[Numpy-discussion] Regression: in-place operations (possibly intentional)

Charles R Harris charlesr.harris@gmail....
Tue Sep 18 14:12:32 CDT 2012


On Tue, Sep 18, 2012 at 1:08 PM, Travis Oliphant <travis@continuum.io>wrote:

>
> On Sep 18, 2012, at 1:47 PM, Charles R Harris wrote:
>
>
>
> On Tue, Sep 18, 2012 at 11:39 AM, Benjamin Root <ben.root@ou.edu> wrote:
>
>>
>>
>> On Mon, Sep 17, 2012 at 9:33 PM, Charles R Harris <
>> charlesr.harris@gmail.com> wrote:
>>
>>>
>>>
>>> On Mon, Sep 17, 2012 at 3:40 PM, Travis Oliphant <travis@continuum.io>wrote:
>>>
>>>>
>>>> On Sep 17, 2012, at 8:42 AM, Benjamin Root wrote:
>>>>
>>>> > Consider the following code:
>>>> >
>>>> > import numpy as np
>>>> > a = np.array([1, 2, 3, 4, 5], dtype=np.int16)
>>>> > a *= float(255) / 15
>>>> >
>>>> > In v1.6.x, this yields:
>>>> > array([17, 34, 51, 68, 85], dtype=int16)
>>>> >
>>>> > But in master, this throws an exception about failing to cast via
>>>> same_kind.
>>>> >
>>>> > Note that numpy was smart about this operation before, consider:
>>>> > a = np.array([1, 2, 3, 4, 5], dtype=np.int16)
>>>> > a *= float(128) / 256
>>>>
>>>> > yields:
>>>> > array([0, 1, 1, 2, 2], dtype=int16)
>>>> >
>>>> > Of course, this is different than if one does it in a non-in-place
>>>> manner:
>>>> > np.array([1, 2, 3, 4, 5], dtype=np.int16) * 0.5
>>>> >
>>>> > which yields an array with floating point dtype in both versions.  I
>>>> can appreciate the arguments for preventing this kind of implicit casting
>>>> between non-same_kind dtypes, but I argue that because the operation is
>>>> in-place, then I (as the programmer) am explicitly stating that I desire to
>>>> utilize the current array to store the results of the operation, dtype and
>>>> all.  Obviously, we can't completely turn off this rule (for example, an
>>>> in-place addition between integer array and a datetime64 makes no sense),
>>>> but surely there is some sort of happy medium that would allow these sort
>>>> of operations to take place?
>>>> >
>>>> > Lastly, if it is determined that it is desirable to allow in-place
>>>> operations to continue working like they have before, I would like to see
>>>> such a fix in v1.7 because if it isn't in 1.7, then other libraries (such
>>>> as matplotlib, where this issue was first found) would have to change their
>>>> code anyway just to be compatible with numpy.
>>>>
>>>> I agree that in-place operations should allow different casting rules.
>>>>  There are different opinions on this, of course, but generally this is how
>>>> NumPy has worked in the past.
>>>>
>>>> We did decide to change the default casting rule to "same_kind" but
>>>> making an exception for in-place seems reasonable.
>>>>
>>>
>>> I think that in these cases same_kind will flag what are most likely
>>> programming errors and sloppy code. It is easy to be explicit and doing so
>>> will make the code more readable because it will be immediately obvious
>>> what the multiplicand is without the need to recall what the numpy casting
>>> rules are in this exceptional case. IISTR several mentions of this before
>>> (Gael?), and in some of those cases it turned out that bugs were being
>>> turned up. Catching bugs with minimal effort is a good thing.
>>>
>>> Chuck
>>>
>>>
>> True, it is quite likely to be a programming error, but then again, there
>> are many cases where it isn't.  Is the problem strictly that we are trying
>> to downcast the float to an int, or is it that we are trying to downcast to
>> a lower precision?  Is there a way for one to explicitly relax the
>> same_kind restriction?
>>
>
> I think the problem is down casting across kinds, with the result that
> floats are truncated and the imaginary parts of imaginaries might be
> discarded. That is, the value, not just the precision, of the rhs changes.
> So I'd favor an explicit cast in code like this, i.e., cast the rhs to an
> integer.
>
> It is true that this forces downstream to code up to a higher standard,
> but I don't see that as a bad thing, especially if it exposes bugs. And it
> isn't difficult to fix.
>
>
> Shouldn't we be issuing a warning, though?   Even if the desire is to
> change the casting rules?   The fact that multiple codes are breaking and
> need to be "upgraded" seems like a hard thing to require of someone going
> straight from 1.6 to 1.7.     That's what I'm opposed to.
>

I think a warning would do just as well. I'd tend to regard the broken
codes as already broken, but that's just me ;)


>
> All of these efforts move NumPy to its use as a library instead of an
> interactive "environment" where it started which is a good direction to
> move, but managing this move in the context of a very large user-community
> is the challenge we have.
>

Chuck
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