[Numpy-discussion] #2522 numpy.diff fails on unsigned integers

Todd toddrjen at gmail.com
Tue Nov 4 09:06:14 EST 2014

On Tue, Nov 4, 2014 at 2:50 PM, Sebastian Wagner <sebix at sebix.at> wrote:

> Hello,
> I want to bring up Issue #2522 'numpy.diff fails on unsigned integers
> (Trac #1929)' [1], as it was resonsible for an error in one of our
> programs. Short explanation of the bug: np.diff performs a subtraction
> on the input array. If this is of type uint and the data contains
> falling data, it results in an artihmetic underflow.
> >>> np.diff(np.array([0,1,0], dtype=np.uint8))
> array([  1, 255], dtype=uint8)
> @charris proposed either
> - a note to the doc string and maybe an example to clarify things
> - or raise a warning
> but with a discussion on the list.
> I would like to start it now, as it is an error which is not easily
> detectable (no errors or warnings are thrown). In our case the type of a
> data sequence, with only zeros and ones, had type f8 as also every other
> one, has been changed to u4. As the programs looked for values ==1 and
> ==-1, it broke silently.
> In my opinion, a note in the docs is not enough and does not help if the
> type changed or set after the program has been written.
> I'd go for automatic upcasting of uints by default and an option to turn
> it off, if this behavior is explicitly wanted. This wouldn't be correct
> from the point of view of a programmer, but as most of the users have a
> scientific background who excpect it 'to work', instead of sth is
> theoretically correct but not convenient. (I count myself to the first
> group)

When you say "automatic upcasting", that would be, for example uint8 to
int16?  What about for uint64?  There is no int128.

Also, when you say "by default", is this only when an overflow is detected,
or always?

How would the option to turn it off be implemented?  An argument to np.diff
or some sort of global option?
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