On 18/10/12 04:41, boB Stepp wrote:
From Programming in Python 3, 2nd edition (p. 22-23):
a = ["Retention", 3, None]
b = ["Retention", 3, None]
a is b
False
b = a
a is b
True
My current understanding is as follows: On the first two lines, two
separate objects are defined, stored in two separate blocks of memory.
Two separate list objects are created. In Python it is rarely helpful to
think about memory usage. Python is abstracted so far from the physical
machine that the connection usually depends on the creator of the
interpreter rather than the language.
These two objects just happen to have the same value, ["Retention", 3,
None], stored in two separate locations.
Yes, but note that its the fact that they are two separate lists that
matters. Even if the contents were the same objects they would still be
two lists:
>>> x = [42]
>>> y = 'spam'
>>> z = None
>>> a = [x,y,z]
>>> b = [x,y,z] # different list, exact same content
>>> a is b
False
>>> a == b
True
>>> a = b
>>> a is b
True
>>> a == b
True
>>>
> a and b, object references (Variables are what I used to call these.
And most folks still do...
I ask: Which implementations of Python do this? In trying to make any
code I write portable across as many platforms as possible, should I
avoid using the identity operator, is (and its opposite, is not),
except when I wish to compare to None?
The point is that you don't know. And, even if you did, the very next
release might change it so you cannot rely on it. That's the real
message - do not assume a particular implementation technique because it
is not guaranteed to be that way or to stay that way.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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