`np.diff` is broken for boolean ndarrays in 1.13.0

Should modify the docstring accordingly, raise a meaningful error, or (preferably) make it work by hidding some logic under the hood.

Traceback:

In [11]: x = np.array([[True, True], [False, False]])

In [12]: np.diff(x)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-12-59225436601c> in <module>()
----> 1 np.diff(x)

/usr/lib/python3.6/site-packages/numpy/lib/function_base.py in diff(a, n, axis)
   1924         return diff(a[slice1]-a[slice2], n-1, axis=axis)
   1925     else:
-> 1926         return a[slice1]-a[slice2]
   1927 
   1928 

TypeError: numpy boolean subtract, the `-` operator, is deprecated, use the bitwise_xor, the `^` operator, or the logical_xor function instead.

In [13]: np.__version__
Out[13]: '1.13.0'

xref https://github.com/scikit-image/scikit-image/issues/2681

[matthew-brett]

Well - I can see the purity argument for disallowing, but it seems to me that it is reasonable to expect diff to work on booleans, and not surprising that scikit image was doing that. So I would have thought a branch on bool was the better route.

[eric-wieser]

can’t see any deprecation warning for numpy 1.12.1 and

That’s because by default DeprecationWarning is not displayed. Add a import warnings; warnings.simplefilter('always') and you’ll see it