[Tutor] is gotchas?

[Kent Johnson]

Hugo González Monteverde wrote:
>>       Hmmm! Hmmm!
>>       Lookee here:
>>       ## console session
>>      >>> a=[1,2]
>>      >>> b=[1,2]
>>      >>> a is b
>>     False
>>      >>> c='1'  ## one byte
>>      >>> d='1'  ## one byte
>>      >>> c is d
>>     True
>>      >>> c='1,2'
>>      >>> d='1,2'
>>      >>> c is d
>>     False
>>
>>     The Hmmm! is emmitted because I'm thinking that if everything is an
>>     object in python, then why does `c is d` evaluate to True when
>>     the assigned value is 1 byte and evaluate to False when the assigned
>>     value is more that 1 byte?
> 
> One and two byte strings are currently optimized in cPython as the same 
> object, referenced multiple times.

It's not just one-byte strings; I think any string that can be a Python 
identifier is interned and will always be the same string.

In [1]: a='abcd'

In [2]: b='abcd'

In [3]: a is b
Out[3]: True
> 
> Note that this is not to be relied upon! Jython, Ironpython, Python3000 
> or cPython itself may break it!

Right. But why do you even care? I don't think I have ever written code 
that compares strings using 'is'; just use ==.
> 
> Still cannot find a reference doc for this....

Probably the source code will be the only reference, this is an 
implementation detail.

Kent