bottlenecks to achieve speed. Many thanks in advance!

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