James Stroud wrote:
Steven D'Aprano wrote:
On Sun, 07 Dec 2008 13:57:54 -0800, James Stroud wrote:
Rasmus Fogh wrote:
ll1 = [y,1]
y in ll1
True
ll2 = [1,y]
y in ll2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()
I think you could be safe calling this a bug with numpy.
Only in the sense that there are special cases where the array
elements are all true, or all false, and numpy *could* safely return a
bool. But special cases are not special enough to break the rules.
Better for the numpy caller to write this:
a.all() # or any()
instead of:
try:
bool(a)
except ValueError:
a.all()
as they would need to do if numpy sometimes returned a bool and
sometimes raised an exception.
I'm missing how a.all() solves the problem Rasmus describes, namely that
the order of a python *list* affects the results of containment tests by
numpy.array. E.g. "y in ll1" and "y in ll2" evaluate to different
results in his example. It still seems like a bug in numpy to me, even
if too much other stuff is broken if you fix it (in which case it
apparently becomes an "issue").
It's an issue, if anything, not a bug. There is no consistent implementation of
bool(some_array) that works in all cases. numpy's predecessor Numeric used to
implement this as returning True if at least one element was non-zero. This
works well for bool(x!=y) (which is equivalent to (x!=y).any()) but does not
work well for bool(x==y) (which should be (x==y).all()), but many people got
confused and thought that bool(x==y) worked. When we made numpy, we decided to
explicitly not allow bool(some_array) so that people will not write buggy code
like this again.
The deficiency is in the feature of rich comparisons, not numpy's implementation
of it. __eq__() is allowed to return non-booleans; however, there are some parts
of Python's implementation like list.__contains__() that still expect the return
value of __eq__() to be meaningfully cast to a boolean.
--
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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