On Fri, Jan 27, 2012 at 2:46 PM, 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? > > Cheers, > > Bartosz > > Don't know if it is recommended, but this is used frequently within matplotlib: if np.prod(a.shape) == 0 : print "Is Empty!" Cheers! Ben Root
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