Rasmus Fogh wrote:
Dear All,
For the first time I have come across a Python feature that seems
completely wrong. After the introduction of rich comparisons, equality
comparison does not have to return a truth value, and may indeed return
nothing at all and throw an error instead. As a result, code like
if foo == bar:
or
foo in alist
cannot be relied on to work.
This is clearly no accident. According to the documentation all comparison
operators are allowed to return non-booleans, or to throw errors. There is
explicitly no guarantee that x == x is True.
I'm not a computer scientist, so my language and perspective on the
topic may be a bit naive, but I'll try to demonstrate my caveman
understanding example.
First, here is why the ability to throw an error is a feature:
class Apple(object):
def __init__(self, appleness):
self.appleness = appleness
def __cmp__(self, other):
assert isinstance(other, Apple), 'must compare apples to apples'
return cmp(self.appleness, other.appleness)
class Orange(object): pass
Apple(42) == Orange()
Second, consider that any value in python also evaluates to a truth
value in boolean context.
Third, every function returns something. A function's returning nothing
is not a possibility in the python language. None is something but
evaluates to False in boolean context.
But surely you can define an equal/unequal classification for all
types of object, if you want to?
This reminds me of complex numbers: would 4 + 4i be equal to sqrt(32)?
Even in the realm of pure mathematics, the generality of objects (i.e.
numbers) can not be assumed.
James
--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095
http://www.jamesstroud.com
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