[wxPython-users] a bug that i just can't figure out how to fix (to do with dc and DrawText (or so i think))

[Christopher Barker]

Jason Wang wrote:

> I gotta look into the difference between is and ==, thanks for the heads 
> up :D

"=="  means: Do these two objects have the same value?

"is" means: Are these the SAME object?

some of your code worked because python does some optimizing by 
"interning" small integers and strings, so if you create two that have 
the same value, they may very well internally be the same object, but 
they may not. Since strings and numbers are immutable, it shouldn't 
matter to you

 >>> x = 4
 >>> y = 4
 >>> x is y
True

 >>> x = 483736
 >>> y = 483736
 >>> x is y
False
 >>> x == y
True

 >>> a = "a"
 >>> b = "a"
 >>> a is b
True
 >>> a = "A longer string"
 >>> b = "A longer string"
 >>> a is b
False
 >>> a == b
True

Since strings and numbers are immutable, it shouldn't matter to you 
whether they are the same, unless you use "is", when you mean "=="!

So, use "is" only when you really want to know if the objects are the 
same object. This isn't very common, except when you are checking if 
something is None:

if x is None:
    ....

This is a good idea because None is a singleton -- every time you set 
something to None, you get the same object, and == doesn't always work 
with None and objects that support "rich comparison" -- which means they 
override == to do something fancy. numpy arrays are an example of this.

-Chris



-- 
Christopher Barker, Ph.D.
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