On 4/27/2011 2:04 PM, Mark Dickinson wrote:
On Wed, Apr 27, 2011 at 10:37 AM, Hrvoje Niksic<hrvoje.nik...@avl.com> wrote:
The other day I was surprised to learn this:
nan = float('nan')
nan == nan
False
[nan] == [nan]
True # also True in tuples, dicts, etc.
That one surprises me a bit too: I knew we were using
identity-then-equality checks for containment (nan in [nan]), but I
hadn't realised identity-then-equality was also used for the
item-by-item comparisons when comparing two lists. It's defensible,
though: [nan] == [nan] should presumably produce the same result as
{nan} == {nan}, and the latter is a test that's arguably based on
containment (for sets s and t, s == t if each element of s is in t,
and vice versa).
I don't think any of this should change. It seems to me that we've
currently got something approaching the best approximation to
consistency and sanity achievable, given the fundamental
incompatibility of (1) nan breaking reflexivity of equality and (2)
containment being based on equality. That incompatibility is bound to
create inconsistencies somewhere along the line.
Declaring that 'nan == nan' should be True seems attractive in theory,
but I agree that it doesn't really seem like a realistic option in
terms of backwards compatibility and compatibility with other
mainstream languages.
I think it should change. Inserting a NaN, even the same instance of
NaN into a list shouldn't suddenly make it compare equal to itself,
especially since the docs (section 5.9. Comparisons) say:
*
Tuples and lists are compared lexicographically using comparison
of corresponding elements. This means that to compare equal, each
element must compare equal and the two sequences must be of the
same type and have the same length.
If not equal, the sequences are ordered the same as their first
differing elements. For example, [1,2,x] <= [1,2,y] has the same
value as x <= y. If the corresponding element does not exist, the
shorter sequence is ordered first (for example, [1,2] < [1,2,3]).
The principle of least surprise, says that if two unequal items are
inserted into otherwise equal lists, the lists should be unequal. NaN
is unequal to itself.
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