On 4/23/2012 12:38 AM, Devin Jeanpierre wrote:
On Sun, Apr 22, 2012 at 9:22 PM, Terry Reedy<tjre...@udel.edu> wrote:
On 4/22/2012 3:43 PM, 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.
There is no ambiguity about the meaning of 'is' in Python. It is always
well-defined. So the suggestion is a nullity. The equivalent function form
in Python is
Bollocks it's well defined. We've already agreed that "1 is 1" may or
may not return True.
'We' have not so agreed. The function should tell the truth. Let me
repeat: 'is' is a function whose two inputs are not eternal mathematical
values but rather are computational implementation objects with finite
lifetimes.
It is wrong to interpret the Python expression '1 is 1' as a
mathematical expression. It simply is not such. Trying to do so only
leads to confusion, as this thread show.
The 'is' function answers a question about the current state of a
particular temporal computational process. Its output is well-determined
given its actual input. Like other introspection functions, it is useful
for testing an implementation.
Then let's just go to the definition:
http://mathworld.wolfram.com/Well-Defined.html
"An expression is called "well-defined" (or unambiguous) if its
definition assigns it a unique interpretation or value.
The definition of 'is' does just that.
The same complaint applies to your suggestion that id() is
well-defined, and I stopped reading there.
In much or most of mathematics, there is little or no concept of
'identify' separate from 'value'. In Python, for Python objects, there
is. This is necessary if Python is to model real life as well as do
mathematical computations. If you do not like it, ignore it.
Since the existence of objects is time-dependent, the mapping from
objects to ids also is. As any one time, the mapping is not only
well-defined but bijective, so there is an inverse mapping from ids to
objects.
--
Terry Jan Reedy
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