On Fri, Jan 27, 2012 at 20:46, Bartosz Telenczuk <b.telenc...@biologie.hu-berlin.de> wrote: > I have been using numpy for several years and I am very impressed with its > flexibility. However, there is one problem that has always bothered me. > > Quite often I need to test consistently whether a variable is any of the > following: an empty list, an empty array or None. Since both arrays and lists > are ordered sequences I usually allow for both, and convert if necessary. > However, when the (optional) argument is an empty list/array or None, I skip > its processing and do nothing. > > Now, how should I test for 'emptiness'? > > PEP8 recommends: > > For sequences, (strings, lists, tuples), use the fact that empty sequences > are false. > >>> seq = [] >>> if not seq: > ... print 'Hello' > > It works for empty numpy arrays: > >>> a = np.array(seq) >>> if not a: > ... print 'Hello" > Hello > > but if 'a' is non-empty it raises an exception: > >>> a = np.array([1,2]) >>> if not a: > ... print 'Hello" > ValueError: The truth value of an array with more than one element is > ambiguous. Use a.any() or a.all() > > One solution is to test lengths: > >>> if len(seq) > 0: > .... ... >>> if len(a) > 0: > ... ... > > but for None it fails again: > >>> opt = None >>> if len(opt): > ... > TypeError: object of type 'NoneType' has no len() > > even worse we can not test for None, because it will fail if someone > accidentally wraps None in an array: > >>> a = np.array(opt) >>> if opt is not None: > ... print 'hello' > hello > > Although this behaviour is expected, it may be very confusing and it easily > leads to errors. Even worse it adds unnecessary complexity in the code, > because arrays, lists and None have to be handled differently. > > I hoped the I managed to explain the problem well. Is there a recommended way > to test for empty arrays?
[~] |5> x = np.zeros([0]) [~] |6> x array([], dtype=float64) [~] |7> x.size == 0 True Note that checking for len(x) will fail for some empty arrays: [~] |8> x = np.zeros([10, 0]) [~] |9> x.size == 0 True [~] |10> len(x) 10 There is no way to test all of the cases (empty sequence, empty array, None) in the same way. Usually, it's a bad idea to conflate the three. -- 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 _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
