Richard D. Moores a écrit :
On Thu, Aug 5, 2010 at 16:15, Rhodri James <rho...@wildebst.demon.co.uk> wrote:
On Thu, 05 Aug 2010 17:07:53 +0100, wheres pythonmonks
<wherespythonmo...@gmail.com> wrote:
You're not testing for equivalence there, you're testing for identity. "is"
and "is not" test whether the two objects concerned are (or are not) the
same object. Two objects can have the same value, but be different objects.
The interpreter can fool you by caching and reusing objects which have the
same value when it happens to know about it, in particular for small
integers, but this is just a happy accident of the implementation and in no
way guaranteed by the language. For example:
"spam, eggs, chips and spam" is "spam, eggs, chips and spam"
True
a = "spam, eggs, chips and spam"
b = "spam, eggs, chips and spam"
a is b
False
a == b
True
I'm wondering if there isn't considerable predictability to that
"happy accident".
There is, indeed, but that's really implementation details.
Note how 1 'word' is treated versus 2:
x =
'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk'
y =
'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk'
x is y
True
x = 'alksjdhflkajshdflkajhdflkjahsdflkj
hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk'
y = 'alksjdhflkajshdflkajhdflkjahsdflkj
hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk'
x is y
False
CPython caches strings that happen to be valid Python identifiers. But
once again, this is an implementation-specific optimization.
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