[Numpy-discussion] clip() with None as argument

[Robert Kern]

On Wed, Sep 8, 2010 at 14:42, Chris Ball <ceball at gmail.com> wrote:
> Robert Kern <robert.kern <at> gmail.com> writes:
>
>>
>> On Tue, Sep 7, 2010 at 15:12, Friedrich Romstedt
>> <friedrichromstedt <at> gmail.com> wrote:
>> > Ah, no need to answer, I do this myself:
>> >
>> > Friedrich, would you please use numpy.inf and -numpy.inf.
>>
>> But if you have an integer array, you will run into the same problem.
>> The result will be upcast to float. I think we would accept a patch
>> that interprets None to be the appropriate extreme bound given the
>> input datatype. This will be tricky, though, since it must be done in
>> C (PyArray_Clip in numpy/core/src/multiarray/calculation.c).
>
> I've been a bit confused about what numpy's clip is
> supposed to support. The documentation
> (e.g. http://docs.scipy.org/doc/numpy/reference/generated/numpy.clip.html)
> doesn't make any mention of supporting only one of min or max, but I
> remember a message some time back from Travis Oliphant that seemed to
> suggest it does
> (http://thread.gmane.org/gmane.comp.python.numeric.general/17844/focus=17877).
>
> Right now, I'm surprised by two aspect's of clip's behavior, both
> demonstrated below...
>
> $ python
> Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41)
> [GCC 4.4.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> import numpy
>>>> numpy.__version__
> '2.0.0.dev8698'
>>>> a = numpy.array([1,2,3,4,5])
>>>> a.clip(2,None)
> array([2, 2, 2, 2, 2], dtype=object)
>
> I'm not sure why the returned array has a dtype of object

The usual type-matching semantics. a.clip() tries to find a common
type that fits `a`, 2 and None. Since None only fits in an object
dtype, that's what you get.

> (although that can be
> avoided by using clip's "out" argument), and also
> why the values are not like this:
>
>>>> numpy.maximum(a,2)
> array([2, 2, 3, 4, 5])

clip(a, low, high) boils down to this in the C code itself (or at
least the code path that it goes down when the input array needs to be
upcast):

  maximum(minimum(a, high), low)

Since all integer objects compare > None based on the default Python
2.x implementation of comparison across types, you would just get the
low value out of that.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco