James Stroud wrote:
Robert Kern wrote:
James Stroud wrote:
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.
Nothing to do with numpy. list.__contains__() checks for identity with
"is" before it goes to __eq__().
...but only for the first element of the list:
py> import numpy
py> y = numpy.array([1,2,3])
py> y
array([1, 2, 3])
py> y in [1, y]
------------------------------------------------------------
Traceback (most recent call last):
File "<ipython console>", line 1, in <module>
<type 'exceptions.ValueError'>: The truth value of an array with more
than one element is ambiguous. Use a.any() or a.all()
py> y is [1, y][1]
True
I think it skips straight to __eq__ if the element is not the first in
the list.
No, it doesn't skip straight to __eq__(). "y is 1" returns False, so (y==1) is
checked. When y is a numpy array, this returns an array of bools.
list.__contains__() tries to convert this array to a bool and
ndarray.__nonzero__() raises the exception.
list.__contains__() checks "is" then __eq__() for each element before moving on
to the next element. It does not try "is" for all elements, then try __eq__()
for all elements.
> That no one acknowledges this makes me feel like a conspiracy
> is afoot.
I don't know what you think I'm not acknowledging.
--
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
--
http://mail.python.org/mailman/listinfo/python-list
