[Python-Dev] Clean way in python to test for None, empty, scalar, and list/ndarray? A prayer to the gods of Python (original) (raw)

Robert Kern robert.kern at gmail.com
Sat Jun 15 00:53:53 CEST 2013

On 2013-06-14 23:31, Robert Kern wrote:

On 2013-06-14 21:55, R. David Murray wrote:

On Fri, 14 Jun 2013 21:12:00 +0200, Martin Schultz <maschu09 at gmail.com> wrote:

2. Testing for empty lists or empty ndarrays:

In principle, len(x) == 0 will do the trick. BUT there are several caveats here: - len(scalar) raises a TypeError, so you will have to use try and except or find some other way of testing for a scalar value - len(numpy.array(0)) (i.e. a scalar coded as numpy array) also raises a TypeError ("unsized object") - len([[]]) returns a length of 1, which is somehow understandable, but - I would argue - perhaps not what one might expect initially Alternatively, numpy arrays have a size attribute, and numpy.array([]).size, numpy.array(8.).size, and numpy.array([8.]).size all return what you would expect. And even numpy.array([[]]).size gives you 0. Now, if I could convert everything to a numpy array, this might work. But have you ever tried to assign a list of mixed data types to a numpy array? numpy.array(["a",1,[2,3],(888,9)]) will fail, even though the list inside is perfectly fine as a list. In general you test whether nor not something is empty in Python by testing its truth value. Empty things are False. Numpy seems to follow this using size, from the limited examples you have given >>> bool(numpy.array([[]]) False >>> bool(numpy.array([[1]]) True numpy does not do so. Empty arrays are extremely rare and testing for them rarer (rarer still is testing for emptiness not knowing if it is an array or some other sequence). What people usually want from bool(somearray) is either somearray.all() or somearray.any(). In the face of this ambiguity, numpy refuses the temptation to guess and raises an exception explaining matters.

Actually, that's a bit of a lie. In the empty case and the one-element case, we do return a bool, False for empty and bool(element) for whatever that one element is. Anything else raises the exception since we don't know whether it is all() or any() that was desired.

-- 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

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