min/max of empty arrays

A similar issue was raised in #2670, however the edge case of (0, ) shape is still not covered. I can go around it with checking for non-zero length in an if statement, but it would be more practical if np.min could handle this, too.

In [24]: [np.min(param[x][1]) for x in fields]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-24-076aa74e3684> in <module>()
----> 1 [np.min(param[x][1]) for x in fields]

/data/wts/pipeline64/python/lib/python2.6/site-packages/numpy/core/fromnumeric.pyc in amin(a, axis, out)
   1893     except AttributeError:
   1894         return _wrapit(a, 'min', axis, out)
-> 1895     return amin(axis, out)
   1896 
   1897 

ValueError: zero-size array to minimum.reduce without identity

In [25]: param[x]
Out[25]: 
{1: array([], dtype=float64),
 2: array([], dtype=float64),
 3: array([], dtype=float64),
 4: array([], dtype=float64)}

In [26]: param[x][1].shape
Out[26]: (0,)

[eric-wieser]

• reduceat doesn’t even use it’s identity - don’t open that can of worms. There’s another issue about that elsewhere
• accumulate perhaps doesn’t need it? np.minimum.accumulate([]) is already well-defined
• accumulateat doesn’t exist

[bsipocz]

OK, I see your point, however can’t see why it behaves differently for different empty arrays (e.g. getting the minimum for axis=1 and axis=0 in #2670’s example). Both are empty, still the minimum will be either an empty array or a ValueError.
To be honest, I would be most happy with a nan as a result to min([]), at least it seems more reasonable than an error.

In [12]: a = numpy.zeros((0,2))

In [13]: a
Out[13]: array([], shape=(0, 2), dtype=float64)

In [14]: np.min(a, axis=1)
Out[14]: array([], dtype=float64)

In [15]: np.min(a, axis=0)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-15-c8666bcd2a35> in <module>()
----> 1 np.min(a, axis=0)

/data/wts/pipeline64/python/lib/python2.6/site-packages/numpy/core/fromnumeric.pyc in amin(a, axis, out)
   1893     except AttributeError:
   1894         return _wrapit(a, 'min', axis, out)
-> 1895     return amin(axis, out)
   1896 
   1897 

ValueError: zero-size array to minimum.reduce without identity