On 12/6/2010 4:17 PM, Steven D'Aprano wrote:
On Mon, 06 Dec 2010 08:59:12 -0800, TomF wrote:
I'm aggravated by this behavior in python:
x = "4"
print x< 7 # prints False
I can't imagine why this design decision was made.
You've never needed to deal with an heterogeneous list?
data = ["Fred", "Barney", 2, 1, None]
data.sort()
Nevertheless, I agree that in hindsight, the ability to sort such lists
is not as important as the consistency of comparisons.
If you're thinking hard about this, I recommend viewing Alexander
Stepanov's talk at Stanford last month:
http://www.stanford.edu/class/ee380/Abstracts/101103.html
He makes the point that, for generic programs to work right, the
basic operations must have certain well-defined semantics. Then
the same algorithms will work right across a wide variety of
objects.
This is consistent with Python's "duck typing", but inconsistent
with the current semantics of some operators.
For example, "+" as concatenation makes "+" non-commutative.
In other words,
a + b
is not always equal to
b + a
which is not good.
Important properties to have across all types:
a + b == b + a
Exactly one of
a > b
a = b
a < b
is true, or an type exception must be raised.
The basic Boolean identities
(a or b) == (b or a)
not (a or b) == (not a) and (not b)
not (not a) == a
should all hold, or an type exception should be raised.
With Python accepting both "True" and "1" as sort of
equivalent, there are cases where those don't hold.
John Nagle
--
http://mail.python.org/mailman/listinfo/python-list
