[Numpy-discussion] [SciPy-user] unique, sort, sortrows

David M. Kaplan David.Kaplan@ird...
Mon Jul 28 07:55:22 CDT 2008


Well, as usual there are compromises in everything and the mgrid/ogrid
functionality is the way it currently is for some good reasons.  The
first reason is that python appears to be fairly sloppy about how it
passes indexing arguments to the __getitem__ method.  It passes a tuple
containing the arguments in all cases except when it has one argument,
in which case it just passes that argument.  This means that it is hard
to tell a tuple argument from several non-tuple arguments.  For example,
the following two produce exactly the same call to __getitem__ :


(__getitem__ receives a single tuple (1,2,3)), but different from:


(__getitem__ receives a single list = [1,2,3]).  This seems like a bug
to me, but is probably considered a feature by somebody.  In any case,
this is workable, but a bit annoying in that tuple arguments just aren't
going to work well.

The second problem is that the current implementation is fairly
efficient because it forces all arguments to the same type so as to
avoid some unnecessary copies (I think).  Once you allow non-slice
arguments, this is hard to maintain.

That being said, attached is a replacement for index_tricks.py that
should implement a reasonable set of features, while only very slightly
altering performance.  I have only touched nd_grid.  I haven't fixed the
documentation string yet, nor have I added tests to
test_index_tricks.py, but will do that if the changes will be accepted
into numpy.  With the new version, old stuff should work as usual,
except that mgrid now returns a list of arrays instead of an array of
arrays (note that this change will cause current test_index_tricks.py to
fail).  With the new changes, you can now do:


The following will work as expected:


But this will not:

mgrid[(1,2,3)]  # same as mgrid[1,2,3], but different from mgrid[[1,2,3]]

Given these limitations, this seems like a fairly useful addition.  If
this looks usable, I will clean up and add tests if desired.  If not, I
recommend adding a ndgrid function to numpy that does the equivalent of
matlab [X,Y,Z,...] = ndgrid(x,y,z,...) and then making the current
meshgrid just call that changing the order of the first two arguments.


David M. Kaplan
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Institut de Recherche pour le Developpement
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Phone: +33 (0)4 99 57 32 27
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