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> I just read in the 'What's New in Python 2.4' document that the None
> data type was converted to a constant:
> http://python.org/doc/2.4/whatsnew/node15.html
>
> """
> # None is now a constant; code that binds a new value to the name
> "None" is now a syntax error.
> """
>
> So, what's the implications of this?
No implications, for any sane code. You can no longer do things like this:
>>> None = 2
SyntaxError: assignment to None
That's all.
> I find the lack of explanation a little puzzling, since I've written code that
> compares a variable's type with the 'None' type. For example, a variable
> would be initialized to 'None' and if it went through a loop unchanged, I
> could
> determine this at the end by using a conditional type(var) == type(None).
Python's None is a singleton, meaning that there is a single, unique
instance of type type(None). So in the case you describe, it's
correct, and idiomatic, to test
if var is None:
instead. Checking type(var) against type(None) will still work, but
was never the best way to do it.
If you have an expression like
var = None
that's fine. You're not changing the binding of name "None" there,
you're changing the binding of name "var".
> What will type(None) return now?
That hasn't changed:
>>> type(None)
<type 'NoneType'>
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