On 4/22/2012 21:43, John Nagle wrote:
On 4/20/2012 9:34 PM, john.tant...@gmail.com wrote:
On Friday, April 20, 2012 12:34:46 PM UTC-7, Rotwang wrote:
I believe it says somewhere in the Python docs that it's undefined and
implementation-dependent whether two identical expressions have the same
identity when the result of each is immutable
Bad design. Where "is" is ill-defined, it should raise ValueError.
A worse example, one which is very implementation-dependent:
http://stackoverflow.com/questions/306313/python-is-operator-behaves-unexpectedly-with-integers
>>> a = 256
>>> b = 256
>>> a is b
True # this is an expected result
>>> a = 257
>>> b = 257
>>> a is b
False
Operator "is" should be be an error between immutables
unless one is a built-in constant.
[...]
I can't think of a single case where 'is' is ill-defined.
You're blaming 'is' for revealing what's really going on. 'is' is /not/
implementation-dependent. It's /what's going on/ that's
implementation-dependent.
"a is b" is true iff 'a' and 'b' are the same object. Why should 'is'
lie to the user?
It does exactly what it says it does: it tells the user whether two
objects are the same.
In C++, for greater efficiency, two objects are compared for equality by
first checking their addresses and then their contents. 'is' would be
perfect for that.
What I don't like is 'isinstance'. I'd prefer an infix operator 'isa' or
'is_a'.
Kiuhnm
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