On Mi, 2014-11-12 at 22:19 -0800, Antony Lee wrote: > I know you can't in general, but this was in a context where I knew > the array contained a single element, which "works" (it checks the > truthiness of the contained element). Of course I didn't consider the > case where the element contained was itself a (non-trivial) array, > thus the finding. > The link you posted doesn't seem to address what magic numpy can do to > make "bool(x)" and "if x:" have different behaviors (FWIW, > "t.__bool__()" also returns True). > Antony >
First sight, sounds like a bug. It should be passing out the error raised by the array inside. Can you open an issue? - Sebastian > 2014-11-12 20:15 GMT-08:00 Michael Sarahan <msara...@gmail.com>: > Hi Antony, > > In general, you can't use numpy arrays in if statements > without using any() or all() or some other means of obtaining > a single boolean value from the whole array. > > I think your confusion is that bool() uses truth testing rules > outlined here: > https://docs.python.org/release/2.5.2/lib/truth.html > > If statements in theory have equivalent behavior, but take > slightly different paths (they don't call bool() directly). > This SO post was enlightening to me: > > http://stackoverflow.com/questions/11885382/is-there-any-difference-between-if-boolx-and-if-x-in-python > > Without looking at Numpy's code, I'd bet Numpy arrays probably > define __bool__ or __nonzero__ in such a way that the > ValueError is raised when it makes sense to do so. > > HTH, > Mike > > > On Tue, Nov 11, 2014 at 12:46 PM, Antony Lee > <antony....@berkeley.edu> wrote: > > I am puzzled by the following (numpy 1.9.0, python > 3.4.2): > > > In [1]: t = array(None); t[()] = array([None, None]) > # Construct a 0d array of dtype object, containing a > single numpy array with 2 elements > > > In [2]: bool(t) > Out[2]: True > > > In [3]: if t: pass > > --------------------------------------------------------------------------- > ValueError Traceback > (most recent call last) > ValueError: The truth value of an array with more than > one element is ambiguous. Use a.any() or a.all() > > > I thought that "if x" simply calls "bool", but > apparently this is not even the case... > > > Antony > > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > > > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion > > > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion
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