Robert Kern wrote:
James Stroud wrote:
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.
You have explained
py> 112 = [1, y]
py> y in 112
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is...
but not
py> ll1 = [y,1]
py> y in ll1
True
It's this discrepancy that seems like a bug, not that a ValueError is
raised in the former case, which is perfectly reasonable to me.
All I can imagine is that something like the following lives in the
bowels of the python code for list:
def __contains__(self, other):
foundit = False
for i, v in enumerate(self):
if i == 0:
# evaluates to bool numpy array
foundit = one_kind_of_test(v, other)
else:
# raises exception for numpy array
foundit = another_kind_of_test(v, other)
if foundit:
break
return foundit
I'm trying to imagine some other way to get the results mentioned but I
honestly can't. It's beyond me why someone would do such a thing, but
perhaps it's an optimization of some sort.
James
--
http://mail.python.org/mailman/listinfo/python-list
