Re: [pygame] Sticky Variables: Terrain Loading

[Casey Duncan <casey@xxxxxxxxxxx>]

On Mar 12, 2008, at 7:54 AM, kschnee wrote:

> > > > > long1 = "water is wet"
> > > > > long2 = "water is wet"
> > > > > long1 is long2
> > False
> > > > > short1 = "water"
> > > > > short2 = "water"
> > > > > short1 is short2
> > True
>
> This really surprises me! I hadn't known about these automatic
> optimizations. Yes, I know that "== != is", but the rest of it is  
> news to
> me.

Python caches short string values (which must not contain a space) as  
an optimization under the assumption that they will be referenced  
repeated (such as for dict keys, which are used often internally by  
the python interpreter). Since short strings without spaces are often  
python identifiers, and referenced very often.

This is an implementation detail of the CPython interpreter (as is the  
reuse of integer objects) which should be of little or no consequence  
to python programs. You should not rely on identity values (e.g.,  
id('foo') or 'foo' is 'foo') for strings, integers and other  
"primitive" types since the interpreter my arbitrarily use singletons  
to represent some values. IOW using the "is" keyword for strings,  
integers, floats, etc. is not useful and potentially confusing. To  
compare these values for equality, only use ==.

"is" is typically useful only for class instances and certain special  
objects like None. Note that:

a is b

is equivalent to (though much faster than):

id(a) == id(b)

-Casey