Seemingly odd 'is' comparison.

[Duncan Booth]

Tobiah <toby at tobiah.org> wrote:

> Subject: Seemingly odd 'is' comparison.

Please put your question into the body of the message, not just the 
headers.

>>>> print float(3.0) is float(3.0)
> True
>>>> print float(3.0 * 1.0) is float(3.0)
> False
>>>>     
> 
> 
> Thanks,
> 
> Tobiah
> 
Your values are already all floats so float() just returns its arguments. 
In other words you can omit it:

>>> 3.0 is 3.0
True
>>> 3.0 * 1.0 is 3.0
False

3.0 used twice in the same compilation unit is the same constant value used 
twice. 3.0 * 1.0 creates a new float value.

Compare with:
>>> n = 3.0
>>> n is 3.0
False

Here two separate compilations result in two separate values.

In general any immutable results of calculations which are the same may or 
may not share the same object and this can vary according to the version of 
Python or the phase of the moon. Only use 'is' when you actually care about 
object identity, don't use it for a shorthand for '=='.