[Numpy-discussion] Overloading numpy's ufuncs for better type coercion?
Christopher Barker
Chris.Barker@noaa....
Wed Jul 22 12:54:04 CDT 2009
Hans Meine wrote:
> In [3]: numpy.add(a, a, numpy.empty((1, ), dtype = numpy.uint32))
> Out[3]: array([144], dtype=uint32)
yes, it sure would be nice to fix this...
> one will often end up with uint8 arrays which cannot be passed
> into many algorithms without an explicit conversion. However, is this really
> a bad problem? For example, the conversion would typically have to be
> performed only once (after loading), no? Then, why not simplify things
> further by adding a dtype= parameter to importImage()? This could even
> default to float32 then.
VIGRA specifically, this sounds like a fine way to go. how ever, for the
broader numpy case:
I want to add two unit8 arrays, and put the results into a int32 array
(for instance). As pointed out this doesn't work:
In [9]: a1 = np.array((200,), dtype= np.uint8)
In [10]: a2 = np.array((250,), dtype= np.uint8)
In [11]: a1 + a2
Out[11]: array([194], dtype=uint8)
As pointed out by others -- this is the "right" behavior - we really
don't want upcasting without asking for it.
However, as above:
In [15]: np.add(a1, a2, np.empty(a1.shape, dtype = np.int32))
Out[15]: array([194])
I am asking for upcasting, so it's too bad I don't get it. The solution
is to upcast ahead of time:
In [17]: np.add(a1.astype(np.int32), a2, np.empty(a1.shape, dtype =
np.int32))
Out[17]: array([450])
or simply:
In [18]: a1.astype(np.int32) +a2
Out[18]: array([450])
Easy enough. The one downside is that an extra temporary in the larger
type is needed (but only one). That could be an issue when working with
large arrays, which I suspect is the case when these issues come up --
no one use the np.add() notation unless you are trying to manage memory
more carefully.
Another way to write this is:
In [19]: a3 = a1.astype(np.int32)
In [20]: a3 += a2
In [21]: a3
Out[21]: array([450])
which I think avoids the extra temporary
This is a pretty special case, ands there ways to accomplish what is
needed. I suspect that's why no one has "fixed" this yet.
-Chris
--
Christopher Barker, Ph.D.
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